Battery Bank Sizing Calculator
Size an off-grid battery bank from daily energy use, days of autonomy, depth of discharge and system voltage — the amp-hours and usable capacity you need.
How it works
rated Wh = (daily Wh × days) ÷ (depth-of-discharge × efficiency) · amp-hours = rated Wh ÷ system voltage
You never use a battery’s full nameplate capacity: flooded lead-acid lasts longest around 50% depth of discharge, and even lithium is usually held back for longevity, so the rated bank must be bigger than the energy you actually draw. Inverter and wiring losses shrink it further, so daily use is divided by both the depth-of-discharge fraction and the efficiency. Dividing the rated watt-hours by the system voltage gives amp-hours, the number batteries are sold in. A higher system voltage (48 V) means fewer amp-hours and thinner cables for the same energy.
Worked example
A cabin uses 3,000 Wh/day, wants 2 days of autonomy, 50% max discharge and 90% efficiency on a 48 V bank: rated Wh = (3,000 × 2) ÷ (0.5 × 0.9) = 6,000 ÷ 0.45 = 13,333 Wh. Amp-hours = 13,333 ÷ 48 ≈ 278 Ah at 48 V, while only 6 kWh is actually delivered to the loads.
Frequently asked questions
What depth of discharge should I use?
For flooded or AGM lead-acid, 50% preserves cycle life; going deeper shortens it sharply. Lithium (LiFePO4) tolerates 80–90% comfortably, so it needs a smaller rated bank for the same usable energy.
Does higher system voltage really matter?
Yes. At 48 V the same energy needs a quarter of the amp-hours of a 12 V bank, which means far less current, thinner and cheaper cables, and smaller losses. Larger off-grid systems almost always run 48 V.
How many days of autonomy do I need?
Two to three days is common for cloudy stretches; critical loads or cloudy climates may want more. More autonomy is safer but costs battery, so size it against your worst realistic run without sun.
Related calculators
Educational estimates only — electrical work must follow the National Electrical Code (NEC) and local amendments, and be done or inspected by a licensed electrician.