How Many Solar Panels Do I Need? Free Calculator and the Maths

Quick answer: Divide your annual electricity use by your region’s peak sun hours, by 365, and by about 0.86 to allow for system losses. That gives the system size in kilowatts. Divide by your panel wattage to get the panel count. A typical American home using 10,800 kWh a year needs roughly 6 to 8 kW, which is about 15 to 20 modern panels and 350 to 500 square feet of usable roof.

What this guide covers

• The calculator
• The maths behind it
• Peak sun hours by region
• Roof area, not just panel count
• Why 100% offset is not always the goal
• Common mistakes
• FAQ

The calculator

Enter your average monthly usage from a recent bill. If your bill only shows dollars, divide the total by your rate per kilowatt-hour first.

The maths behind it

The whole calculation is four steps, and it is worth understanding rather than trusting a black box.

Step one, annual consumption. Take twelve months of bills, not one. Summer and winter differ enormously, and a single month will mislead you in either direction.

Step two, peak sun hours. This is not hours of daylight. It is the number of hours per day that would deliver your location’s total daily solar energy at a standard 1,000 watts per square metre. A location with 5 peak sun hours receives the equivalent of five full-strength hours, spread across a much longer day.

Step three, system losses. A panel rated 400 watts does not deliver 400 watts into your house. Inverter conversion, wiring resistance, heat, soiling, mismatch and downtime all take a share. A commonly used industry default is around 14% total losses, which is the figure this calculator uses by default. Raise it if your roof is dusty, hot or partly shaded.

Step four, panel count. Divide the system size by the wattage of the panel you plan to install, then round up.

Written out: system kW = annual kWh divided by (peak sun hours times 365 times 0.86).

Peak sun hours by region

These are broad regional averages for a fixed array at a reasonable tilt. Your own figure depends on the specific roof plane, and a flat or north-facing surface will fall well below the regional number.

Roof area, not just panel count

Panel count answers half the question. The other half is whether the panels fit.

A modern residential panel is roughly 21 square feet. Add spacing between rows, setbacks from roof edges for fire access, and gaps around vents, chimneys and skylights, and you should plan on roughly 24 to 26 square feet of roof per panel in practice.

Only count usable roof. A plane that faces north in the northern hemisphere contributes very little. A plane shaded for four hours a day contributes far less than its area suggests, as explained in tree shading and solar panels.

If the roof cannot take the system you need, the options are a smaller array with partial offset, higher-efficiency panels that produce more per square foot, or a ground-mounted array.

Why 100% offset is not always the goal

Sizing to exactly cancel your annual consumption made sense under traditional net metering, where every exported kilowatt-hour was credited at retail value.

Under newer export tariffs that credit exports at a much lower avoided-cost rate, surplus generation is worth far less. Sizing beyond your self-consumption then produces energy you effectively sell at a discount.

Three things should adjust the target. Your export tariff, whether you plan to add a battery, and whether you plan to electrify heating or buy an electric vehicle. The last one pushes the target up, sometimes substantially.

Read net metering explained and time-of-use rates before fixing the number.

Common mistakes

Using one month’s bill. Always use twelve.

Ignoring planned load changes. An electric vehicle can add 3,000 to 4,000 kWh a year. A heat pump adds more. Size for the house you are about to have.

Assuming every roof plane counts. Orientation and shading matter more than raw area.

Forgetting inverter limits. The array must also fit the inverter’s voltage and current windows. See string sizing.

Skipping efficiency work first. Cutting consumption is cheaper per kilowatt-hour than generating it. See home energy audits.

FAQ

How many solar panels does an average house need?
A home using around 10,800 kWh a year typically needs 6 to 9 kW depending on region, which is roughly 15 to 23 panels at 400 watts each.

How much roof space do I need?
Plan on about 24 to 26 square feet per panel once you allow for spacing and fire setbacks. A 7 kW system therefore needs somewhere near 450 square feet of usable roof.

What are peak sun hours?
The number of hours per day equivalent to full-strength sunlight at 1,000 watts per square metre. It is a way of compressing a whole day’s solar energy into a single figure.

Why does the calculator subtract losses?
Because inverter conversion, heat, wiring, soiling and mismatch all reduce delivered energy. Around 14% total losses is a common industry default.

Should I size for 100% of my usage?
Not always. If exports are credited well below retail, sizing closer to your self-consumption often produces a better return.

Do higher wattage panels mean fewer panels?
Yes, and slightly less roof area, but the cost per watt is what determines value, not the count.

Will an electric vehicle change my answer?
Considerably. Typical home charging adds several thousand kilowatt-hours a year, so include it before sizing.

Where to go next

Run the numbers on return with our solar payback period calculator, then read solar panel cost and how to vet a solar company before requesting quotes.

Irradiance data and system modelling assumptions are published by the National Renewable Energy Laboratory and the U.S. Department of Energy.