ComfortCalc

Solar Panel Size Calculator

Your daily energy usage, divided by how much a panel actually produces given your location's sun exposure.

Daily electricity usage (kWh)

Peak sun hours per day

Panel wattage (W)

Panels needed

Peak sun hours vary by location and season — check a solar irradiance map for your area rather than assuming a national average.

System size: 6,000 W

What you entered

Daily electricity usage (kWh): 30Peak sun hours per day: 5Panel wattage (W): 400

1
Daily usage in watt-hours
30 × 1000= 30000 Wh
2
Required system wattage
30000 ÷ 5 peak sun hours= 6000 W
3
Number of 400W panels
⌈6000 ÷ 400⌉= 15 panels (6000 W system)

Result

Panels needed: 15

Covering 30 kWh/day of usage at 5 peak sun hours needs about 15 panels (a 6000 W system).

Solar array size by daily usage and sun hours

Array size is daily kilowatt-hours divided by peak sun hours. Panel count rounds up, so installed capacity is usually a little above the calculated need.

Daily usage	Peak sun hours	Panel wattage	Panels needed	Installed capacity
20 kWh	4	400 W	13	5,200 W
30 kWh	3	400 W	25	10,000 W
30 kWh	4	400 W	19	7,600 W
30 kWh	4	550 W	14	7,700 W
30 kWh	5	400 W	15	6,000 W
50 kWh	5	400 W	25	10,000 W

Peak sun hours is not daylight hours: it is the equivalent hours of full-strength sun, typically 3 to 4 in northern Europe and the UK, and 5 to 6 in the American southwest. The same 30 kWh household needs 25 panels at 3 sun hours and 15 at 5. Real systems also lose 10 to 20% to inverter losses, wiring, heat, and soiling.

Peak sun hours, not daylight hours

'Peak sun hours' means the equivalent hours of maximum solar intensity, not total daylight — a location might have 12 hours of daylight but only 5 peak sun hours, since morning and evening sun is much weaker than midday sun.

This is a rough sizing estimate

Real solar system design also accounts for panel efficiency losses, inverter efficiency, shading, roof orientation, and seasonal variation — a solar installer's site assessment will refine this baseline estimate significantly.

Frequently asked questions

I use 30 kWh/day and get 5 peak sun hours — how many 400W panels do I need?

About 15 panels. Each 400W panel produces roughly 2 kWh/day (400W x 5 hours = 2,000 Wh). To generate 30 kWh, you need 30 / 2 = 15 panels. Real-world system losses (inverter, wiring, temperature) add about 15%, so plan for 17-18 panels.

What are 'peak sun hours' and how do I find mine?

Peak sun hours are the equivalent hours of full-intensity sunlight per day, not total daylight hours. The US averages 4-6 peak sun hours — Arizona gets 6-7, the Pacific Northwest gets 3-4. Search 'peak sun hours' for your zip code to find your local figure.

How much roof space do I need for solar panels?

A standard residential panel is about 17.5 sq ft. For 15 panels, you'd need roughly 265 sq ft of unshaded, south-facing roof space. Panels can also be mounted on ground racks if roof space is limited.

Does this account for cloudy days and winter?

The peak sun hours figure already averages seasonal and weather variation. However, daily production varies — summer may produce 50% more than winter. Net metering lets grid-connected systems bank summer surplus against winter shortfalls.