Solar Panel Sizing Calculator

Daily energy need (kWh) is divided by location peak sun hours to get raw system size in kW, then adjusted upward by a system efficiency factor (typically 0.80) to account for inverter losses, temperature derating, and soiling. Panel count is the adjusted system size divided by individual panel wattage, rounded up to a whole number.

Roof orientation is applied as a production derate factor — south-facing at 1.0, east/west at ~0.85, and north-facing at ~0.65 — before the panel count calculation.

How many solar panels does it take to power a house?

The average US home uses about 10,500 kWh per year. At 400W per panel and 4.5 peak sun hours per day, each panel produces roughly 657 kWh annually. That means a typical home needs 15–20 panels for a 100% offset. Homes with high usage, poor sun, or less efficient panels may need 25–30. Enter your actual bill into the calculator above for a location-specific estimate.

How do I calculate how many solar panels I need?

Divide your daily energy consumption (kWh) by your location's average peak sun hours per day, then divide by the panel efficiency factor (typically 0.75–0.80 to account for inverter losses, temperature, and soiling). This gives you system size in kW. Divide by individual panel wattage (usually 400–430W today) to get panel count. The calculator handles all of this automatically when you enter your monthly bill or daily kWh.

Does roof orientation affect how many panels I need?

Yes, significantly. A true south-facing roof at the optimal tilt angle produces the most energy — roughly 100% of theoretical maximum. East or west-facing roofs produce about 80–85% as much. North-facing roofs in the northern hemisphere are the least productive at 60–70%. If your roof is not south-facing, you may need 15–25% more panels to hit the same annual offset.

What is a solar panel offset percentage?

Offset percentage is how much of your annual electricity consumption your solar array covers. A 100% offset means your panels produce as much as you consume over the year (you still draw from and export to the grid seasonally). Many homeowners target 80–100% offset. Going above 100% only makes financial sense if your utility pays a good export rate — otherwise excess production is not compensated.

How much roof space do solar panels require?

A modern 400W solar panel measures roughly 6.5 ft × 3.5 ft — about 22 square feet. A 7kW system (17–18 panels) needs approximately 380–400 square feet of usable roof area. Usable roof area excludes setbacks from edges, skylights, vents, and shaded zones. Most installers require a minimum of 200 sq ft of unshaded south-facing roof for a cost-effective residential installation.