Peak sun hours are the equivalent hours of full 1,000 W/m² sunshine per day. In the US you can find them for your location with NREL's PVWatts calculator; use a winter figure if you live there year-round.
How the battery bank calculator works
BatteryBankCalc adds watts × hours × quantity for every load to get watt-hours per day. Battery watt-hours = daily Wh × days of autonomy ÷ (usable depth of discharge × inverter efficiency), and amp-hours = that ÷ system voltage. Batteries in series = system voltage ÷ battery voltage; strings in parallel = amp-hours needed ÷ battery amp-hours, rounded up. Inverter = sum of all load watts × (1 + headroom), rounded up to a standard size. Solar array = daily Wh ÷ inverter efficiency ÷ (peak sun hours × system efficiency); controller amps = installed panel watts ÷ system voltage × 1.25.
Usable capacity by battery type
| Battery | Typical usable depth of discharge | Usable Wh from 12 V 100 Ah |
|---|---|---|
| Lithium iron phosphate (LiFePO4) | 80% | 960 Wh |
| AGM / gel lead-acid | 50% | 600 Wh |
| Flooded lead-acid | 50% | 600 Wh |
Batteries in series for each system voltage
| System | 12 V batteries in series | Current for a 1,000 W load |
|---|---|---|
| 12 V | 1 | 83.3 A |
| 24 V | 2 | 41.7 A |
| 48 V | 4 | 20.8 A |
Depth of discharge, inverter efficiency (90%), solar system efficiency (80%) and the 1.25 controller factor are common planning figures, not product specifications; follow your battery, inverter and controller manuals, and size cables and fuses for the current. A licensed electrician should connect any system to house wiring. Results are estimates.
Frequently asked questions
How many days of autonomy do I need?
Commonly 2 to 3 days for off-grid homes in sunny areas and more in cloudy climates, unless you have a backup generator.
How many solar panels do I need off-grid?
Daily Wh ÷ inverter efficiency ÷ (peak sun hours × system efficiency) ÷ panel watts. 2,150 Wh/day at 4.5 sun hours needs about 660 W, or two 400 W panels.
Should the array be bigger than one day's use?
Yes if you want to recharge a deep bank after cloudy days; many designers add 25% or more.
Is BatteryBankCalc free?
Yes, with no sign-up or ads.
Are my loads saved?
Yes, in this browser only.