Solar Panel Calculator
Calculate the system size, panel count and roof area a home solar array needs from your monthly kWh, your region's peak sun hours and the panel wattage you plan to use.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- System size
- 8.4 kW
- 21 panels of 400 W. Sized to cover 10,800 kWh a year at 4.3 peak sun hours and 14% losses
- Panels needed
- 21 panels
- 400 W module each. Panels are bought whole, so the array rounds up from 8 kW
- Roof area needed
- 441 sq ft
- 21 sq ft per module, before walkways and setbacks. A fire access path is usually required at the ridge and along one side
- Estimated production
- 11,338 kWh per year
- About 945 kWh a month against your 900 kWh of use
- Installed cost
- Enter a price per watt
- Typically $2.50 to $3.50 per watt installed in 2026 before incentives. A licensed installer and the utility interconnection govern the final design
Solar Panel formula
System size (kW) = Annual kWh / (365 x Peak sun hours x (1 - Losses))
Panels = System size (kW) x 1,000 / Panel watts, rounded up
Roof area = Panels x Module area (about 21 sq ft for a 400 W module)
Annual production = Installed kW x 365 x Peak sun hours x (1 - Losses)Worked example
A Phoenix house uses 1,400 kWh a month, which is 16,800 kWh a year. At 5.5 peak sun hours and 14 percent losses the system size is 16,800 / (365 x 5.5 x 0.86) = 9.73 kW.
In 450 W modules that is 22 panels, an installed 9.9 kW, and about 484 sq ft of roof. Production comes to roughly 17,092 kWh a year, slightly over the 16,800 used, and at $3.00 per watt installed the system is about $29,700 before any incentive.
Quick reference
| Monthly kWh | Southwest 5.5 psh | Midwest 4.3 psh | Northwest 3.7 psh |
|---|---|---|---|
| 500 kWh | 3.5 kW, 9 panels | 4.4 kW, 12 panels | 5.2 kW, 13 panels |
| 900 kWh | 6.3 kW, 16 panels | 8.0 kW, 21 panels | 9.3 kW, 24 panels |
| 1,200 kWh | 8.3 kW, 21 panels | 10.7 kW, 27 panels | 12.4 kW, 31 panels |
| 1,800 kWh | 12.5 kW, 32 panels | 16.0 kW, 41 panels | 18.6 kW, 47 panels |
From a power bill to a panel count
Sizing an array is one division. Take the year's kilowatt hours, divide by 365 days times the peak sun hours where you live times one minus the system losses, and you have the kilowatts of panel that cover the load. Peak sun hours are not daylight hours: they are the daily equivalent of sun at full strength, roughly 5.5 in Arizona and 3.7 in western Washington.
Losses are the other honest term. Fourteen percent is the PVWatts default and covers inverter conversion, wiring, soiling, mismatch, shading and the fact that panels lose output as they heat. Then divide by the panel wattage and round up, because panels come whole. A 400 W module occupies about 21 square feet, so the roof area follows straight from the count.
What the roof and the panel decide
The roof plane matters as much as its area. South facing is the reference; east or west gives up roughly 15 to 20 percent, and north is usually not worth it. Pitch changes it again, and a shading obstruction that clips one module can pull down a whole string unless the system uses microinverters or optimizers. NREL PVWatts models an exact address with pitch and azimuth and is worth running before signing anything.
Check the roof itself before the array. Asphalt shingles last 20 to 25 years and panels last longer, so an array on a 15 year old roof means paying to remove and reset it later. Fire code also wants access paths, generally along the ridge and at least one side, which is why a roof rarely fits as many panels as its bare square footage suggests.
What this calculator leaves out
The exact solar resource for your address, roof pitch and azimuth, which PVWatts models properly, and any shading analysis. It does not size the inverter or the conductors, check the main panel busbar against NEC 705.12(B), or price the electrical upgrades a small service may need. It also leaves out net metering rules, which vary by state and utility and decide what exported power is worth, the interconnection application, permits, and battery storage. On incentives, note that the 30 percent federal Residential Clean Energy Credit under section 25D ended for systems placed in service after December 31, 2025; state, utility and local programs still vary and change often.
Code limits that apply
- NEC 690.12
Rapid shutdown so conductors inside the array boundary drop to 80 V within 30 seconds
If you miss it: An array without module-level shutdown fails inspection and leaves firefighters a live roof
- NEC 705.12(B)
The 120 percent rule: busbar rating times 1.2 caps the sum of the main and the PV breaker
If you miss it: A 7.6 kW inverter on a 100 A panel with a 100 A main exceeds the busbar and needs a supply-side tap
- IRC R324.6
Roof access paths and setbacks for rooftop PV, generally at the ridge and one side
If you miss it: A roof filled edge to edge is refused by the fire marshal and panels come back off
- IRC R324.4
The roof structure carries the added dead load of the array and its rails
If you miss it: An older 2x4 truss roof needs an engineer's letter before the permit is issued
- NEC 690.13
A readily accessible PV disconnect, marked, at a location the utility accepts
If you miss it: No exterior disconnect means the utility refuses to energize the interconnection
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- Sizing from a summer bill. Use twelve months averaged, or the array covers August and falls short all winter.
- Ignoring shade. A single tree over one panel drags a whole string down unless the system uses microinverters or optimizers.
- Filling the roof plane to the edges: fire codes generally require access paths at the ridge and setbacks from the eave.
- Assuming the 30 percent federal credit still applies. It ended for systems placed in service after December 31, 2025.
- Skipping the roof age. Reroofing under an existing array costs several thousand dollars to remove and reset the panels.
- Forgetting net metering rules changed in several states; excess production may credit at a wholesale rate, not retail.
Key facts
- Peak sun hours are the daily equivalent of full-strength sun, roughly 5.5 in the Southwest and 3.7 in the Pacific Northwest.
- The PVWatts default for system losses is 14 percent, covering inverter conversion, wiring, soiling, shading, mismatch and heat.
- A 400 W residential module is about 21 square feet, so wattage per square foot barely changes between 400 and 500 W panels.
- The average US household uses about 900 kWh a month, which is roughly an 8 kW array in the Midwest and 6 kW in the Southwest.
- The 30 percent federal Residential Clean Energy Credit under section 25D does not apply to property placed in service after December 31, 2025.
Frequently asked questions
How many solar panels do I need for a house?
For an average 900 kWh a month in the Midwest, about 21 panels of 400 W, or an 8.4 kW system over roughly 441 sq ft of roof. The same house in the Southwest needs about 16 because the sun resource is stronger.
What are peak sun hours?
The daily equivalent of sunlight at full strength, about 1,000 watts per square meter. A location with 4.3 peak sun hours gets the same daily energy as 4.3 hours of full-strength sun, spread across the whole day.
Is there still a 30 percent federal solar tax credit?
Not for new residential systems. The Residential Clean Energy Credit under section 25D does not apply to property placed in service after December 31, 2025. State, utility and local incentives still exist and vary widely, so check what your utility offers.
How much roof space does a solar array need?
About 21 square feet per 400 W panel, so a 21 panel system is roughly 441 square feet. Allow more, because fire access paths at the ridge and along one side take usable roof out of the plan.
Sources and references
- Residential Clean Energy Credit, section 25D status and dates (IRS)
- Solar Energy Technologies Office, residential photovoltaic basics (US Department of Energy)
- Residential solar module specifications and dimensions (Silfab Solar)
Next steps for this project
The usual order for a electrical & plumbing project. See all electrical & plumbing calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.