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 100–200Ah of lithium; daily-use and full-time systems near 300–400Ah; 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 ≈ 83Ah 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 100–200Ah of lithium covers a light weekend in a van or RV (our RV Starter Kit is 200Ah), full-time RV setups use 300–400Ah, and boats start around 400Ah because the fridge, electronics and autopilot run around the clock at anchor.

• 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.

Voltage alone will not tell you how much usable energy is left. What a shunt measures, and what it cannot see.

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.