How to Calculate Solar Panel Output: Sizing a System for Your Actual Energy Use

A 400-watt panel does not produce 400 watts for eight hours a day. It produces 400 watts only under perfect sun at 77°F — real rooftops see 60-80% of that. Sizing a solar array around nameplate wattage rather than real-world output is the single most common mistake homeowners make, and it usually shows up as a system that misses its target by a third. Here is the calculation that gets it right.

The Core Formula

Daily Output (kWh) = System Size (kW) × Peak Sun Hours × Performance Ratio

• System size — total panel watts ÷ 1,000. Ten 400W panels = 4 kW.
• Peak sun hours — the daily equivalent hours of full-strength sun. Ranges from ~3.0 (Seattle) to ~5.5 (Phoenix).
• Performance ratio — losses from heat, wiring, inverter, soiling, and shading. Typically 0.75-0.85.

Worked Example: A 4 kW System in Two Cities

Location	Peak Sun Hours	Performance Ratio	Daily kWh	Monthly kWh
Phoenix, AZ	5.5	0.80	17.6	~528
Seattle, WA	3.0	0.80	9.6	~288

The same 4 kW array delivers nearly double the energy in Phoenix. If your household uses about 900 kWh a month, you would need roughly a 7 kW system in Phoenix but a 12 kW system in Seattle to cover the same bill. You can model the panel count and area quickly with the Solar Calculator, which factors sun hours and roof orientation into the estimate.

Step-by-Step Sizing

1. Pull your average monthly kWh from a recent utility bill (or take the annual total ÷ 12).
2. Divide by 30 to get daily kWh needed.
3. Divide daily kWh by (peak sun hours × performance ratio) to get required system kW.
4. Multiply required kW by 1,000, then divide by your chosen panel wattage for the panel count.
5. Check roof space: each 400W panel needs about 20-22 square feet. Use the Area Calculator to confirm your usable roof area after subtracting vents, chimneys, and setbacks.

Frequently Asked Questions

Do solar panels work on cloudy days? Yes, but output drops to roughly 10-25% of clear-sky production. They need daylight, not direct sun, but strong direct sun produces far more.

How much does heat reduce output? Panel efficiency falls about 0.3-0.4% per degree Celsius above 25°C (77°F). A hot 35°C rooftop can shave 10-15% off nameplate rating — which is exactly why the performance ratio exists.

What about snow and dust? A dirty or snow-covered array can lose 15-30% until cleared; heavy snow can drop output to near zero until it slides off.

Practical Takeaway

Never size a system on nameplate watts alone. Multiply by your local peak sun hours and a realistic performance ratio of 0.75-0.85 to get the energy you can actually expect. Then check that the array fits your roof — energy plans that ignore physical space are plans that never get installed.