Size an off-grid, RV or backup battery bank from your daily energy use. The calculator applies days of autonomy, inverter efficiency, depth of discharge and cold-temperature derating, then tells you the amp-hours needed and how many batteries to wire in series and parallel.
Last updated:
Energy use
Add up watts × hours for each appliance per day
days
Days the bank must run with no charging (cloudy days)
%
Battery
%
Ah
Optional: leave empty to skip the battery count
Enter your values to see the result.
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How to use the Battery Bank Size Calculator
Enter your daily energy use in Wh or kWh (watts × hours of use for each device, added up).
Set the days of autonomy you want without any charging, and the inverter efficiency.
Choose the battery type and system voltage; keep the recommended depth of discharge or set your own.
Enter the battery you plan to buy (voltage and Ah) to get the series and parallel count.
Read the required capacity, the battery count and the actual autonomy, then compare voltages in the table.
Battery bank sizing formula
Capacity (Wh) = Daily Wh × Days of autonomy ÷ Efficiency ÷ Depth of discharge × Temperature factor
Capacity (Ah) = Capacity (Wh) ÷ System voltage
Example: a cabin using 3,000 Wh a day, 2 days of autonomy, a 90% efficient inverter and LiFePO4 batteries at 80% DoD needs 3,000 × 2 ÷ 0.90 ÷ 0.80 = 8,333 Wh. At 24 V that is 347 Ah. With 12 V 100 Ah batteries you need 2 in series × 4 parallel strings = 8 batteries (400 Ah at 24 V, 9.6 kWh).
Typical depth of discharge and cycle life
Battery type
Design DoD
Round-trip efficiency
Notes
Flooded lead-acid
50%
about 80–85%
Needs watering and ventilation; cheapest per Ah
AGM
50%
about 85–90%
Sealed, maintenance-free
Gel
50%
about 85%
Tolerates deep cycling, needs lower charge voltage
LiFePO4
80–90%
about 95%+
Long cycle life, built-in BMS, light weight
Lithium-ion NMC
80%
about 95%
High energy density, common in packaged home batteries
Lead-acid temperature multipliers
Battery temperature
Multiplier
26.7 °C / 80 °F
1.00
21.2 °C / 70 °F
1.04
15.6 °C / 60 °F
1.11
10.0 °C / 50 °F
1.19
4.4 °C / 40 °F
1.30
−1.1 °C / 30 °F
1.40
−6.7 °C / 20 °F
1.59
These widely used solar-design multipliers oversize a lead-acid bank to make up for reduced capacity in the cold. Check your battery's datasheet for exact figures.
Frequently asked questions
How do I calculate battery bank size?
Battery capacity (Wh) = daily energy use × days of autonomy ÷ system efficiency ÷ depth of discharge. Divide by the system voltage to get amp-hours. For 3,000 Wh/day, 2 days, 90% efficiency and 80% DoD: 3,000 × 2 ÷ 0.9 ÷ 0.8 = 8,333 Wh, or 347 Ah at 24 V.
What depth of discharge should I use?
For lead-acid (flooded, AGM, gel), 50% is the common design limit for good cycle life. LiFePO4 and other lithium batteries are usually designed for 80% or more. A lower DoD means a bigger bank but longer battery life.
How many days of autonomy do I need?
Off-grid solar systems commonly use 2 to 3 days in sunny climates and 3 to 5 days where long cloudy periods are normal. Systems with a backup generator or grid connection often use 1 day or less.
Should I use 12 V, 24 V or 48 V?
Higher voltage halves the current for the same power, which means thinner cables and lower losses. A common rule of thumb is 12 V up to about 1–2 kW of inverter power, 24 V up to about 3–4 kW, and 48 V above that.
How do I wire batteries for a given voltage?
Batteries in series add their voltages (two 12 V batteries make 24 V) while keeping the Ah the same. Strings in parallel add their Ah at the same voltage. Use identical batteries of the same age in a bank.
Why does temperature matter for lead-acid batteries?
Lead-acid batteries deliver less capacity when cold. Solar design guides use a multiplier, for example about 1.19 at 10 °C (50 °F) and 1.59 at −7 °C (20 °F), to oversize the bank. Lithium batteries are less affected but often cannot be charged below freezing.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.