How to size a house battery bank
List every DC device, multiply its current by the hours it runs per day, and add them up to get daily amp-hours. Multiply by the number of days you want to go without charging, then divide by the usable fraction of the battery type.
Example: weekend at anchor
80 Ah per day for 2 days on LiFePO4: 80 × 2 ÷ 0.9 = 178 Ah, so two 100 Ah batteries. On lead-acid the same boat needs 320 Ah (four 100 Ah batteries) because only half is usable.
Typical 12 V loads
| Device | Amps | Hours/day | Ah/day |
|---|---|---|---|
| 12 V fridge (compressor) | 4–5 | 8–12 (cycling) | 30–50 |
| Chartplotter / MFD | 1–3 | 8 | 8–24 |
| VHF radio (receive) | 0.5 | 8 | 4 |
| LED anchor light | 0.2 | 10 | 2 |
| LED cabin lights | 1–2 | 4 | 4–8 |
| Autopilot | 2–6 | 6 | 12–36 |
| Phone / tablet charging | 1–2 | 3 | 3–6 |
Frequently asked questions
How many amp-hours do I need on a sailboat?
Most coastal cruisers use 80–150 Ah per day. Two days of autonomy on lithium needs roughly 200–350 Ah.
Should the starting battery be part of the house bank?
No. Keep a separate starting battery, isolated with a battery switch, combiner or DC-DC charger, so you can always start the engine.
Is LiFePO4 worth it on a boat?
Usually yes: nearly twice the usable capacity per rated Ah, about half the weight, and many more cycles. It needs a compatible charger and alternator protection.