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How to Size an Off-Grid Solar Battery Bank
Battery sizing is three numbers: daily use, days without sun, and how far you are willing to drain it.
Direct answer: Multiply your daily energy use in kWh by the number of cloudy days you want to cover (days of autonomy), then divide by the usable depth of discharge. A cabin using 3 kWh/day with 2 days of autonomy and 80% usable lithium needs about 7.5 kWh of battery capacity.
Worked calculation
| Step | Example | Result |
|---|---|---|
| 1. Daily use | Lights 0.3 + fridge 1.2 + pump 0.3 + electronics 0.2 | 2.0 kWh/day |
| 2. Add inverter losses (×1.1) | 2.0 × 1.1 | 2.2 kWh/day |
| 3. Days of autonomy | 2 cloudy days | 4.4 kWh |
| 4. Usable depth of discharge | LiFePO4 at 80% | 4.4 ÷ 0.8 = 5.5 kWh |
| 5. Round up for cold and ageing | +20% | ≈ 6.6 kWh bank |
So a 6.6–7 kWh lithium bank. If you use lead-acid at 50% depth, the same load needs 4.4 ÷ 0.5 = 8.8 kWh, plus a 20% ageing margin — about 10.5 kWh. That is why lead-acid looks cheap per amp-hour and expensive per usable kWh.
Daily usage first
Do not size a battery until you have measured or estimated loads honestly. A watt-meter on each appliance for a week beats any rule of thumb. The biggest surprise is usually a fridge, a pump, or a phantom load from an inverter left on.
Days of autonomy
- 1 day: grid-tied backup or a cabin with a generator.
- 2–3 days: typical off-grid cabin in a mixed climate.
- 5+ days: remote sites with no generator and long winter cloud cover.
Each extra day roughly doubles cost. Most people are better off with two days of battery and a small generator than seven days of battery.
Battery types compared
| LiFePO4 | Lead-acid (AGM/flooded) | |
|---|---|---|
| Usable depth | 80–90% | 50% |
| Cycle life | 3,000–6,000 | 500–1,200 |
| Cost per usable kWh | Lower over life | Higher over life |
| Maintenance | None | Watering, equalising (flooded) |
| Cold performance | Needs above-freezing charging | Better tolerance, less capacity |
Frequently asked questions
Can I mix old and new batteries?
No. Mixing ages and chemistries causes uneven charging and shortens the whole bank's life.
Does cold reduce capacity?
Yes. Lithium loses usable capacity below freezing and should not be charged below 32°F without a heated battery. Lead-acid loses roughly 20% at 20°F.
How many batteries is 10 kWh?
With 100 Ah 12.8 V lithium units (1.28 kWh each), about eight batteries. With 200 Ah 12 V lead-acid (2.4 kWh gross, 1.2 usable), about eight to nine.
Start with the off-grid solar beginner's guide, and compare backup options in solar vs generator.
Frequently asked questions
Can I mix old and new batteries?
No. Mixing ages and chemistries causes uneven charging and shortens the bank's life.
Does cold reduce capacity?
Yes. Lithium loses usable capacity below freezing and should not be charged below 32°F without heating. Lead-acid loses roughly 20% at 20°F.
How many batteries is 10 kWh?
With 100 Ah 12.8 V lithium units (1.28 kWh each) about eight batteries; with 200 Ah 12 V lead-acid (1.2 kWh usable) about eight to nine.