How to Choose a Power Inverter

How to Choose a Power Inverter

When creating any off-grid power system for your tiny house, besides choosing the charge controller and battery bank, it’s also important to know how to choose a power inverter. The inverter’s main function is to convert DC power to AC power; additionally, inverters are part voltage step-up devices which can covert a lower DC voltage (from 12/24/48 volt battery banks) to a higher AC voltage (which is 120/240 volts as required in most countries). Given that most devices run on AC power, power inverters are needed to make the battery banks’ DC power compatible with home appliances.

In Section I, I’ll first go over what inverter features and settings to consider when choosing an inverter. In Section II, I’ll cover the different AC output waveforms of inverters (especially pure and modified sine waves); and how they affect the quality of the AC output waves. Finally, at the end of the Section II, I’ll list down which devices can be used with pure and/or modified sine wave inverters.

Before you continue, here is a collapsible Table of Contents menu for easier navigation throughout this post (using anchor links). If you click ‘Top (⇑)’ next to any of the section and sub-section headers, it will take you back to this menu. For some of my longer future posts, I will add similar Table of Contents navigation menus.

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I.Basics of Inverters (top⇑)

Before choosing any power inverter, one needs to determine which inverter features and settings are needed in the off-grid power system. Below are the main inverter features and settings you need to consider when selecting the appropriate inverter.

Power Capacity (top⇑)

The power capacity is the maximum amount of continuous power(in watts) that can be drawn and outputted by the inverter. This is basically the maximum load the inverter can handle.

For example, an inverter with a power capacity of 500 Watts can theoretically draw a maximum of 500 watts of DC and ouput nearly 500 watts of AC into the turned-on devices. However, if the turned-on devices together need to draw 700 watts, the inverter won’t be able to provide 700 Watts because 500 Watts is the limit.

Also, the inverter can be damaged if there is too much wattage. Due to these situations, proper sizing of the inverter is important. The way to do inverter sizing was explained in my earlier post on battery bank sizing. When choosing any inverter, the inverter’s power capacity should be your first consideration.

Surge Power Capacity (top⇑)

Surges are power spikes used to start up engines/devices as many of them require more power starting up than when needed to run;  the surge time ranges from few seconds to up to 15 minutes. The surge power capacity is the maximum total surge power that an inverter can accommodate for multiple device starting up at once.

For example: there is an inverter with a surge capacity of 700 watts and there are 2 AC devices with surge capacity of 400 watts each. The total combined surge of the 2 devices is 800 watts. Because 700 < 800, the 700 watt surge capacity of the inverter is unsuitable. The inverter’s surge capacity has to be sized up. Redoing the example, if the inverter’s surge capacity was instead 900 watts, then both devices can surge at once.

The previous inverter(w/700 watts surge capacity) could work if each device surges one at a time, but sizing generally requires taking into account the worst case scenario which is where all attached AC devices (w/surge ability) experience a startup surge at once; similar to when sizing for continuous power capacity where all AC devices run at once (from previous subsection).

Lastly, because a device’s surge output is higher than it’s continuous power output, the inverter’s surge power capacity needs to be higher than its continuous power capacity by roughly 2-3+ times. Basically, any inverter needs to be sized to account for both total continuous power and total surge power of all connected AC devices. Sizing for surge capacity was also explained in the battery bank sizing post mentioned in the previous subsection.

Battery Voltage Handling (top⇑)

Any inverter will specify what battery bank voltages can work with the inverter. Many are rated to work with battery banks with an equivalent voltage of 12, 24, or 48 VDC (volts DC); some inverters have multiple voltage settings. The voltage has to match the voltage choice of the battery bank.

There are a few high-end ones that can work with 120, and even 240 VDC. These one are quite expensive and aren’t really needed in most systems.

Conversion Efficiency (top⇑)

When converting DC to AC power, the conversion isn’t 100% efficient and some of the original power is “lost”. To start with, a tiny portion of the DC power is used for the power consumption of the inverter as it is an electrical device (especially modern ones). This can more or less be accounted for during battery bank sizing.

However, most of the inefficiency naturally comes from wire heating as well as the sum of the inefficiencies of various working components inside the power inverter. Depending on the inverter, the conversion efficiency ranges from 70% to 98%. This is a VERY important consideration because power will be lost during the DC-to-AC conversion; hence, both the renewable power source and battery bank need to be sized(or scaled) up to offset losses. Ideally, an inverter with high efficiency should be picked to the minimize the amount of additional sizing (or scaling) needed.

Again referring to my earlier post on battery bank sizing, it also explains how to apply an inverter’s efficiency value.

Total Harmonic Distortion(THD) (top⇑)

Total harmonic distortion(or THD) is a property of the output AC wave of an inverter. In easy terms, it means the more the AC output wave deviates from a pure sine wave, the greater the total harmonic distortion(or THD). The closer the inverter’s output wave matches a pure sine wave, the less THD there is. Mathematically speaking, the THD value is the ratio/percentage that an AC output wave deviates from the ideal form (in this case a pure sine wave).

Inverters with low THD are more expensive than inverters with higher THD due to the fact that higher quality components are needed to smooth out THD . This is basically a trade-off. More on relationship between different wave forms, THD percentage, and which types of inverters to use based on THD will be covered later in Section II of this post.

Low vs High Frequency Inverters (top⇑)

The frequency here refers the internal operating frequency of the inverters, not the frequency of the output waveform which is always 50-60 Hz(which is the national standard used in power lines and for nearly all home devices).

Low-frequency(LF) inverters use high-speed switches to convert DC-to-AC power, but drives the switches at low frequencies which can range to at most few hundred hertz . LF inverters tend to be large and bulky because low-frequency transformers are large, so space has to be a consideration. LF inverters also have high surge capacity(about 4-8 times continuous power capacity). Also, because the internal frequency ranges are often within the human hearing’s decibel range, there can be an acoustical buzz (which can be a bother).

High-frequency(HF) inverters’s frequency tends to be in the kHz region like around 20-50 KHz or more. The size of these inverters are much more compact than low frequency inverters via smaller transformers. Due to these HFs being above human hearing range, there is less to no acoustical noise. Surges capcity in HF inverters are smaller(about 2-4 times continuous power capacity); the initial surges are smaller and shorter lasting than those of low-frequency inverters. Lastly, due to HF inverters having additional components to convert LF input to HF internally, HF inverters are slightly less robust than LF inverters.

To better understand the difference between inverters using high or lower internal operating frequencies, check this handy link.

Idle Power Consumption (top⇑)

Because inverters are electronic devices, they will use small amounts of power throughout the entire day just to be on standby when no loads are running. This value usually ranges from 5-15 Watts depending on the inverter. In battery bank sizing, the inverter’s idle power consumption needs to be accounted for. Note that LF inverters use less idle power than HF ones due to fewer components. Ideally, the inverter’s idle power should be as little as possible.

Other Auxiliary Considerations (top⇑)

  • Internal Protection: there needs to be internal circuitry to sense when to disconnect or regulate at certain points; like when the battery bank’s charge is too low or if the load is too much for the inverter. Many inverters have circuit breaker capabilities. Also, some inverters should have grounding capability for additional protection.
  • Automatic On/Off: when this feature is active, the inverter is normally off. However, when any connected AC device is turned on and attempts to draw a certain amount of watts, the inverter is tripped on. Otherwise, when all AC devices are turned off, the inverter is tripped off. For example, the trip maybe set to something like 10 watts; if a turned-on device uses 10 watts or more, the inverter automatically turns on. However, there are times when the automatic on/off feature maybe cumbersome to deal with. In that case, this feature can be switched over to always-on idle power.
  • Phantom loads: this is when a load(s) draws power, even though it is turned off. This occurs because there is still a completed circuit from being plugged in and connected; the drawn power is wasted. Some inverters may have a built-in capability to prevent phantom loads from occurring. If not, the AC device needs to be unplugged when no in use. Better yet, switches should be installed at outlets to avoid phantom loads.

Right now, everything that has been explained so far are factors to consider when choosing a power inverter. What hasn’t been covered yet are the AC output waveforms and how they affect inverter choice. This will all be covered in Section II with great detail.

II.Pure vs Modified Sine Wave Inverters (top⇑)

The above factors listed so far are all considerations when picking any inverter. One of the most important considerations not yet covered is picking an inverter based on the AC output’s waveform. The waveform determines what AC devices can be run and under what circumstances. This means a poorly selected power inverter can damage the connected AC devices. When selecting the actual inverter, it mainly come down to pure vs modified sine wave inverters.

This section also uses math equations (with commentary) to explain the shape and behavior of the AC output wave forms. Terms like VAC (volts AC), VAC RMS, and VAC-Peak are used in these equations. When dealing with inverters’ specifications and off-grid power systems, these electrical terms are bound to show up. By seeing how they’re used in the below AC output waveforms’ analysis, you should know what to make of their values when you finally see them in real-life.

A.Wave Forms (top⇑)

Square Waves (top⇑)

While inverters with output square waves do exist, none of them are worth using even if they’re free. A square wave’s general appearance is shown in the diagram below:Square Wave Inverter
Because of the sudden voltage changes from fixed positive to fixed negative and vice-versa, these inverters don’t have any ability to regulate the AC output voltage. This means that the AC output voltage cannot be kept fixed at 120 VAC RMS which is the required to run American appliances (other countries use 240 VAC RMS).

The fact that the AC output voltage cannot be regulated is shown in the above Graph’s legend where the different VDC values are converted to different VAC RMS values. By the way, the VAC RMS (root mean square) is the “effective voltage” in place of the continuously varying AC voltage. To understand how the VAC RMS changes in square wave inverters, view the math logic below.

The varying VAC RMS can be explained with the following equation:
Average Voltage(VAC RMS) = (Area under half a cycle(V*ms)) ÷ (half-cycle time(ms))

For the 100 VAC Peak for 10.5 VDC, the equation is filled as follows:
Average Voltage = (8 ms(width) × 100 V(height)) ÷ (8 ms) = 100 VAC RMS

For the 120 VAC Peak for 12.6 VDC:
Average Voltage = (8 ms(width) × 120 V(height)) ÷ (8 ms) = 120 VAC RMS

For the 140 VAC Peak for 14.7 VDC:
Average Voltage = (8 ms(width) × 140 V(height)) ÷ (8 ms) = 140 VAC RMS

Basically, in order for square wave inverters to be viable in real-life, output voltage would have to be 120 VAC RMS at all times since American appliances can only operate at 120 VAC RMS (some other countries use 240 VAC RMS as the standard). However, this is unrealistic because the input VDC value (=12.6 VDC in the graph) cannot be kept fixed at all times and can vary for any number of reasons.

Because the average AC Voltage isn’t consistently at 120 VAC RMS, even simple AC loads like motors or lightbulbs will fail prematurely. Due to high variation in the VAC RMS value, square waves will have a really high amount of harmonic distortion(more will be explained later in this post). The THD for the AC output waves of square wave inverters is >40% which is considered very high and “unclean”. In conclusion, avoid square wave inverters for any usage as it is outdated.

Modified Sine Waves (top⇑)

Modified sine waves (MSWs), in comparison to square waves, are smoother. The appearance of a modified sine wave is shown below:
Modified Sine Wave Inverter Waveform
In the above diagram, the Modified Sine Waves (MSWs) consist of instantaneous increases and decreases in voltage together with the signal sitting periodically at 0 Volts. Actual MSWs looks very similar to the above waveforms if not exactly.

Unlike square waves, the varying widths of the “pulses”(where the signal doesn’t sit at 0 volts) inside the MSWs enable the inverter to have some ability to regulate the VAC RMS voltage. This means that the output can have a consistent 120 VAC RMS regardless of the VDC input(s). To see how the MSW inverter regulates the output to be 120 VAC RMS, refer to the math logic below.

The VAC RMS can be explained with the following equation:
Average Voltage(VAC RMS) = (Area under half a cycle(V*ms)) ÷ (half-cycle time(ms))

For 9.5 VDC input with output 120 VAC peak:
Average Voltage = (8 ms(width) × 120 V(height)) ÷ (8 ms) = 120 VAC RMS

For 12.6 VDC input with output 160 VAC peak:
Average Voltage = (6 ms(width) × 160 V(height)) ÷ (8 ms) = 120 VAC RMS

For 15.7 VDC input with output 200 VAC peak:
Average Voltage = (4.8 ms(width) × 200 V(height)) ÷ (8 ms) = 120 VAC RMS

Because the output is consistently 120 VAC RMS, the MSW inverters are suited for real-life use in off-grid power systems. However, because the shape of MSWs still deviates far from the shape of pure sine waves, the total harmonic distortion (THD) is still significantly high and will affect sensitive devices. For MSW inverters, the THD of the output wave is generally around 20-40%; which is less than the THD in the output for square wave inverters.

While MSWs can be damaging to some devices, there are other devices that can work with MSW inverters. More of this will be covered in the eventual comparison between pure vs modified sine wave inverters later in this post.

Stepped Sine Waves (top⇑)

In stepped wave forms, instead of having a single positive or negative pulse with an “off” period in-between, a stepped sine wave inverter produces a series of different voltage levels which can be arranged to produce a waveform with a shape similar to a “stepped temple” as shown below.Stepped Sine Wave InverterRegarding the numbers of steps in the waveform from above, it varies as the battery voltage changes. At higher battery voltages(VDC), there are fewer, but taller steps; at lower battery voltages(VDC), there are many shorter steps.

For example, in the above waveform, there are 6 steps for the battery input of 17.2 VDC. To understand why, look at this calculation:

6 steps × 17.2 volts = 103.2 volts

However,  7 steps × 17.2 volts = 120.4 volts.

Because 7 steps will cause it to exceed 120 volts, the inverter’s waveform will have 6 steps at 17.2 VDC input. From using the above logic, let’s try using 10.5 VDC battery input since 17.2 VDC is rather unrealistic(but was shown for demonstrative purposes):

120 volts ÷ 10.5 volts = 11.429 steps(round down)

11 steps × 10.5 volts = 115.5 volts

Basically, when the input battery bank voltage is at 10.5 VDC, the wave form will have 11 steps.

With regards to stepped sine waves, the general THD for the output wave is around 5-15%. The produced wave form has a much lower THD than any MSW inverters. Plus, this wave form offers good performance along with high DC-to-AC conversion efficiency (as much as >90%).

The reason stepped sine waves have low THD is due to the fact that stepped waves deviate little from pure sine waves in terms of shape(see diagram in next section). Also, if the area underneath 1/2 cycle is taken, it would be close to 120 VAC RMS.

Because stepped sine waves have low THD and are quite close to 120 VAC RMS, inverters with these wave form outputs are often considered “clean” enough for virtually all household appliances except possibly the most sensitive of devices (which may then require pure sine waves). The latter is unlikely to be found in any normal households let alone off-grid tiny houses. Lastly, steeped AC sine waves are “clean” enough to be sent back to an electrical municipal company for credit. As a result, inverters that produce stepped sine waves are commonly grouped together with inverters that create real pure sine waves as “pure” sine wave inverters.

Pure Sine Waves (top⇑)

The pure sine waves are smooth and continuous and shaped like real sine waves. Below is a diagram of pure sine waves(in comparison with a stepped sine wave):Pure Sine Wave vs Stepped Sine WaveIn pure sine waves, the voltage rises and falls smoothly with a phase angle that changes smoothly. Also, the polarity changes instantly when it crosses 0 Volts; just like in the diagram above.

With regards to the above diagram, a pure sine output wave should consistently be of 120 VAC RMS.

With regards to the sine wave, the Vrms = peak voltage × (1/(√2)).

Vrms = 170 VAC × (1/(√2)) = 120.2082 VAC RMS ≈ 120 VAC RMS

Pure sine waves also have the lowest THD among the different wave forms. Although pure sine waves by name should not have any THD; in actuality, pure sine waves inverters produce output waves with THD of 0-5%. This low THD means virtually any device(even really sensitive ones) can run without problems. However, pure sine wave inverters are the most expensive.

B. What is Harmonic Distortion? (top⇑)

Harmonic Distortion In-Depth (top⇑)

The output sine wave of an inverter is not a completely pure sine wave; it’s a complex wave made up of the main sine wave plus additional waves that act like “noise”. The additional waves(harmonics) which act like “noise” is not really noise; but these harmonics are a result of electrical topology inefficiencies(or system non-linearities) that result in an output AC wave that doesn’t completely match a sine wave.

As for how these complex waves are described:
First, there is a pure sine wave at a fundamental frequency(f1). These distortions are measured in terms of added overtones(or harmonics) to the original pure sine wave with f1. An overtone is any frequency higher than the fundamental frequency. The frequencies of the added overtones are integer multiples of the pure sine wave at the fundamental frequency(f1). Because MSWs are a variation of square waves, the integer multiples are always odd numbers (with the even harmonics equaling zero). Also, the amplitude for the odd harmonics follows this equation: A1*(1/n) where A1 is the 1st harmonic(or original) amplitude and n is the current harmonic.

An example fundamental(1st harmonic) sine wave could be:
V1p*sin(2π*f1*t) where V1p is peak voltage

The 3rd harmonic would be:

The 5th harmonic (and beyond) would be:
V5p*sin(2π*(5*f1)*t) and etc….

The final complex function is:
V(t) = V1p*sin(2π*f1*t) + V3p*sin(2π*(3*f1)*t)+ …….+ Vnp*sin(2π*(n*f1)*t)

In the above equation, the number of odd harmonics will depend on the waveform; some of the odd harmonics can be skipped while all the even harmonics equal zero.

Below is a graph showing how a square-like complex waveform breaks into the fundamental waveform plus its harmonics:Total Harmonic Distortion DefinitionFrom the above graph, it’s evident to assume that as more harmonics are added to a pure sine(fundamental) wave, the closer the distorted wave resembles a modified sine or even square wave. On the other hand, fewer added harmonics will allow the distorted wave to resemble much closer the original pure sine wave.

The above was a simple example, the breaking down of the complex waveforms for modified and stepped sine waves may be slightly different.

Finally, as to how THD is calculated, it is done using the formula below:
\mbox{THD} = {\sqrt{V_2^2 + V_3^2 + V_4^2 + \cdots + V_n^2} \over V_1}

In the above formula, the inputs are the RMS voltages of the matching harmonic’s function. Hence, this is why the complex wave form in broken into harmonics to obtain their amplitudes to fit into this formula. The THD output will be a ratio ranging from 0 to 1. The ratio can be multiplied by 100 to get THD as a percentage. The THD shown as a percentage is what is shown on the specs sheets for any inverter; the lower the better.

Effects of Harmonic Distortion on Devices (top⇑)

As discussed earlier, harmonic distortion is most prevalent in square, modified, and stepped sine wave inverters(excluding square waves from discussion).

Below is a general graph showing square, modified, and pure sine-wave(no stepped sine unfortunately). This graph will to be used to help with illustrating the effects of THD. Pure Sine Wave vs Modified Sine Wave InverterThere exists many appliances that rely on reading the AC waveform of the input power. Because high THD distorts the AC waveform, this throws off any appliances’ circuitry which relies on reading the waveform.

Listed below are 3 common ways that high THD can disrupt waveform-reading circuits in appliances:

  1. High THD disrupts circuits that rely on detecting the phase angles of the AC power’s waveform. These circuits sense phase angles using the gradual increases and decreases in the waveform’s voltage. Because phase angles “shifts” a waveform, some circuits read those phase angles to adjust relevant speed(like motor speed) and voltage controls in response to the detected “shift”. In pure sine waves, the phase angle changes are smooth(or smooth enough in steeped sine waves).
    In MSWs and square waves, the phase angles change sharply in accordance with the sharp changes in voltage levels. For these waveforms, the circuit will only notice the phase angle change after the sudden voltage drop/increase; this delays needed adjustments by the circuit(s) later than it should compared to relying on the gradual increases and decreases of voltages found in pure/stepped sine waves. Lastly, the “sitting periods” found in MSWs also confuse phase angle sensing circuits.
  2. High THD disrupts circuits that utilize the instantaneous switching between positive and negative voltages for timing control. There are many circuits which rely on counting how many times the voltage crosses zero cleanly. If these types of circuits use MSWs to collect ‘crossing counts’, the parts of the MSWs that periodically sit on zero volts can easily confuse the circuits as multiple ‘crossing counts’ even though the waveform technically didn’t cross zero volts and change polarity. This can cause devices to malfunction.
  3. Waveforms associated with high THD can cause connected devices to overheat. To explain this, first know that when AC power courses though electrical components, heat gets released in proportion to its voltage. Hence, when the voltage gradually increases positively or negatively from zero, the temperature changes slowly and causes minimal to no harm to the appliance. In stepped sine waves, while there are small jumps in voltage level, voltage changes are still gradual overall and shouldn’t cause noticeable harm to devices.
    Regarding MSWs and square waves, however, there are large instantaneous jumps between relatively high voltage levels which cause sudden temperature changes; these sudden temperature changes will eventually damage sensitive circuits found in many home appliances. Lastly, voltage levels for modified sine waves(and squares) are also relatively high for a longer period of time than sine waves which only have high voltages mainly at the peak area; this also contributes to heating.

In Conclusion: If your selected appliance(s) has complex circuitry and relies on the shape of the AC power’s waveform in some shape or form, it’s likely unsuited for Modified Sine Wave inverters. However, you can still ask the manufacturer regarding how much THD the device can handle and match that against a power inverter’s spec sheet.

C.When to use Pure or Modified Sine Wave Inverters (top⇑)

I will first list down AC appliances that can work well (for at least short-term) on MSW inverters. Then, I’ll list down AC appliances which have sensitive circuits and should likely be used on pure sine wave inverters. Lastly, next to each listed appliance, there is some commentary on why said device works well with the MSW or pure sine wave inverter.

Appliances that CAN run on Modified Sine Wave Inverters (top⇑)

It’s already apparent that any device will work better and last longer on pure sine wave inverters. However, one maybe forced to use a MSW inverter because it’s much cheaper, robust, and more widespread. Below are devices that generally work at least moderately long on MSW inverters:

  • Resistive Loads work fine with MSWs because resistive loads only want the 120 VAC RMS from the MSWs. This type of load simply uses electricity to create heat. However, be wary that newer resistive loads may have electrical circuits and microprocessors which depend on the AC waveform. Common resistive loads include:  Incandescent bulbs, Electric HeatersToaster, Ovens, Stoves, Clothes Iron, and Electric Radiators.
  • Other devices that work potentially well with MSWs are chargeable cordless appliances. Virtually all cordless appliances rely on DC power stored from their rechargeable battery pack. When a connected charger converts AC-to-DC power for the battery pack, the existing harmonic distortion gets “smoothed out”. Everyday cordless appliances include:
    • Digital devicesCellphones, Smartphones, Tablets, Digital Camcorders, Digital Cameras, and etc.
    • Cordless yard & power toolsHandheld Drills, Handheld Saws, Hedge Trimmers, Chainsaw, Leaf Blowers, and etc.
    • Cordless indoor items: Electric Shavers, Egg Beaters, and etc.
  • Larger digital devices can run on MSW inverters if it uses a SMPS(switched-mode power supply). SMPS is a component which efficiently converts AC/DC to the needed DC voltage. Devices such as Computer Workstations and certain LCD TVs have SMPS. However, the noisier DC can slightly reduce the lifespan of the silicon circuits; as well as create “distortion lines” in the screens. Despite being able to work, it’s generally safer to run these devices on Pure Sine Wave inverters.
  • Appliances using AC motors could run on MSWs. However, MSWs often cause AC motors to heat up. The impact of this varies depending on the THD tolerance as well as durability of motor components. Appliances using simple AC motors like cooling fans and ceiling fans can work with MSWs for longer periods of time. If the AC motor has complex features like automatic speed control, using any MSWs is undesirable.
  • Appliances using DC motors (including brushed and brushless motors) will run since these do not rely on any AC waveform. As implied earlier, DC motors are primarily found in appliances that utilize DC battery packs (unless the corded appliance has a built-in rectifier). This includes the cordless appliances which use motors from the earlier list.
  • Appliances using universal motors will also run on MSW inverters. Similarly, universal motors(which runs on both DC and AC power) do not depend on AC waveform characteristics. The main strength of universal motors is that it can generate high start torque and high RPM. Appliances using universal motors include: Vacuum cleaners, Leaf Blowers, Weed Trimmers, non-cordless motorized power tools (like various saws and drills), and etc.
  • Older appliances (like old Tube TVs) that don’t rely on microprocessors will perform much better on MSW inverters than newer appliances which have embedded microprocessors and chips. For example: a vacuum cleaner with only an ON/OFF switch (and manual switches for speed control) will perform better with MSW inverters than vacuum cleaners which have microprocessors for automatic speed control. Any corded appliance that uses complex circuitry for timing, speed, or another form of control will rely on the AC waveform in some way. This means any of the previously listed corded appliances will no longer work on MSW inverters if that version of the appliance uses microprocessors.
  • Any appliance whose THD tolerance can handle the THD generated by the MSW inverter. As earlier stated, the appliance’s THD tolerance can be determined via specification or by contacting the manufacturer.

Appliances that MUST run on Pure Sine Wave Inverters (top⇑)

As already stated, virtually all AC appliances will run smoothly on Pure Sine Waves (PSWs). Hence, I will only list the appliances that must run on PSWs (otherwise, these will get damaged by MSWs for even short-term use). Here are the well-known appliances:

  • Inductive Load appliances (including those using induction motors (a.k.a. AC motors)) must run on pure sine waves. Popular induction appliances include any Refrigerators/Freezers, Washers, Dryers, Air Conditioners, Microwaves, and etc. 
  • Devices that are considered non-linear loads . Non-linear loads are devices that add harmonic distortion the incoming AC waveform when entering the device(s). With MSWs’ already high THD, more harmonic distortion is undesired. Examples of non-linear appliances include Printers, Fax Machines, Electronic Lighting Ballasts, Computers, and HD TVs.
  • Any appliances with built-in clocks like VCRs, DVD players, Coffee Makers, Bread Makers, Microwaves, Plugged-in Clocks, and etc. need PSWs due to the timing depending on reading the AC “crossing counts”.
  • Light Dimmers and other output voltage control devices that rely on phase angle change sensing.
  • Ceiling fans, motors, and other devices with rotation speed control which rely on phase angle change sensing.
  • Most brands of Compact Fluorescent Light (CFL) bulbs. All fluorescent light require the use of lighting ballasts(which are non-linear devices) to regulate the voltage and current; otherwise, the fluorescent lights would rapidly draw current, overheat, and then burnout. CFL bulbs are basically compact fluorescent lighting tubes with a compact ballast built-into each bulb.
  • Most brands of Light-emitting Diode (LED) bulbs. Similar to CFL bulbs, most LED bulbs are not suited for high harmonic waves. Additionally, there are some LED bulbs which have built-in motion sensors and light dimmers (as explained, these non-linear functions don’t work well with MSWs).
  • High intensity discharge (HID) lamps like Metal Halide lamps. These can be damaged by high THD. Most manufacturers(and their spec sheet) should be able to tell someone what the THD % tolerance is.
  • Nearly all devices utilizing microprocessors must run on pure sine waves. As stated earlier, besides things like computers and HD TVs which have built-in adapters to convert AC-to-DC for internal use, microprocessors can be found even in items not traditionally having a microprocessor(s) like plug-able flashlights or stoves.
  • Corded-versions of all the “cordless” devices listed earlier for MSW inverters. Since corded appliances do not have a rectifier to smooth out any harmonics, the appliances will directly receive the AC waveform’s harmonics. Hence, PSWs are a safe bet for corded AC appliances.
  • Other Devices with a THD % tolerance less than the THD of the output waveforms of available MSW Inverters. Check with the manufacturer(and/or with relevant spec sheet)
The superiority of pure over modified sine waves is clear. However, because using Modified Sine Wave inverters is more complicated due to their high THD; I’d like to add some ending commentary on MSW inverter usage.

Final Commentary on Modified Sine Wave Inverters (top⇑)

Saying it again, Pure Sine Wave (and Stepped Sine Wave) inverters will work for all AC appliances because their AC waveforms are virtually clean of harmonics. Whenever possible, always use a Pure Sine Wave(PSW) over a Modified Sine Wave(MSW) inverter.

However, I also stated that there are pros to using a MSW inverter when a PSW inverter is not available. First off, MSW inverters are about 2-3 times cheaper or more than PSW inverters due to the fact producing MSWs needs less sophisticated components; the fewer components also leads of overall greater DC-to-AC conversion efficiency than with PSW inverters. Lastly, for temporary or small scale use, using a MSW inverter(s) can be more cost-effective.

I have heard from others that MSW inverters work well with most AC home appliances. However, the word “most” is too vague to give any clear idea of what appliances can run on MSWs without first having to check each chosen appliances’ THD tolerance. Hence, I created the above 2 lists earlier to respectively list which appliances can at least work moderately long on MSWs and which appliances must run on PSWs.

If you ever need an MSW inverter for a period longer than short-term or emergency use, consider connecting DC-powered versions of certain appliances to the battery bank; in order to bypass the MSW inverter. For example, there exist fridges/freezers on the market that can run on DC power. Similarly for lighting purposes, there exist versions of fluorescent(CFL) and LED bulbs that can also run on DC power.

While MSW inverters are still attractive due to their low costs; one trend to watch out for is that technological progression will continue to make PSW inverters more affordable (and encourage wider usage).

Finally, always check the THD generated by the MSW inverter in the specifications. Some will produce output waves closer to square waves while others can be closer to stepped sine waves.

III.Ending (top⇑)

By now, you should know how to choose a power inverter. Everything covered is summarized in the following steps:

  1. First determine if the power inverter(s) will be used for a residential home, off-grid tiny house, or for some small scale/temporary application(s).
  2. Next, determine the loads and their power and time usages. The load types (and financial resources) determine if one should use a Pure or Modified Sine Wave inverter(s). By know, you should know the pros and cons of each as well as the impacts of THD.
  3. Determine the max. power and surge power capacities needed for the inverter. As explained, these can be determined as demonstrated in my earlier post on battery bank sizing.
  4. Finally, determine any additional features, setting, and specifications needed for the inverter depending on the system’s needs. For example: Don’t pick a 12 volt inverter for a 24 volt battery bank.
With all the information given in this post, you can now make informed decisions for each of the above steps and decide which power inverter(s) to get. Ultimately, this post will benefit anyone who needs to select an inverter for their off-grid system or anyone who needs to know how to operate an inverter in an existing system.

If you liked this post, make sure to share(via social media bar) and subscribe for email updates(if you already haven’t). Leave a comment below regarding your thoughts, ideas, and experiences regarding the use of inverters and their place in tiny house off-grid power systems.


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  1. Header Image: Copyrighted composite image created from:
  2. Square Wave Inverter Graph: Original Image created and owned by this website. PLEASE read the “Content Reuse & Attribution Policy” from the bottom with regards to copyright and reuse from this site.
  3. Modified Sine Wave Inverter Graph: Same as #2.
  4. Stepped Wave Inverter Graph: Same as #2.
  5. Pure vs Stepped Wave Comparison: Same as #2.
  6. Distorted Wave Breakdown: Same as #2.
  7. Three waveforms for THD comparison: Same as #2.
  8. Inverter on Hidden Pinterest Image(visible only with Pinterest Browser Button): I reserve ALL RIGHTS to this image as I created this for my Pinterest sharing needs. This composite pin image consists of:
  9. Facebook/Google+/etc. Post ImageI also own all rights to this image as I made this for my other social media sharing needs. This is made up of the first two used in the above hidden Pinterest image for this page.
How to Choose a Power Inverter. #Inverters are needed to convert DC power into the AC power needed to run household appliances. To choose the best #powerinverter for you DC #batterybank and #electrical system, you need know about the general features and settings of inverters as well as the differences between #ModifiedSineWave and #PureSineWave inverters. Click through to learn what you need for choosing your power #inverter.

4 Responses

  1. Amit Misal says:

    hi Bobby,
    Just visited your site while surfing the internet and found the article about “How to Choose a Power Inverter for Off-Grid Systems” very helpful. I myself wanted to purchase an inverter for my house because I live in a place where there are power cuts for most of the time.
    However, I had no knowledge of the necessary technical specifications and that was why I was not able to purchase one. Now from your site, I now know exactly what specifications to look for and ask for in my new inverter. I would like to thank you for making this article.

    • Bobby Kundu says:

      I’m sorry to hear about you having to put up with power cuts. I know that one of the causes of power cuts are faulty electrical components; including faulty inverters. I am glad that my article has been helpful if a faulty inverter was what was bothering you. I wish you luck in taking care of your power cut issue. Finally, if you have any questions, feel free to comment or use the contact form.

  2. bioelectrobot says:

    This is a great article. I always enjoy reading about electricity. I study mathematics, however, my knowledge of electricity is quite limited. Great use of equations and graphs to help the reader better understand the concepts. This is a most interesting article indeed. Excellent explanation of stepped sine waves. I like your explanation of Total Harmonic Distortion. This is an important concept and you’ve described it very well in a short paragraph. It’s also nice to read the detailed THD information you’ve provided with the associated equations. You’ve put some time and consideration into this article, and I appreciate that. The novice and the expert alike can benefit from the information you’ve posted here.

    • Bobby Kundu says:

      I am glad you liked the post/article. The reason I posted this in the first place is because the inverter is an important component in renewable energy systems. I posted this article because it could provide some help when understanding and selecting an inverter. If you or anyone else who reads this ever gets involved with a renewable energy system(s) and come upon the inverter portion of the system(s), I hope this article’s content would be of help to you and others.

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