How Many Solar Panels Do I Need for Off-Grid?
- Formula: Daily load (Wh) ÷ peak sun hours ÷ panel efficiency factor = minimum panel watts.
- Peak sun hours vary by location: 3.5h (Pacific NW) to 6.5h (Arizona).
- Always add 25–30% to your calculated minimum to account for losses and cloudy days.
- A typical off-grid cabin needs 400–1,200W of panels. A full home needs 3,000–8,000W.
- More panels = faster battery charging = more autonomy. Oversizing panels is rarely a mistake.
The formula
Sizing your solar array comes down to one core calculation:
Panel watts = Daily load (Wh) × 1.25 ÷ Peak sun hours
The 1.25 multiplier accounts for system losses (inverter, wiring, temperature derating). Peak sun hours is the key variable that changes by location.
Panel calculator
Peak sun hours by US region
| Region | Peak sun hours | Example cities |
|---|---|---|
| Pacific Northwest / Alaska | 3.0–4.0h | Seattle, Portland, Anchorage |
| Northeast | 3.5–4.5h | Boston, New York, Philadelphia |
| Midwest / Great Lakes | 4.0–4.5h | Chicago, Detroit, Minneapolis |
| Southeast | 4.5–5.5h | Atlanta, Charlotte, Nashville |
| Florida | 5.0–5.5h | Miami, Tampa, Orlando |
| Texas | 5.0–6.0h | Houston, Dallas, San Antonio |
| California | 5.5–6.5h | LA, San Francisco, Sacramento |
| Southwest / Desert | 6.0–7.0h | Phoenix, Las Vegas, Albuquerque |
| Mountain West | 5.0–6.0h | Denver, Salt Lake City, Boise |
Worked examples
Example 1: Weekend cabin (Pacific NW)
- Daily load: 800 Wh
- Peak sun hours: 3.5h
- Minimum panels: 800 × 1.25 ÷ 3.5 = 286W → round up to 400W (1× 400W panel)
- Recommended: 400W × 1.25 = 500W → 2× 400W panels for safety margin
Example 2: Full-time cabin (Texas)
- Daily load: 2,400 Wh
- Peak sun hours: 5.5h
- Minimum panels: 2,400 × 1.25 ÷ 5.5 = 545W → round up to 600W
- Recommended: 800W (2× 400W panels) for margin and cloudy day buffer
Example 3: Off-grid home (Arizona)
- Daily load: 8,000 Wh
- Peak sun hours: 6.5h
- Minimum panels: 8,000 × 1.25 ÷ 6.5 = 1,538W → round up to 1,600W
- Recommended: 2,400W (6× 400W panels) at 48V system
How panel wiring affects your array
How you wire your panels (series vs parallel) affects your system voltage and charge controller choice — but not the total wattage needed. See our series vs parallel wiring guide for full details.
| Wiring | Voltage | Current (amps) | Best for |
|---|---|---|---|
| Series | Multiplies | Same as one panel | Higher voltage MPPT systems, long cable runs |
| Parallel | Same as one panel | Multiplies | 12V systems, partial shading situations |
| Series-parallel | Middle ground | Middle ground | Large arrays, flexible configuration |
Common mistakes to avoid
- Using summer sun hours. Always size for your worst month. December in Seattle gets 1.8h peak sun — not the 4.5h annual average.
- Forgetting temperature derating. Panels lose ~0.4% efficiency per °C above 25°C. In hot climates add another 10% to your panel requirement.
- Undersizing to save money. Panels are the cheapest part of the system per kWh produced. Undersizing means your batteries never fully charge, dramatically shortening their life.
- Not accounting for shading. Even partial shading on one panel can dramatically reduce output. If shading is unavoidable, use micro-inverters or power optimisers.
Frequently asked questions
Can I add more panels later?
Yes — expanding a solar array is straightforward as long as your charge controller can handle the additional input. Many people start small and add panels as budget allows. Just ensure your controller has headroom. See our MPPT controller guide.
Is it better to oversize my panel array?
Generally yes. Extra panels mean faster battery charging, more buffer on cloudy days and the ability to expand loads later. The cost of oversizing panels is much lower than the cost of an undersized system that leaves you without power.
What size panels should I buy — 200W, 400W or larger?
400W monocrystalline panels offer the best value per watt in 2026. Larger panels (500–600W) are becoming common and offer lower installation cost per watt but require a higher-voltage MPPT controller.