How Many Solar Panels Do I Need for My House?
The formula, step by step
- Find your daily energy use. Daily kWh = monthly kWh × 12 ÷ 365.
- Divide by your sun hours and system losses. System size (kW) = daily kWh ÷ (peak sun hours × performance ratio). Performance ratio accounts for real-world losses (heat, wiring, dust) and is typically ~80%.
- Convert to panel count. Panels = system size (in watts) ÷ panel wattage.
Worked example
- Monthly usage: 900 kWh → daily usage: 900 × 12 ÷ 365 = 29.6 kWh/day
- Peak sun hours: 4.5/day, performance ratio: 80%
- System size: 29.6 ÷ (4.5 × 0.8) = 8.2 kW
- Panels needed (400W each): 8,200 ÷ 400 = ~21 panels
What changes the number
| Factor | Effect |
|---|---|
| More sun (sunny climate) | Fewer panels needed for the same usage |
| Higher-wattage panels | Fewer panels for the same system size |
| Lower export payment from utility | Sizing for less than 100% offset can be smarter |
| Planning to add an EV or heat pump | Size for your future usage, not just today's |
Should you size for 100% of your usage?
Often yes, but not always. If your utility pays close to the retail rate for exported power (net metering), maximizing your system size makes sense. If export payments are low, a system sized closer to your daytime usage — with a battery to store the rest — can give a better return. Our solar export earnings calculator and solar vs. battery guide help work out which applies to you.
Frequently asked questions
How many solar panels do I need for an average home?
For a home using about 900 kWh/month with average sun (4.5 peak-sun-hours/day), you need roughly an 8 kW system, or about 20 panels at 400W each. Usage, sunlight, and panel wattage all shift this number, so calculate your own instead of using a rule of thumb.
Should I size solar to cover 100% of my usage?
Not always. If your utility pays little for exported power, sizing closer to your daytime usage often gives a better return than maximizing total offset.
How much roof space do I need?
Roughly 18–22 sq ft (1.7–2 m²) per panel. A 20-panel system needs about 360–440 sq ft of usable roof area.