How Many Solar Panels Do You Need?
The question isn't really "how many panels" — it's "how many watt-hours do you need per day?" The number of panels is just the answer to that question divided by your available sunlight and individual panel wattage.
The Formula
Number of Panels = (Daily Wh × 1.25) ÷ Peak Sun Hours ÷ Panel Wattage
The 1.25 multiplier accounts for real-world system losses (inverter efficiency, wire losses, controller overhead, panel soiling). Peak sun hours vary by location (3–7 hours depending on region and season). Panel wattage is the rated output of each individual panel.
Quick Reference by Use Case
| Use Case | Daily Wh | Panels (100W) | Panels (200W) | Panels (400W) |
|---|---|---|---|---|
| Shed lights + phone charging | 300 | 1 | 1 | 1 |
| RV (basic: fridge, lights, charging) | 1,000 | 3–4 | 2 | 1 |
| Off-grid cabin (moderate use) | 2,000 | 6–8 | 3–4 | 2 |
| Small off-grid home | 5,000 | 16–20 | 8–10 | 4–5 |
| Average US home (full offset) | 30,000 | Not practical | Not practical | 20–25 |
Based on 5 peak sun hours and 25% efficiency buffer. Your actual numbers will vary with location, season, and panel orientation.
Factors That Change the Number
Location: Phoenix (7+ peak sun hours) needs fewer panels than Seattle (3–4 peak sun hours) for the same energy output. The difference can be 50% more panels for cloudy regions.
Panel orientation and tilt: South-facing panels at optimal tilt (roughly equal to your latitude) produce the most annual energy. Flat-mounted panels lose ~10% efficiency. East or west-facing panels lose 15–20%.
Shading: Even partial shade on one panel can significantly reduce output — especially in series-wired configurations. If shading is unavoidable, you may need to oversize your array to compensate.
Seasonal variation: If you need reliable year-round production (full-time off-grid), size panels for your worst month (typically December/January). Summer production will exceed your needs, but you'll maintain adequate output through winter.
Use NREL's PVWatts Calculator with your exact coordinates for precise location-specific estimates. Then use our sizing formulas to translate production into component specifications.
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