Key takeaways
- Start from an honest daily watt-hour load, not a guess at "how many batteries."
- Battery count = required amp-hours ÷ one battery's amp-hours, always rounded up.
- Depth of discharge drives bank size — lithium's deeper DoD means fewer batteries.
- Wire in series for voltage, parallel for capacity; derate for cold and age.
From a daily load to a battery bank
A battery bank isn't sized in "batteries" — it's sized in energy. Begin with how many watt-hours you burn in a day, which the off-grid load calculator gives you by adding up each appliance's watts times hours. Everything else is arithmetic from that one number.
First decide your days of autonomy: how many cloudy, no-sun days the bank alone must carry your loads. Multiply daily watt-hours by that figure to get the usable energy you need stored. But you can't safely drain a battery to empty, so divide by the depth of discharge (DoD) — the fraction the chemistry tolerates per cycle — to get the total battery energy you must install.
Convert that energy into amp-hours by dividing by your system voltage (12, 24, or 48 V). Finally, divide the required amp-hours by the amp-hour rating of a single battery and round up — that's your count, before you account for the series and parallel wiring needed to actually reach system voltage.
The formulas
Run the whole chain in the battery bank sizing calculator, which handles the rounding and wiring for you.
How chemistry changes the answer
The single biggest lever is usable depth of discharge. The same daily load can need a very different nameplate capacity depending on how deep you're allowed to cycle each battery.
| Chemistry | Usable DoD | Effect on bank size |
|---|---|---|
| Flooded lead-acid | ~50% | Needs ~2× the stored energy as usable energy — largest bank. |
| AGM (sealed lead-acid) | ~50% | Same ~2× penalty; smaller footprint, no maintenance. |
| LiFePO₄ (lithium) | 80–100% | Uses nearly all capacity — roughly half the nameplate Ah for the same load. |
Worked example: 3,000 Wh per day, 2 days
Suppose you draw 3,000 Wh/day and want 2 days of autonomy. Usable energy needed is 3,000 × 2 = 6,000 Wh. With flooded or AGM lead-acid at 50% DoD, total battery energy is 6,000 ÷ 0.5 = 12,000 Wh. At a 24 V system that's 12,000 ÷ 24 = 500 Ah, so 500 ÷ 100 = five 100 Ah batteries.
Switch to LiFePO₄ at 80% DoD and total battery energy drops to 6,000 ÷ 0.8 = 7,500 Wh, or 7,500 ÷ 24 ≈ 313 Ah — about three to four 100 Ah lithium batteries for the same job. Same load, far less hardware.
Wiring, temperature, and age
Amp-hours and voltage aren't interchangeable in the field — wiring decides which you get. Batteries in series add their voltages (two 12 V in series make 24 V at the same Ah); batteries in parallel add their amp-hours (two 100 Ah in parallel make 200 Ah at the same voltage). To hit 500 Ah at 24 V from 12 V/100 Ah units you'd build series-parallel strings, so your real count is often a clean multiple of the strings, not the bare division.
Then derate. Lead-acid loses usable capacity in the cold — well below freezing it can deliver far less than its rating — and every chemistry fades with age and cycles. Build in a margin (often 10–20%) so the bank still covers your loads years from now and on the coldest nights. If you mainly want to confirm a finished bank carries your loads, check it against the battery backup runtime calculator.
Frequently asked questions
How many batteries do I need to go off-grid?
Multiply daily watt-hours by days of autonomy, divide by depth of discharge, convert to amp-hours at your system voltage, then divide by one battery's Ah and round up. A 3,000 Wh/day load over 2 days at 50% DoD needs ~500 Ah at 24 V — five 100 Ah batteries.
What depth of discharge should I use to size a bank?
~50% for flooded and AGM lead-acid to protect cycle life, and 80–100% for LiFePO₄. A deeper usable DoD means less total capacity for the same load, which is why lithium banks are smaller.
Do I need fewer batteries with lithium than lead-acid?
Usually yes. LiFePO₄ uses 80–100% of capacity versus ~50% for lead-acid, so the same usable energy needs roughly half the nameplate amp-hours — fewer or smaller batteries, though each costs more.