Key numbers
- Battery kWh = daily kWh × days ÷ usable fraction ÷ inverter efficiency
- LiFePO4: 80% usable in this calculator; lead-acid/AGM: 50%
- Array sized for the worst month with a 75% derate
- Controller amps = array watts ÷ battery volts × 1.25
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How to use the Off-Grid Solar Calculator
- 1Enter your numbersFill in daily energy use (kWh), days of autonomy (no sun), battery type, battery bank voltage, peak sun hours (worst month) (h) and panel size (W). Every field starts with a typical value you can overwrite.
- 2Check the assumptionsAssumes 90% inverter efficiency and a 75% array derate for off-grid charging losses. Size for the worst month, not the yearly average.
- 3Read the resultsThe calculator shows battery bank, at 48 v, solar array, charge controller (min.) and inverter (continuous, rule of thumb), and updates as you type.
- 4Use the resultCompare it with your bill, a quote or a product spec sheet before you buy or install anything.
Results update as you type. Back to the calculator ↑
This off-grid solar calculator sizes the four main parts of a stand-alone system: the battery bank, the solar array, the charge controller and the inverter. It doubles as a solar battery bank calculator, giving storage in both kWh and amp-hours (Ah) at 12, 24 or 48 volts. Enter your daily energy use, how many sunless days you want to ride through, your battery chemistry and your worst-month peak sun hours.
How the off-grid system is sized
Each component has its own formula.
- Battery bank (kWh) = daily kWh × days of autonomy ÷ usable fraction ÷ 0.90 inverter efficiency
- Battery bank (Ah) = battery kWh × 1,000 ÷ bank voltage
- Solar array = monthly need (daily kWh × 30.42 ÷ 0.90) ÷ 30.42 ÷ sun hours ÷ 0.75 derate, rounded up to whole panels
- Charge controller (A) = array watts ÷ bank voltage × 1.25
- Inverter (kW) floor = daily kWh ÷ 4, then size up to your largest simultaneous load
The usable fraction is 80% for lithium iron phosphate (LiFePO4) and 50% for lead-acid or AGM. The array derate is 75%, lower than a grid-tied system’s (PVWatts assumes 14% losses for grid-tied arrays, Dobos, 2014), because battery charging adds losses.
Worked example with the default inputs
Defaults: 5 kWh a day, 2 days of autonomy, LiFePO4, 48 V bank, 3.5 sun hours in the worst month, 400 W panels.
- Battery: 5 × 2 ÷ 0.80 ÷ 0.90 = 13.9 kWh
- At 48 V: 13,889 Wh ÷ 48 = 289.4, rounded up to 290 Ah
- Array: 5 ÷ 0.90 ÷ 3.5 ÷ 0.75 = 2.12 kW, which is 6 × 400 W panels
- Charge controller: 2,116 W ÷ 48 V × 1.25 = 55.1, shown as 56 A minimum
- Inverter: at least 1.3 kW, more if large loads run together
Off-grid sizing reference table
All rows use 2 days of autonomy, a 48 V bank, 3.5 sun hours and 400 W panels.
| Daily use | Battery (LiFePO4) | Battery (AGM) | Array | Controller |
|---|---|---|---|---|
| 2 kWh (cabin) | 5.6 kWh · 116 Ah | 8.9 kWh · 186 Ah | 0.85 kW (3 panels) | 23 A |
| 5 kWh (small home) | 13.9 kWh · 290 Ah | 22.2 kWh · 463 Ah | 2.12 kW (6 panels) | 56 A |
| 10 kWh (full home) | 27.8 kWh · 579 Ah | 44.4 kWh · 926 Ah | 4.23 kW (11 panels) | 111 A |
For comparison, the average US home used 855 kWh a month in 2023 (EIA), about 28 kWh a day. Most off-grid homes run on far less by switching heating, cooking and hot water to propane or wood.
What affects the result
Daily energy use. Every kWh you cut shrinks the battery, array and cost. List each appliance with the electricity usage calculator before you size anything.
Worst-month sun hours. Use the lowest monthly value for your site, not the annual average. NREL’s PVWatts shows monthly solar radiation for US locations.
Battery chemistry. Lithium costs more up front but needs a smaller bank and usually lasts many more cycles. Lead-acid capacity also shrinks at high discharge rates, while lithium-ion is much less affected (Doerffel & Sharkh, 2006). Check the cycle rating on the battery spec sheet.
Days of autonomy. Each extra day adds one full day of storage. A backup generator lets you keep this number low.
Panel tilt. For winter-heavy use, tilt panels steeper. The solar panel angle calculator gives the winter angle for your latitude.
Common off-grid sizing mistakes
The most common mistake is sizing from annual average sun hours, which leaves the system short every winter. The second is forgetting inverter idle draw, which can add several hundred watt-hours a day on a system that runs 24 hours. The third is mixing old and new batteries in one bank, which shortens the life of all of them.
Tips for a reliable off-grid build
- Check the controller result against real products with the solar charge controller calculator, which also checks string voltage.
- Estimate how long the bank lasts during an outage with the battery runtime calculator.
- Leave room to add panels later; it is cheaper than adding batteries.
- Have a licensed electrician review wiring, fusing and grounding.
References
- Dobos, A. P. (2014). “PVWatts Version 5 Manual.” National Renewable Energy Laboratory, NREL/TP-6A20-62641. PDF
- Doerffel, D., & Sharkh, S. A. (2006). “A critical review of using the Peukert equation for determining the remaining capacity of lead-acid and lithium-ion batteries.” Journal of Power Sources, 155(2), 395–400. doi:10.1016/j.jpowsour.2005.04.030
For a full walk-through of components and costs, read the off-grid solar system guide, and see how stand-alone solar fits into renewable energy as a whole.
FAQ
Frequently asked questions
How big a battery bank do I need for off-grid solar?
Multiply your daily kWh by the days of autonomy, then divide by the usable fraction of the battery and by inverter efficiency. For 5 kWh a day and 2 days with lithium batteries, that is 5 × 2 ÷ 0.80 ÷ 0.90 = 13.9 kWh, or about 290 Ah at 48 V.
What are days of autonomy?
Days of autonomy are the number of days the batteries must carry your loads with no useful solar input, such as during a storm. Cabins with a generator backup often use 1 to 2 days; remote homes without a backup often plan for 3 or more.
Why does lead-acid need a bigger battery bank?
Lead-acid and AGM batteries last much longer if you use only about half their capacity each cycle, so the calculator counts 50% as usable. Lithium iron phosphate tolerates deeper cycling, so 80% is counted. The same 5 kWh-a-day load needs 22.2 kWh of AGM but 13.9 kWh of LiFePO4.
Should I use a 12 V, 24 V or 48 V battery bank?
Higher voltage means lower current for the same power, which allows thinner cables and smaller charge controllers. 12 V suits small vans and sheds, 24 V mid-size cabins, and 48 V most whole-home off-grid systems. The default 2.12 kW array needs 56 A at 48 V but over 220 A at 12 V.
Why size the array for the worst month?
Off-grid systems have no grid to fall back on, so production must cover use in the darkest month, usually December or January in the northern hemisphere. Sizing to the annual average leaves you short exactly when days are shortest.
How big should my off-grid inverter be?
Size the inverter's continuous rating to the largest set of loads that run at the same time, plus surge capacity for motors such as well pumps and fridges. The calculator shows a floor of daily kWh ÷ 4, which is 1.3 kW for 5 kWh a day, but your peak loads decide the final size.
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Last updated October 25, 2026. Part of our renewable energy toolkit. Found an error? Tell us.