How to Size a Battery Bank
Why Battery Sizing is Needed?
Knowing how to size a battery bank is one of the most important calculations in designing any viable off-grid power system for any off grid tiny house. I will use a long example in steps to explain the basic calculations needed to size a battery bank; and we will assume that deep-cycle lead acid (DCLA) batteries will be used in said battery bank.
First off, the DCLA battery bank should not be sized too small. If sized too small, the battery bank can overcharge often and reduce its lifespan; overcharging can result in a build up of gasses and cause the battery(-ies) to burst. A good charge controller can prevent overcharging.
Another issue with an under-sized battery bank is the risk of over-depletion from power usage. For DCLA batteries, there is a depletion threshold that should not be crossed. For example, most DCLA batteries have 50% max discharge threshold(aka Depth of Discharge(D.O.D)); depleting a battery beyond the rated D.O.D significantly shortens the batteries’ lifespan. At 50% discharge, the DCLA battery bank should be recharged.
On the other hand, if battery bank to sized too big, the battery bank is less likely to have passed the D.O.D threshold and will have a longer lifespan overall. However a larger battery bank risks being undercharged due to it being harder to fully charge; if a battery bank is undercharged for too long, the overall battery capacity will be affected.
When DCLA batteries are discharged, lead sulfate(PbSO4) forms around the battery’s anode and cathodes. If the DCLA batteries are left discharged for too long, the PbSO4 crystallizes and shortens the life of the DCLA batteries. When the DCLA battery bank is fully charged without being overcharged, there is no more lead sulfate and the battery bank is in a neutral and safe state.
To prevent the problems associated with an under-sized or over-sized battery bank, it’s important to stick to an approximate daily usage of watts based on a chosen lifestyle. Then, the battery bank has to be appropriately sized to accommodate this lifestyle. These 2 related points are the basis of the sizing process.
I will break down the sizing process into steps and explain them. Finally, at the end, I will provide commentary on each of the steps in order to account for some real-world variables.
I.Determine the Approximate Daily Usage of Selected Household Items/Devices
First is to take all electrical items and determine their power ratings. Their power rating(or Watt usage) should be labeled in said electrical items by their respective manufacturer. Also, 100% efficiency of these items is assumed for ease of calculations. Here is a sample categorized list of electrical items/appliances that can be found in a tiny home with each of their power ratings along with a likely daily usage time for later calculations:
Lighting and Heating/Cooling:
- 10 LUNO A19 Non-Dimmable LED light bulbs (3 in bathroom, 3 in bedroom, and 4 in kitchen/living room): 9 Watts/hour for each bulb. Daily usage is listed later in post.
- 1 Frigidair Air conditioner (Model: FFRA0511R1): this model uses 450 Watts/hour; usage of 3 hours/day should be standard use during summer.
- 1 Danby 2-Door Apartment Refrigerator (Model: DPF073C1BSLDD): 343 kWh/year according to Energy Star. Assuming continuous use (=24 hrs/day), this spreads out to 39.2 Watts/hour.
- 1 Westinghouse Black Microwave (Model: WCMH900B): uses 900 Watts/hour; estimated maximum usage is 30 minutes(=0.5 hr) per day.
- 1 Mr. Coffee 4-Cup Switch Coffee Maker: rated at 600 Watts. One brewing for 4-cups of coffee takes up to 10 minutes (≈0.167 hr); one brewing of 4 cups is enough for one individual per day.
- 1 SPT Countertop Dishwasher (Silver): Uses 424 Watt-hr per heavy wash load (for 100 min ≈ 1.667 hr); also uses 14 liters of water per heavy load wash according to the user manual’s Wash Cycle Table section. Power usage rate is roughly 254.4 Watts/hour. It’s assumed that heavy wash is once per day; in actuality, it maybe once per few days.
- 1 Giantex Portable Compact Twin Tub Washing Machine and Spinner: This device is basically a combined washer and spinner that is portable. The former is used for washing; then the spinner is used for drying the same load. Washing uses 300 Watt per wash cycle load of 15 min(=0.25 hr). The spinner uses 110 Watt for one dry cycle of 5 min; however, because the washing tub is nearly twice the size of spinner/dryer tub, it may take 2 drying cycles to dry the entire wash load. Hence, the spinner uses 110 Watt for 10 min(≈0.167 hr) to dry the entire load.
- 1 iClever BoostCube 2-Port USB Wall Charger: 24 Watts/hour; used to charge USB-connected wireless devices like iPhone or Samsung Galaxy; 1 hour needed at most to charge iPhone and iPad devices.
- 1 ZOZO Universal Laptop charger: This charger utilizes 45-90 Watts/hour depending on what Laptop brand is used. Hence, a mean value of 70 Watts/hour for 8 hours will be assumed. If one runs an online business, keeping the laptop charger plugged in for 8 hours isn’t unusual.
- 1 Satellite Internet System: ~60 Watts/hour; this is as close as an average value I can get when it comes to the modem/hot spot being powered on. It is assumed that the dish will be on a fixed tripod mount instead of an automated mount (which would use additional power). The system will be on for 8 hours as this matches the laptop charger (hence internet) use. I will explain more about satellite internet options for tiny houses in a later blog post.
Note: There are probably more items, but these would make the bulk of energy consumption. Additionally, the above usage for some items assume maximum possible usage; this is to make sure the eventual battery bank is sized to accommodate the “worst case” in terms of usage. For example, the Frigidair Air conditioner is still accounted for even though it would only be used for summer. For another example, I assume a daily heavy wash load for the SPT Countertop Dishwasher even though the actual wash load maybe less heavy and only once per 2-3 days.
II.Using the Above Values, Determine Inverter Size:
Because the battery power is in DC, the inverter converts to AC power which is what nearly all appliances only accept. The inverter size has to be such that it can take a full load of all electrical appliances working at once. A smaller size could result in a bottleneck. The sizing is the sum of the power/wattage from all the devices being used at once (as shown below):
|Electrical Appliance||Quantity||Power Rating(W)||Total(W)|
|Minimum Inverter Size**||2897.6|
|LUNO A19 LED light bulbs (bathroom)||3||9||27|
|LUNO A19 LED light bulbs (bedroom)||3||9||27|
|LUNO A19 LED light bulbs (living room)||4||9||36|
|Frigidair Air Conditioner||1||450||450|
|Danby Apartment Refrigerator||1||39.2||39.2|
|Westinghouse Black Microwave||1||900||900|
|Mr. Coffee 4-Cup Switch Coffee Maker||1||600||600|
|SPT Countertop Dishwasher||1||254.4||254.4|
|Giantex Compact Twin Tub Washing Machine and Spinner (washing load)*||1||300||300|
|Giantex Compact Twin Tub Washing Machine and Spinner (drying load)*||1||110||110|
|iClever BoostCube Wall Charger||1||24||24|
|ZOZO Universal Laptop Charger||1||70||70|
|Satellite Internet System(Generic)||1||60||60|
As shown above, the inverter size sum was calculated to be about 2897.6 Watts(≈3 kW). Purchasing an inverter with a higher size like 3.5 or even 4 kW should be considered in order to take into account a possible margin of error in case the actual power ratings of some appliances are a little higher than stated.
*Note #1: The washer and drying spinner on this device can run at the same time; hence, the two functions are listed separately for the inverter sizing.
**Note #2: In the above table, I only did the inverter sizing for standard power ratings. There is another inverter sizing that accounts for surge power ratings. This will be explained near the end of this post.
III.Calculate Daily Energy Usage from the Earlier List
The output of daily power usage is measured in Watt-hour(W-hr) or KiloWatt-hour(kW-hr). Watt-hour is an energy unit equivalent to 1 Watt of power expended over 1 hour. Similarly, a KiloWatt-hour is equal to 1 KiloWatt used over 1 hour.
The daily power usage is calculated as follows:
Watt-hour(W-hr) = Total(W) x Daily Usage(hours)
KiloWatt-hour(kW-hr) = [Watt-hour(W-hr)] / 1000
The Total(W) for each of the items is found on the table from earlier. Also, the Daily Usage(hours) values for each item is found in the item list in the beginning. The daily power usage of each item (in Watt-hour) values are in the table below. Afterwards, the total daily use of power is the sum of each items’ individual daily energy use (W-hr and/or kW-hr).
|Electrical Appliance||Total(W)||Daily Usage(hours)||Watt-Hours|
|LUNO A19 LED light bulbs (bathroom)||27||2||54|
|LUNO A19 LED light bulbs (bedroom)||27||3||81|
|LUNO A19 LED light bulbs (living room)||36||5||180|
|Frigidair Air Conditioner||450||3||1350|
|Danby Apartment Refrigerator||39.2||24||940.8|
|Westinghouse Black Microwave||900||0.5||450|
|Mr. Coffee 4-Cup Switch Coffee Maker||600||0.167||100|
|SPT Countertop Dishwasher||254.4||1.667||424|
|Giantex Compact Twin Tub Washing Machine and Spinner (for Washing)||300||0.25||75|
|Giantex Compact Twin Tub Washing Machine and Spinner (for Drying)||110||0.167||18.37|
|iClever BoostCube Wall Charger||24||1||24|
|ZOZO Universal Laptop Charger||70||8||560|
|Satellite Internet System(Generic)||60||8||480|
From above, the approximate daily usage of 4737.17 W-hr is converted to 4.74 kW-hr.
IV.Decide Number of Autonomous Days
In this example, the battery bank will be configured to run autonomously for 3 days; meaning that the example battery bank can provide 3 days worth of power without recharging and without damaging the deep-cycle battery bank via over-depletion.
Because the issue of over-depletion is being accounted for only 3 days of continuous usage without recharge; that means that the depth of discharge(D.o.D) won’t even reach close to 50% in normal circumstances where each day’s worth of power usage is continually recharged by the renewable power system. This enables a long life for the battery bank.
Note: Later this post, I will bring up an important issue regarding autonomous days as additional commentary.
V.The Battery Bank Size is Determined
First, know that depth of discharge(DoD) is the percentage a battery capacity has been used up. The lower the repeated DoD of the battery bank, the more battery life, or cycles(recharge and discharge), the bank will have. The greater the repeated DoD, the less battery life, and cycles, there is for the battery bank. A common DoD range for lead acid batteries is about 40%-80%. A 50% DoD will be used as it is a common and maximum recommended DoD.
First is to take the daily watt usage and the days of autonomy and multiply them to get the 50% DoD of the battery bank.
4.74 kW-hr * (3 days) ≈ 14.22 kW-hr for 50% DoD
14.22 kW-hr * (100%/50%) = 28.44 kW-hr for the full watt capacity of the battery bank
To determine amp-hours, the previous value has to be divided by the battery system voltage(we will use 12 Volts here) to get total battery bank capacity in Amp-hrs.
28440 Watts-hour / 12 Volts = 2370 Amp-hours
Above, I used 12 volts for the battery bank as the assumed solar panels used here usually provide 12 volts. A higher voltage battery bank could be used if the system allows(like 24 & 36 volts), but there are legal restrictions that prohibit a battery bank from going over 48 Volts due to dangers associated with higher voltages.
VI.The Battery Wiring Configuration gets Determined:
Before choosing the batteries needed to satisfy 2167 Amp-hours, you will need to know how different wiring configurations affect the overall voltage and storage capacity of the battery bank. There are only three wiring configurations which are series, parallel, and series-parallel hybrid.
Before explaining each of the wiring configurations using diagrams, all the DCLA batteries used here will be assumed to be of the same type (Flooded, AGM, or Gel), same brand, same voltage rating, and exact same amp-hr storage capacity for optimization and to prevent uneven charging and discharging. Hence, all batteries will be assumed to be FLA(flooded lead acid) batteries with 12 Volts rating and 200 Amp-hr (Ah) power storage capacity each.
Here are the 3 three battery bank wiring configurations diagrams below:
1.)Series: A diagram for lead acid batteries in series is shown belowAny segment of the battery bank in series will have the sum of the voltages added up, but the Amp-hours(Ah) from both batteries will NOT add up to increase overall storage size. In the above series segment, the total voltage is 24 volts and the storage capacity is only 200 A-hr.
2.)Parallel: A diagram for the lead acid batteries in parallel is shown belowUnlike in series where only the voltages add up; this time, only the charge capacity from each segment adds up. The result is that voltage of this section is still 12 Volts, but the total power storage capacity would be increased to 400 A-hr.
3.)Series-Parallel Hybrid: Below is a diagram that shows a hybrid of series and parallel circuitsThe series-parallel is most common as it has the best of both worlds. From the above circuits, each series branch adds up to 24 volts each with the 200 amp-hr for each branch. When the branches are brought into parallel, 24 volts stays the same, but the battery capacity adds up with a total of 400 amp-hr. Overall, the above diagram showcases a battery bank of 24 volts and 400 amp-hours.
When finally putting together the battery bank; all three configurations need to be kept on mind in order to optimize both the battery bank capacity and voltage to the system requirements.
While all the steps have been done, there is more that can be done at each of these steps in order to do a sizing that closer to real life sizing. Here are some additional considerations when sizing for a real battery bank.
Real life Sizing Considerations for each Step
Under step “I”: For the items list, I mostly chose appliances that can be considered both necessary and the bulk of energy usage. I mainly excluded:
- common low-wattage items like electric shavers, dryers, and electronic clocks which can be added at one’s discretion.
- high-wattage luxury items like a TV set which is not essential to living.
- and high-wattage items that can be replaced with alternatives like electric stoves and vacuum cleaners.
For real-life battery bank sizing, two important things that weren’t included in the item list (but should have been) are the power consumption values of the charge controller (~5 watts/hr) and inverter (~10 watts/hr in sleep mode). Check the device manuals for the actual values.
Next, because renewable power can generate a limited amount of watts at once, one should consider using non-electric alternatives to replace high wattage electrical items that have necessary functions. For example, electric stoves can be replaced with propane and wood burning stoves for cooking. Choosing non-electrical alternatives depend on whether one can provide proper power to electrical alternatives or tolerate possible inconveniences that come with non-electrical alternatives.
Additionally, one needs to consider the impact of the seasons on the item list throughout the year and as well as on renewable power generation. For example, a solar array would be able to generate enough power to support the high-watt Frigidair air conditioner during the summer. Then in winter, even though the air conditioner is no longer used until next summer, the foggy winter days reduce solar power and can then impact how one rations his/her item and energy usage.
Lastly, as I stated in the beginning, I only used the items’ advertised power ratings for all sizing calculations. Before doing any real-life sizing, one should first measure the actual power rating of each listed item as the actual power ratings can deviate from the advertised power ratings for any number of reasons.
Under step “II”: Besides the inverter sizing for the standard (aka continuous) power ratings of devices, there is also an inverter sizing for the “surge power” of devices. A device’s surge power rating is the power used by a device during its start up; also, surge values are usually higher because more power is needed for starting-up than running.
The total surge power is the addition of all the surge ratings of the devices with surge capacity. For ease of calculation and as a rule of thumb the inverter’s surging size would be about 2-3 times of the inverter sizing for the continuous power ratings. Using the table from Step II as reference, the surge-accounted inverter size would be ~6000-7000 Watts (or 6-7 kW).
Under step “IV” (no comments for step “III”): the days of autonomy accounted for was set to 3 days. However, if continuous power usage exceeds even more than 1 day (due to recharging being unavailable), the battery bank will end up being continuously undercharged. This is because the renewable power source is usually scaled to recharge the battery bank for only a little over 1 day’s worth of power usage. Scaling the renewable power source to account for more than 1 day’s worth of power would be both costly and excessive for most days.
Under step”V”: Another potentially important aspect of battery sizing is taking into account the inverter’s inefficiency, when DC power is converted to AC via inverter, there is a slight drop in overall power during the conversion process. If an inverter only has 85% efficiency, added power would be needed to compensate for the loss as calculated:
1000 Watts/(efficiency/100%) = 1000 Watts/(0.85) ≈1176.5 Watts
From above, if 1000 watts of power was needed for an AC appliance, then 1176.5 Watts would need to be drawn from the battery bank as the 15% conversion loss (aka 85% efficiency) of 1176.5 Watts would result in the needed 1000 Watts (=1176.5 Watts * 0.85).
If the above math is then applied to the battery bank’s capacity(amp-hr), the entire battery bank can be further sized to take into account inverter efficiency. This calculation is as follows:
(2370 Amp-hours)/0.85 ≈ 2788 Amp-hours
With this this, the new battery bank size that accounts for both autonomous days and inverter inefficiency is 2788 Amp-hours. The reason the old battery bank sizing capacity can simply be divided by 0.85(=85% efficiency) is because all the computations from the beginning were scalar.
In the appliances list, you may have noticed that I haven’t used any DC devices. If there were any DC-powered devices in the appliances list, those DC devices can be directly connected to the battery bank if their voltages matched(like a 12V DC device for a 12V battery bank). This direct connection would bypass the inverter’s inefficiency. However, if there is a DC voltage difference between the device(s) and battery bank, then a voltage converter would be needed. In that case, the conversion efficiency of the voltage converter would need to be accounted for; a good voltage converter would have an efficiency of 90% or higher.
In total, the chosen inverter should account for total continuous power, surge power, and the inverter efficiency (which should be specified with the inverter product).
Under step “VI”: In the production of DCLA batteries, there is a common trend; DCLA batteries are higher capacity for lower voltages and high voltage for lower capacities. Here are some FLA(flooded lead acid) battery models from these brands:
- Trojan (model: SCS225): 12 volts and 130 amp-hr.
- Rolls Surrette (model: 6 CS 25P): 6 volts and 820 amp-hr.
- Trojan (model: L16RE-2V): 2 volts and 1100 amp-hr.
As shown above, the voltage and power capacity (amp-hr) are inversely related. Also as stated before, one should use multiples of only one battery model in any of the three wiring configurations. Even if multiple battery types within a wiring configuration theoretically yield the right voltage and amp-hr; the charging and discharging can end up uneven due to the battery models’ different attributes. This can lead to long-term damage to some batteries within the battery bank.
Next, when choosing the best battery model, one should aim to keep the number of batteries as few as possible (for ease of maintenance) while making sure the wiring satisfies both the voltage and amp-hr needs. The dimensions of the batteries should also be taken into account when building the storage space needed to hold the battery bank.
Lastly, when making the storage space, the battery bank should be kept at a stable temperature(like at room temp.); and the storage space needs some venting to release any gasses coming from the lead acid batteries. The buildup of these flammable gases can lead to an explosion.
Stationary VS Mobile Tiny Houses
One last and very important point I would like to make is that this entire battery bank sizing process was more suited for tiny homes built upon land than tiny homes on wheels. With a property, there would be more needed space to install the larger off-grid power apparatus needed to support multiple comfort appliances. This would be much harder for mobile tiny homes because:
- limited space on tiny house roofs to install solar panels (wind and hydro power is completely unviable)
- and the limited available space inside a tiny house to store a large battery bank
For mobile tiny homes, a smaller solar energy apparatus limits the amount of comfort appliances that can be powered. Additionally, the limited battery bank size also restricts power usage as well as limit possible autonomy days; in order to avoid the 50% depth of discharge limit of most battery banks.
In the case of mobile tiny homes, the best option might be to use an electrical system similar to an RV’s where the battery is manually charged at road stops like RV stations and parks. Then, the solar panels on the roof can be used for supplemental power; as well as power critical low-load items like lighting, laptop chargers, and the internet (for communication needs).
By now you should know how to size a battery bank, just remember that sizing a battery bank comes after:
- determining the daily usage since it cannot exceed the daily generated renewable power
- accounting for conditions that affect the implementation of a renewable power system (like if the tiny house is on land or on wheels).
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- Header Image: Derived image created from “Goombagarin battery bank for solar array” by sridgway via Flickr
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- Parallel Circuit Diagram Image: Same as image #2
- Series-Parallel Hybrid Circuit Diagram Image: Same as image #2
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