How to Choose a Power Inverter
When creating an off-grid tiny house power system, 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. Given that nearly all devices run on AC power, an inverter is essential to making the DC power from the battery bank compatible with home appliances.
In Section I, I’ll first go over what kind of inverter specs one should analyze when choosing an inverter. In Section II, I’ll cover the different AC-output wave forms of inverters (including 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.
Post Table of Contents:
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- Basics of Inverters
- Pure vs Modified Sine Wave Inverters
I.Basics of Inverters (top⇑)
Before choosing the inverter, one needs to determine what settings and specifications are needed so the inverter can satisfy the needs of the off-grid power system. Below are the primary inverter settings and specifications that need to be determined when choosing the 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 basically the is 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 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 is explained in my earlier post on battery bank sizing. When choosing an inverter, the inverter’s power capacity must be considered first.
Surge Power Capacity (top⇑)
Surges are power spikes(in wattage) to start up engines/devices as many of these kinds of engines/devices require a higher startup power than when needed to run; a surge is usually for a short time like 15-20 minutes. The surge power capacity is the maximum surge wattage that the inverter can supply for multiple device startups 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 surge capacity of the 2 devices is 800 watts. Because 700 < 800, the 700 surge capacity of the inverter is unsuitable. The inverter’s surge capacity has to sized up. Redoing the example, if the inverter 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 requires to take into account the worst case where all attached AC devices(w/surge ability) are accounted in the situation where all such devices experience a star-up surge at once, similar to when sizing for continuous power capacity (previous subsection).
Lastly, because a device’s surge capacity is higher than it’s continuous power capacity, the surge power capacity of the inverter is higher than its power capacity by 2-3 or more times. Together with the power capacity of the inverter, the surge capacity value must also be taken into account. Sizing for surge capacity is also explained in the battery bank sizing post mentioned from the previous subsection.
Battery Voltage Handling (top⇑)
An inverter will specify what nominal 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 inverter have multiple voltage settings. The voltage has to match the nominal voltage choice of the battery bank.
There are a few high-end ones that can work with 120, and even 240 VDC. The latter are quite expensive and isn’t really needed in residential systems.
Conversion Efficiency (top⇑)
When converting DC power to AC power, some of the DC power is “lost” because a tiny portion of the DC power is used to power-on the inverter as it is an electrical device (especially modern ones).
A bit of the inefficiency naturally comes from wire heating. Depending on the inverter, the conversion efficiency ranges from 70% to 98%. This is another important consideration because power will be lost during a conversion from the battery bank and the renewable energy system and battery bank has to be sized to make up for losses. Ideally, an inverter with high efficiency can be selected the minimize additional sizing of the battery bank and renewable power source.
Again referring to the post on battery bank sizing, it also explains how to apply an inverter’s efficiency value.
Total Harmonic Distortion(THD) (top⇑)
Inverters with low THD are more expensive than inverters with higher THD due to the fact more and/or higher quality components are needed to smooth out the THD . There 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 tend to 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 are 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.
If one wishes to understand inverters and topology more from the perspective of high vs low internal operating frequency. 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.
Some Other Auxiliary Considerations (top⇑)
- Internal Protection: there needs to be internal circuitry to sense when to disconnect/regulate at certain points like if the battery bank is too low in charge or if the load is too much for the inverter. Many inverters have circuit breaker capabilites. Also, some inverters should have grounding capability.
- Automatic On/Off: when this feature is active, the inverter is normally off. When an AC device is powered on and needs about 50 watts, the inverter trips on. When the all AC devices are turns off, the inverter trips off. The trip may be set to somthing like 10 watts; if a device that requires less than 10 watts won’t trip the inverter and won’t automatically turn on. Also, there are device times that may trip on making automatic on/off unappealing to some. This can be switched to always-on idle power.
- Phantom loads: this is when an AC device(s) draws power, even though it is off, because there is still a completed circuit from being plugged in; 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.
While all that have been listed are all factors to consider in inverter selection, the one that hasn’t been covered are waveforms and how they affect inverter choice. This will all be covered in Section II in in-depth detail for long term benefit of the reader.
II.Pure vs Modified Sine Wave Inverters (top⇑)
The above factors listed so far are all considerations when picking an inverter. One of the most important considerations not yet covered is to pick an inverter based on the AC output’s waveform. The waveform determines what AC devices can be run and under what circumstances. A poorly selected inverter in this regard can damage the connected AC devices. When selecting an actual inverter, it will 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, your should know what to make of their values when you see them.
A.Wave Forms (top⇑)
Square Waves (top⇑)
While inverters that output AC power as square waves do exist, none of them are worth using even if they are free. A square wave’s appearance can be shown in the diagram below:
Also to note from the above Graph’s legend, the VAC-Peak values are about 10x of the input battery VDC value as inverters are part voltage-step up devices.
When the battery power is about 10.5 VDC, the inverter’s output is 100 VAC RMS*. Because the output voltage of the battery bank can vary, 12.6 VDC can become about 120 VAC RMS, and 14.7 VAC can become about 140 VAC RMS.
Despite the changing DC battery input and VAC peak. The average AC power is not consistently 120 VAC RMS.
Note*: RMS(root mean square) for now can be seen as the “effective voltage” in place of a continuously varying AC voltage. Also, american devices/appliances are required to receive 120 VAC RMS in order to function.
This can be shown 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 140 VAC Peak for 14.7 VDC, again the equation is filled as follows:
Average Voltage = (8 ms(width) * 140 V(height)) / (8 ms) = 140 VAC RMS
By same logic, the average voltage of the 120 VAC for 12.6 VDC should be about 120 VAC RMS.
Because the average ac voltage isn’t consistently at 120 VAC RMS, this can cause even simple AC loads like motors or lightbulbs to fail prematurely. Due to the high variation in the VAC RMS value, square waves have a really high amount of harmonic distortion(will explain more later in this post). The THD for the AC output waves of square wave inverters are >40% which is very high and “unclean”. Hence, avoid square wave output inverters at all costs.
Modified Sine Waves (top⇑)
Modified sine waves, in comparison, are not smooth. The appearance of a modified sine wave(MSW) is shown below:
From the above diagram, it can be seen that there are instantaneous increases and decreases in voltage and the signal sits periodically at 0 volts. Actual MSWs look similar to that in the above diagram if not exact.
Unlike square waves, the varying length of the ‘pulses’ of the wave enables the inverter to have at least some ability to regulate the VAC RMS voltage.
Following the average voltage(VAC RMS) equation:
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
Using the same equation for 9.5 VDC with 120 VAC peak, the average voltage is again 120 VAC RMS.
Hence, MSW inverters are able to regulate their AC voltage.
Although the harmonic distortion is much lower than with square wave inverters, it is still significantly high and will affect sensitive devices.
Generally, for MSWs, the THD of the output wave is generally around 20-40%. Although there is less THD in the output than for square wave inverters, the “cleaness” of MSWs is still low. However, while MSWs can be damaging to some devices, there are still applications for these inverters. More on this will be covered in the eventual comparison between pure sine waves vs modified sine waves 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.
Regarding 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 wave form, 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 deviates the least from pure sine waves in terms of shape(see diagram in next section). Also, if the area underneath 1/2 cycle was 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 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 also 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):
In 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.
B. What is Harmonic Distortion? (top⇑)
Harmonic Distortion In-Depth (top⇑)
The output sine waves of an inverter is not a completely pure sine wave; it is 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 to 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 in the case of inverters) to the original pure sine wave with f1. An overtone is any frequency higher than the fundamental frequency. The frequencies of harmonics, specifically, are integer multiples of the pure sine wave signal at the fundamental frequency(f1). Because MSWs are a variation of square waves, the integer multiples are always odd numbers. 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 current amplitude.
An example fundamental(1st harmonic) sine wave maybe V1p*sin(2π*f1*t); V1p is peak voltage
The 2nd harmonic would be V3p*sin(2π*(3*f1)*t).
The 3rd harmonic 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*(2*f1)*t)+ …….+ Vnp*sin(2π*(n*f1)*t); the number of odd harmonics will depend on the waveform; some odd harmonics can be skipped.
Below is a diagram illustrating how a square-like complex waveform breaks into the fundamental waveform plus its harmonics:
If more harmonics were added to the above waveform, it would look more like a square 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:
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 multipled by 100% to get THD as a percentage. The THD shown as a percentage is what is shown on the specs sheets for 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).
There are three dangers associated with waveforms that have high THD:
- First, there are circuits that sense phase angles from the gradual increases and decreases in the waveform’s voltage for speed and voltage control. Because a phase angles “shifts” a waveform, some circuits/devices read the phase angle(s) to adjust relevant speed and voltage control. In pure sine waves, the phase angle changes are smooth(or relatively smooth in the case of steeped sine waves). In MSWs and square waves, the phase angles changes sharply in accordance with the sharp changes in voltage levels(due to high THD). In 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)/device(s) later than it would for ones that would rely on the gradual increase/decrease of voltage. For MSWs, in addition, the “sitting periods” also confuse phase angle sensing circuits.
- Second, circuits that rely on timing control can rely on the voltage to instantaneously switch from positive to negative and vice-versa. 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 the zero volts and change polarity. This can cause devices to malfunction.
- Third, regarding the the different waveforms, there is a danger of heating associated with waveforms with high THD. When the voltage gradually increases positively or negatively from zero, the gradual temperature changes causes minimal to no harm. 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 instantaneous large jumps between relatively high voltage levels which cause sudden temperature changes that are bad for many sensitive circuits and made-for-on-grid devices on the long run. Also, 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.
C.Which and Where to Use (top⇑)
There are circumstances where modified sine wave inverters are better than pure(and stepped) sine wave inverters; and vice-versa.
Devices that Work with Pure and/or Modified Sine Wave Inverters (top⇑)
First, some devices that work better with Pure Sine Wave over MSW Inverters:
- Built-in clock in devices like VCR, coffee makers, bread makers, microwaves, plugged-in clocks, and etc due to the timing depending on reading the AC ‘crossing counts’ for timing.
- Output voltage control devices like light dimmers that rely on phase angle change sensing
- Ceiling fan/motors that rely on phase angle change sensing to control rotation speed
- Devices that use radio frequency signal carried out via AC Distribution wiring due to the high THD creating interference.
- Devices that utilize microprocessors. In modern times, besides things like computers and HD TVs, microprocessors can be found even in items not traditionally having a microprocessor(s) like plug-able flashlights or stoves.
- Devices that run on lower frequency(-ies); due to MSWs having harmonics which consists of higher frequencies, a fair amount of the power cannot be used resulting in more power being drawn by such devices.
- Inductive Loads like microwaves and inductive motors and run hotter, more noisily, and less efficiently. This is because such loads depends on the electromagnetic energy to do work; which in turns depends on the sine wave input. If the sine wave’s form is changed, it affects the inductive loads’ effectiveness. Check with manufacturer.
- Capacitance loads also have trouble with MSWs due to sine wave shape. The capacitor(s) prefers being charged gradually via smooth sine waves; it dislikes sudden charging via sharp increase in voltage.
- Devices that are considered non-linear loads. Non-linear loads are devices that add harmonic distortion the incoming AC waveform before powering the device(s). with MSWs’ high THD, more harmonic distortion is not good.
- 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.
- Other Devices with a THD % tolerance less than the THD of the output waveforms of the MSW Inverter. Check with the manufacturer(and/or with relevant spec sheet)
Devices that do work well with MSW Inverters:
- Built-in or separate clocks that do not rely on ‘crossing counts’ of the AC waveform. Clocks that use a crystal(s) for timing is an example.
- Resistive Loads like heaters and incandescent light bulbs work fine with MSWs because these types of loads only want the 120 VAC RMS from the MSWs.
- TVs, laptops, and speakers with its own power supply. These devices with built-in power supplies run on DC power. When the AC signal is brought in via power chord(with a built-in rectifier), the DC power that goes into the power supply(ies) has already smoothed out much of the “noise”.
- Certain universal motors(can use both AC and DC) can work if it’s robust enough to handle the extra heat generated from modified sine waves. Some DC/AC fans(without microprocessor or variable speed functions) can work this way. For fancier motors like the ones used in fridge compressors, heat increases are bad and the MSW inverters aren’t warranted here.
- Other linear loads. Linear loads can work well with MSWs because linear load devices don’t add additional harmonics to the incoming AC waveform. Examples of linear loads are the resistive loads, some universal motors, and some inductive loads.
- Other Devices/Circuits that don’t rely too much on the properties of the AC waveform and are not electrically complicated.
- Other Devices/Circuits with a THD % tolerance that is greater than or within the THD range of the MSW inverter’s AC output wave.
The most of device considerations in the first list can work with MSWs albeit for short-term with little to no damage or long-term with a significantly shorter lifespan. Others may simply not start or blow up shortly after starting. The latter list of device consideration are often suitable for long term use on MSWs.
Other Considerations (top⇑)
Regarding pure sine wave(and stepped sine wave) inverters, because their waveforms are clean, they can work with virtually any electronic device without too much concern because there is virtually no device that could fail to run on “clean” AC waves.
While this statement should imply that pure sine wave (PSW) inverters should be used at all times, there are cases where modified sine waves inverters are merited use. For one thing, MSW inverters are about 2-3 times cheaper or more than PSW inverters due to the fact producing MSWs needs less sophisticated components than PSWs. Also, for devices that can handle MSWs, it converts DC more efficiently for than PSW inverters.
PSW inverters are better for full residential systems where there is variety of devices including sensitive devices that have low THD tolerance and/or if one wants no inefficient performances in any devices what so ever.
For certain small scale uses and/or temporary use, MSW inverters can be much more economical. If only a simple lighting array and/or a heater were to be used, which don’t rely on waveform shape, an MSW inverter can be used for the long run. While MSWs can power most common AC devices like fridges and TVs, most of these devices will likely have a shorter lifespan due to the problems associated with MSWs(and other high THD waves) if used for too long.
Also, if certain devices(like chest fridges/freezers) were to be used temporarily away from a power grid or from a residential renewable power system that uses a PSW inverter, an MSW inverter connected to a battery would be economical as short-term usage shouldn’t cause lasting harm to the device.
Lastly, while PSW and MSW inverters both have their uses currently, MSW inverters are trending to become obsolete as smaller cheaper PSW inverters will emerge in the future.
By now, knowing how to choose a power inverter should not be too difficult a task. Define whether it is to be used for a full residential, off-grid tiny house, or for some small scale/temporary application(s). Define what type of loads (appliances) will be powered. Which inverter should be picked with relation to sizing? Does size and weight matter? Especially, is the waveform critical? These are some of the questions to be answered.
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