The formula
Worked example
A van uses 1,200 Wh a day. For one day of autonomy with LiFePO4 (90% usable) at 12 V and 90% efficiency: 1,200 ÷ 0.9 ÷ 0.9 ÷ 12 = about 123 Ah, so a 150 Ah or 2 × 100 Ah lithium bank. With AGM at 50% usable, you'd need about 222 Ah, which is why lead-acid banks are so much bigger and heavier for the same usable energy.
How many days of autonomy?
- Weekend van or boat with shore power or a DC-DC charger: 1 day.
- Full-time van life with solar: 1.5–2 days, so a cloudy day doesn't flatten you.
- Off-grid cabin or home: 3–5 days, or fewer with a backup generator.
Choosing a system voltage
Keep to 12 V for most vans and small boats, because so many appliances are 12 V. Above roughly 2–3 kWh of storage or a 2,000 W inverter, 24 V or 48 V halves or quarters the current and lets you use much thinner cable.
Frequently asked questions
How many batteries do I need for off-grid living?
It depends on your daily use. A modest off-grid cabin using 3 kWh a day with 3 days of autonomy on LiFePO4 needs about 11 kWh of batteries, which is roughly 460 Ah at 24 V. Enter your own numbers above for an exact figure.
Can I mix old and new batteries?
It's best not to. In a bank, batteries should be the same type, capacity, brand and age, or the weaker one drags the others down.
Should I size for winter or summer?
Size for the worst season you'll actually use the system in. Winter solar output in the UK can be a fifth of summer, so more battery or another charging source helps.