Battery Bank Calculator
Size an off-grid battery bank from daily use and days of autonomy.
Battery systems store dangerous energy and require correct overcurrent protection, ventilation and disconnects per NEC Article 706. Have the installation designed and inspected by a licensed professional.
How this calculator works
amp hours = (daily Wh × days ÷ depth of discharge) ÷ bank voltage
Depth of discharge is what separates nameplate capacity from usable capacity. A 100 Ah lead-acid battery only yields 50 Ah before damage; the same nameplate in lithium yields 80 to 90.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- IEEE Std 1013 (Sizing Lead-Acid Batteries for Stand-Alone PV Systems) 2019 — the depth-of-discharge method that separates nameplate capacity from usable capacity
Cited to a specific edition on purpose — table numbers and values move between cycles. Confirm against whichever edition is enforced locally.
Worked example
With daily energy use 5000 , bank voltage 48 , usable depth of discharge 0.8 , days of autonomy 2 , this calculator returns 260.42 .
Bank size for common off-grid loads
Each row is a link that fills in the calculator above. Send one and the other end sees the same figures you do.
| Job | Size | Bank capacity needed |
|---|---|---|
| Cabin — lights and a fridge Lead-acid, 50% usable |
2000 × 12 × 0.5 × 2 | 666.7 |
| Same cabin on lithium | 2000 × 12 × 0.8 × 2 | 416.7 |
| Van conversion | 1200 × 12 × 0.8 × 2 | 250.0 |
| Small off-grid house | 5000 × 48 × 0.8 × 2 | 260.4 |
| House with three days of cloud cover | 5000 × 48 × 0.8 × 3 | 390.6 |
Depth of discharge is what separates the same nameplate into half a bank or most of one.
Frequently asked questions
How many amp hours do I need?
Multiply daily watt hours by days of autonomy, divide by depth of discharge, then divide by bank voltage. A 5 kWh daily load with two days autonomy on a 48 V lithium bank needs about 260 Ah.
What depth of discharge should I plan for?
Fifty percent for flooded and AGM lead acid — going deeper destroys them quickly. Eighty to 90% for LiFePO4, which tolerates deep cycling with little loss of life.
Should I use a 12, 24 or 48 volt bank?
Forty-eight volts for anything over about 3 kW. Higher bank voltage means proportionally less current for the same power, which allows far smaller and cheaper cable and reduces losses.
How many days of autonomy do I need?
Two days is the usual off-grid figure, giving one full cloudy day of margin. Three or more where winter cloud is persistent, or size in a generator instead — batteries are the expensive way to cover rare events.