Off Grid Living WisdomHow Many Solar Panels Do I Need? Solar & Battery Calculator
How many solar panels do I need? This free solar and battery calculator answers it in seconds. Enter your daily energy use, your region's sun hours and your panel size, and it sizes the solar array, the battery bank and the charge controller for an off-grid cabin, a homestead or home backup.
How the solar calculator works
- Daily energy is how many watt-hours you use per day. If you don't know, the generator size calculator adds it up from your appliances.
- Solar array = daily watt-hours ÷ (peak sun hours × 0.75). The 0.75 covers real-world losses: heat, dust, wiring, battery charging and inverter losses.
- Battery bank = daily watt-hours × days of backup ÷ usable depth ÷ 0.9 inverter efficiency.
- Charge controller = array watts ÷ battery voltage × 1.25 safety margin.
Worked example: 3,000 Wh a day with 4 sun hours needs 3,000 ÷ (4 × 0.75) = 1,000 watts of panels, so three 400 W panels. Two days of backup on LiFePO4 needs about 7,400 Wh of battery, or roughly 155 Ah at 48 V.
Peak sun hours by US region
| Region | Yearly average | December |
|---|---|---|
| Desert Southwest (AZ, NV, NM) | 6–7 | 4–5 |
| Texas, Southern Plains, coastal California | 5–6 | 3.5–4.5 |
| Southeast and Florida | 4.5–5.5 | 3.5–4.5 |
| Rocky Mountains | 5–6 | 3–4 |
| Midwest and Upper Midwest | 4–4.5 | 2–2.5 |
| Northeast and New England | 3.5–4.5 | 1.5–2.5 |
| Pacific Northwest (west of the Cascades) | 3.5–4 | 1–1.5 |
Approximate. For your exact address, use the free PVWatts calculator (pvwatts.nlr.gov) in the US, or PVGIS elsewhere.
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Frequently asked questions
How many solar panels do I need to run a house off grid?
Divide your daily watt-hours by your peak sun hours × 0.75, then by your panel size. A home using 15,000 Wh a day with 4 sun hours needs about 5,000 watts, or 13 panels of 400 W. One using 30,000 Wh needs about 25. Sizing for winter can take two to three times more.
How big a solar battery do I need?
Multiply your daily watt-hours by the days of backup you want, divide by the battery's usable depth (0.9 for LiFePO4, 0.5 for lead-acid) and by 0.9 for inverter losses. Two days of 3,000 Wh on LiFePO4 is about 7,400 Wh.
Should I size for summer or winter?
For year-round off-grid living without a generator, size for December, which can mean two to three times more panels in northern states. Most off-grid homes size for spring and fall and run a generator for the darkest weeks; it's far cheaper.
What battery voltage should I use?
12 V for vans and small cabins up to about 1,500 W, 24 V up to about 3,000 W, and 48 V for homes. Higher voltage means thinner wire and less loss.