Why Sizing Matters
Buying too few solar panels means your batteries won't fully recharge on typical days, leaving you short of power. Buying too many wastes money and space. Getting the sizing right requires a simple but methodical approach — and it's more accessible than most people expect.
Step 1: Calculate Your Daily Energy Consumption (Wh/day)
List every electrical device you plan to run and estimate its daily usage:
| Device | Wattage (W) | Hours/Day | Daily Wh |
|---|---|---|---|
| 12V fridge/cooler | 45W avg | 24 | 1,080Wh |
| Laptop | 65W | 4 | 260Wh |
| LED lighting | 20W total | 4 | 80Wh |
| Phone charging | 10W | 2 | 20Wh |
| Fan | 30W | 6 | 180Wh |
| Total | 1,620Wh/day |
Add a 20–25% buffer for inefficiencies (cable losses, inverter conversion loss, battery charge/discharge loss). So: 1,620 × 1.25 = ~2,025Wh/day needed from solar.
Step 2: Determine Your Peak Sun Hours
Peak sun hours (PSH) is the average number of hours per day when sunlight intensity is strong enough to produce rated panel output. This varies by location and season:
- Southwest USA (Arizona, Nevada): 5.5–7 PSH in summer
- Pacific Northwest / UK / Northern Europe: 2.5–4 PSH average
- Most of continental USA/Europe: 4–5.5 PSH in summer, less in winter
For planning, use a conservative figure. If you expect 4.5 PSH and want year-round reliability, consider planning for 3.5–4 PSH.
Step 3: Calculate Required Solar Array Size
The formula is simple:
Required Solar Watts = Daily Wh Needed ÷ Peak Sun Hours
Using our example: 2,025Wh ÷ 4 PSH = ~506W of solar panels needed.
You'd round up to a practical configuration, such as four 160W panels (640W total) to give headroom for cloudy days.
Step 4: Match to Battery Bank Size
Your battery bank should store at least 1–2 days of consumption without solar, especially for nights and overcast days. Using our 2,025Wh/day example:
- 1-day reserve: ~2,000–2,500Wh battery bank
- 2-day reserve: ~4,000–5,000Wh battery bank
For LFP batteries (recommended for off-grid), you can safely use 80–90% of rated capacity. For lead-acid, limit to 50% depth of discharge to preserve battery life.
Common Off-Grid Scenarios and Typical Solar Needs
- Weekend van life (minimal loads): 200–400W solar, 500–1,000Wh battery
- Full-time van/skoolie with fridge and electronics: 400–800W solar, 2,000–4,000Wh battery
- Off-grid cabin with moderate loads: 1,000–3,000W solar, 10–20kWh battery bank
- Portable camping with power station: 100–400W folding/rigid panels, matching station capacity
Tips for Getting More From Your Panels
- Track the sun: Manual tilting and re-angling panels during the day can add 20–30% more daily output.
- Avoid shade at all costs: Even partial shading dramatically cuts output in series-wired arrays.
- Use an MPPT charge controller: More efficient than PWM, especially in lower-light conditions.
- Clean panels regularly: Dust and pollen can reduce output by 5–15% over time.
Final Takeaway
Solar sizing is math, not guesswork. Work out your daily consumption, find your local peak sun hours, apply the formula, and build in a buffer. Starting a bit over-sized is far less painful than running out of power on day two of a trip.