What Size Battery Bank Do I Need?

Size your battery bank from your loads, not from a product page. Add up the watt-hours you use in a day, decide how many days of autonomy you want, then divide by your system voltage to get amp-hours. A weekend RV setup often lands near 100Ah of lithium; daily-use systems near 150–300Ah; off-grid homes are sized per site.

• Battery listings all show amp-hours, but nobody explains how many you need.
• You do not want to buy twice — undersized now, replaced next season.
• Someone told you “just get the biggest one” and it did not sound like engineering.

List each load, its watts, and its hours per day. Multiply and add:

Example only — your loads will differ, and that difference is the whole point of sizing.

Divide by system voltage: 1,000 Wh ÷ 12V ≈ 84Ah of usable capacity per day. LiFePO4 batteries can routinely use about 80% of their rated capacity, so one day of autonomy needs roughly a 105Ah bank — which is why 100Ah lithium is the classic weekend starting point, and why full-time setups step up to 150–300Ah and add a second charging source.

• Air conditioning and induction cooking — these move you into inverter territory and can multiply the bank size. On 12V and full-time use, heavy AC loads usually mean a system review, not a bigger battery.
• Days of autonomy — cloudy-day buffer. Two days of autonomy doubles the bank or demands better charging.
• System voltage — larger off-grid systems often move to 24V or 48V so the same power flows at lower current.

The APS Power Guide asks about your actual loads and travel pattern and does this math for you — then recommends a right-sized kit, or routes you to a human review when your loads deserve one.