Sizing a battery bank
An off-grid cabin, an RV or a home backup system needs batteries big enough to cover the hours, or days, when the sun is not shining. Too small and you run out at night or in a cloudy spell; too large and you pay for capacity you never use and may never fully recharge.
The method
- Daily energy use in watt-hours: add up each appliance’s watts × hours of use per day.
- Days of autonomy: how many days the batteries must cover without sun, typically 1 to 3.
- Usable depth of discharge (DoD): how much of the battery you can safely use.
- Efficiency: inverter and wiring losses, typically about 90%.
Battery capacity (Wh) = daily use × days ÷ (DoD × efficiency)
Amp-hours = Wh ÷ system voltage
Worked example
A cabin uses 3,000 Wh a day. You want 2 days of autonomy on a 24 V system with lithium batteries used to 80%, and an inverter at 90% efficiency.
- Capacity = 3,000 × 2 ÷ (0.80 × 0.90) ≈ 8.33 kWh
- At 24 V: 8,333 ÷ 24 ≈ 347 Ah
- With 12 V 100 Ah batteries: 2 in series per string × 4 strings in parallel = 8 batteries
- Solar array to recharge the daily use with 4.5 peak sun hours: 3,000 ÷ (4.5 × 0.75) ≈ 889 W
Battery chemistry
| Type | Usable DoD | Cycle life | Notes |
|---|---|---|---|
| LiFePO4 (lithium iron phosphate) | 80–100% | 3,000–6,000 | Light, no maintenance, built-in management system |
| AGM or gel lead-acid | 50% | 500–1,500 | Cheaper up front, heavy, losses in cold |
| Flooded lead-acid | 50% | 1,000–2,000 | Needs watering and ventilation |
Choosing the system voltage
| Daily load or inverter size | Voltage |
|---|---|
| Up to about 1 kW | 12 V |
| 1–3 kW | 24 V |
| Above 3 kW, whole-home | 48 V |
Higher voltage halves the current for the same power, which means thinner cables and lower losses. Most new home battery systems run at 48 V.
Tips
- Cut the load first: LED lighting, an efficient fridge and avoiding electric heating can halve the battery bank you need.
- Count winter sun hours when sizing the array for year-round use; they can be half the summer value or less.
- Add a generator for long cloudy spells rather than buying days of extra batteries.
- Avoid mixing old and new batteries or different capacities in the same bank.
- For grid-tied homes, a battery is mainly for backup or for shifting solar power to the evening under time-of-use rates.