Battery Bank Size Calculator
The calculator sizes the bank so it can carry your load through the backup period without ever being discharged past its safe limit.
Enter Values
How to use this calculator
- Enter your daily energy use in watt-hours per day (add up each device's watts × hours run).
- Set the system voltage (12, 24 or 48 V), the days of autonomy you want, and the usable depth of discharge for your battery chemistry.
- Optionally add a system efficiency to cover inverter and wiring losses, then read off the required Ah and kWh.
How it works
The calculator sizes the bank so it can carry your load through the backup period without ever being discharged past its safe limit. It first works out the energy the bank must deliver: daily energy use × days of autonomy, then divides by the system efficiency to allow for round-trip, inverter and wiring losses.
That delivered energy is then grossed up by the usable depth of discharge, because you only draw part of a battery's rated capacity. In formula form: rated Ah = (daily Wh × days) ÷ (efficiency × DoD ÷ 100 × 100 ÷ system voltage). Rated kWh is the rated Ah × system voltage. Because different chemistries tolerate very different discharge depths (lithium ~80–100%, lead-acid ~50%), the DoD input has a large effect on the answer.
Worked example
2400 Wh/day, 12 V, 2 days, 50% DoD, 85% efficiency. Energy over autonomy = 2400 × 2 = 4800 Wh. Divide by 85% efficiency = 5647 Wh. Divide by 50% DoD = 11294 Wh rated. Divide by 12 V = about 941.2 Ah. So you need roughly a 941 Ah, 11.29 kWh bank (round up to whole batteries), delivering about 5.65 kWh of usable energy.
Common mistakes
- Sizing to 100% depth of discharge for lead-acid batteries — use about 50% or the bank wears out fast and falls short in practice.
- Entering amp-hours instead of watt-hours for the daily load — multiply amps by hours and by voltage to get Wh first.
- Ignoring inverter and wiring losses — leave efficiency at 100% and the bank is undersized; 80–90% is typical for a real off-grid system.
Frequently asked questions
What depth of discharge should I use?
Match your battery chemistry. Flooded and AGM lead-acid last much longer if you only use about 50% of capacity. LiFePO4 and other lithium banks commonly allow 80–100% depth of discharge — check the manufacturer's spec sheet.
How many days of autonomy do I need?
Autonomy is how long the bank runs your load with no charging (cloudy days for solar). Two to three days is common for off-grid homes; critical systems use more. More days means a bigger, more expensive bank.
Does higher system voltage change the battery bank size?
It changes the amp-hours, not the energy. A 48 V bank needs a quarter of the Ah of a 12 V bank for the same kWh, which means thinner cabling and lower current — the stored kWh is the same either way.
Related tools
- Battery Life Calculator
- Watt-hours to Amp-hours Calculator
- Solar Output Calculator
- Inverter Efficiency Calculator
- Capacitor Parallel Calculator
- Transformer Ratio Calculator
Explore more in Electrical, Electronics, Solar & Energy.
Tip: Enter any known values to calculate the remaining results.
All calculations run in your browser. Your inputs are never saved or transmitted.