Solar Panel Calculator

How to Calculate Solar Panels for Your Home?

Start with your monthly electricity bill and rate. Divide the bill by the rate to find monthly kilowatt-hour (kWh) usage. Divide monthly kWh by 30 for daily usage. Divide daily usage by the average peak sun hours in your area to find the required system size in kilowatts. Divide the system size by the wattage of individual panels for the panel count. The calculator above performs all of these steps, also estimating annual savings, system cost, payback period, and carbon offset from your inputs.

How Many Solar Panels Does a Typical Home Need?

The average US home uses approximately 900 kWh per month. In an area with 5 peak sun hours per day, this requires a 6 kW system: 900 / 30 / 5 = 6 kW. Using 400-watt panels, that translates to 15 panels. Actual needs vary widely. A small apartment or efficient home may need only 8-12 panels. A large home with electric heating, an EV charger, and a pool pump might need 25-35 panels. The calculator sizes the system to your specific electricity consumption rather than using averages, providing a much more accurate estimate for your situation.

What Are Peak Sun Hours?

Peak sun hours represent the equivalent number of hours per day when solar irradiance averages 1,000 watts per square meter (the standard used to rate panel output). A location receiving 5 peak sun hours does not necessarily see 5 hours of bright sunshine. It means the total solar energy received throughout the day equals what 5 hours of peak-intensity sun would deliver. Phoenix averages 6-7 peak sun hours. New York averages 4-4.5. Seattle averages 3.5-4. Miami averages 5-5.5. Your local peak sun hours directly determine how many panels you need: fewer sun hours means more panels to generate the same energy.

How Much Do Solar Panels Cost?

The average installed cost for residential solar is $2.50-$3.50 per watt before incentives. A 6 kW system costs $15,000-$21,000 before the federal tax credit. The federal Investment Tax Credit (ITC) covers 30% of the installed cost, reducing a $18,000 system to $12,600 out of pocket. State rebates, utility incentives, and local programs may reduce the cost further. Panel prices have dropped more than 80% since 2010. Financing options include solar loans (owning the system with monthly payments), leases (using the system without owning it), and power purchase agreements (buying the electricity the system produces at a fixed rate).

What Is the Payback Period for Solar?

The payback period is how long it takes for cumulative electricity savings to equal the system cost. Divide the net system cost (after incentives) by annual savings. A $12,600 system saving $1,800 per year pays back in 7 years. After payback, the electricity is essentially free for the remaining 18-25 years of the system lifespan. The payback period depends on electricity rates (higher rates = faster payback), sun hours, system cost, and available incentives. In states with high electricity rates and good solar resources (California, Hawaii, Massachusetts), payback can be under 6 years. In states with low rates and fewer sun hours, it may take 10-12 years.

How Much Roof Space Do Solar Panels Need?

A standard residential solar panel measures approximately 65 x 39 inches (about 17.5 square feet). A 15-panel system needs roughly 263 square feet of unshaded, south-facing roof space. Panels are mounted in rows with small gaps between them and setbacks from roof edges and ridges required by fire code. Effective roof area for solar is typically 60-80% of the total roof area after accounting for obstructions (vents, chimneys, skylights), shading, and setbacks. A roof assessment (which most solar installers provide for free) determines exactly how many panels fit on your specific roof and which sections receive the best solar exposure.

Do Solar Panels Work on Cloudy Days?

Solar panels produce electricity from daylight, not direct sunlight alone. On overcast days, panels generate 10-25% of their rated capacity depending on cloud thickness. Rain and heavy cloud cover reduce output to 5-15%. Panel technology continues to improve at capturing diffuse light. Annual production accounts for all weather conditions through the peak sun hours metric, which already factors in cloudy days for your region. Germany, one of the world leading solar markets, has fewer peak sun hours than most US states, demonstrating that solar works effectively even in less sunny climates.

How Much CO2 Does Solar Offset?

The average US electricity grid produces about 0.85 pounds of CO2 per kWh. A 6 kW solar system generating 9,000 kWh per year offsets approximately 7,650 pounds (3.5 metric tons) of CO2 annually. Over a 25-year system life, that is roughly 87 metric tons of avoided emissions. This is equivalent to taking about 1.8 cars off the road or planting approximately 4,000 trees. The environmental impact compounds as the grid cleans up over time, because each kWh of solar displaces whatever marginal generation source would have run instead, which is frequently a natural gas or coal plant.

Frequently asked questions