A camper planning a 10-day off-grid trip made a common mistake: they bought a 100W solar panel and 100Ah battery based on an online "solar calculator" that didn't account for their location's peak sun hours. In Scandinavia in October, peak sun hours average 1.5–2 per day, not the 5 hours assumed by the calculator. Their panel could only deliver 150–200 Wh/day instead of the calculated 500 Wh, depleting the battery by day 3. The correct setup for their actual location was a 250W panel and 200Ah battery — 2.5× more expensive than what they bought.
The core formula for solar sizing is: Required panel watts = Daily Wh consumption ÷ Peak sun hours × Correction factor. The correction factor (typically 1.25–1.5) accounts for panel efficiency losses, inverter inefficiency, wiring losses, and battery charge/discharge losses.
Peak Sun Hours by Region (Annual Average)
| Region | Peak sun hours/day | 100W panel output |
|---|---|---|
| Sahara / Arabian Desert | 6–8 hours | 480–640 Wh/day |
| Southern US / Mediterranean | 5–6 hours | 400–480 Wh/day |
| Morocco / Northern Africa | 5–6 hours | 400–480 Wh/day |
| Central Europe | 3–4 hours | 240–320 Wh/day |
| UK / Northern France | 2.5–3.5 hours | 200–280 Wh/day |
| Scandinavia (summer) | 4–5 hours | 320–400 Wh/day |
| Scandinavia (winter) | 0.5–1.5 hours | 40–120 Wh/day |
Battery Sizing: The Missing Half of the Calculation
The panel produces energy; the battery stores it. Battery capacity must cover at least 2–3 days of consumption without any solar input (cloudy days, overnight use). A 100Ah 12V battery stores 1,200 Wh but only 840 Wh is usable — lead-acid batteries should not be discharged below 50% without reducing lifespan. LiFePO4 batteries can be discharged to 20%, making 100Ah LiFePO4 = 960 Wh usable.
This Tool's Estimate vs. Professional Sizing
The calculator here provides a ballpark estimate based on your inputs and your region's average solar resource data. For a permanent home installation worth thousands of dollars, commission a site survey — a professional installer will measure actual roof shading, check structural load capacity, assess local grid interconnection rules, and calculate realistic payback periods. The online estimate is a starting point for conversations, not a final specification.