What Is a Solar Panel Size Calculator and How Does It Work?
A solar panel size calculator estimates the system capacity in kilowatts (kW) and the number of panels needed to generate a target share of your electricity use. It divides your daily energy need by the location's peak sun hours and an efficiency factor that accounts for real-world losses, then divides by each panel's wattage.
The sizing formula is: System Size (kW) = Daily Usage (kWh) ÷ (Peak Sun Hours × (1 − System Losses)) and Number of Panels = System Size (W) ÷ Panel Wattage (W), rounded up to a whole panel. Daily usage is monthly kWh × 12 ÷ 365, and one peak sun hour equals 1 kWh/m² of solar irradiance.
According to the U.S. Energy Information Administration (EIA), the average US home uses roughly 900 kWh of electricity per month. The National Renewable Energy Laboratory (NREL) PVWatts model assumes about 14% total system losses from inverter, wiring, soiling, and temperature effects, which is the default used here.
How to Use This Solar Panel Size Calculator
Enter your electricity use and site details; the panel count, system size, production, and roof area update instantly. Here's what each field means:
- Monthly Usage (kWh): Your average monthly consumption from the last 12 utility bills, not the dollar amount.
- Peak Sun Hours: Equivalent hours per day of full 1,000 W/m² sunlight at your location, typically 3–7 in the US and 2.5–3.5 in the UK and northern Europe.
- Panel Wattage: The rated output of one panel. Modern residential panels commonly range from 350 W to 450 W.
- Panel Area: Surface area of one panel, around 17–21 sq ft (1.6–2.0 m²) for common residential models.
- System Losses: Combined inverter, wiring, shading, dirt, and temperature losses. Use 14% unless your installer quotes otherwise.
- Energy Offset Goal: The share of your usage you want solar to cover. Use 100% for full coverage or lower if roof space is limited.
- Installed Cost per Watt: Your quoted price before incentives, used for a rough budget estimate.
Results work with kWh and kW, which are used worldwide. Roof area is shown in both square feet and square metres, and costs use the currency set in your site settings, so the tool applies equally in the US, UK, Europe, and Australia.
How Many Solar Panels Do You Need by Usage?
The table below shows the number of 400 W panels needed at 4.5 peak sun hours and 14% losses for a 100% offset.
| Monthly Usage | System Size | 400 W Panels |
|---|---|---|
| 400 kWh | 3.4 kW | 9 |
| 600 kWh | 5.1 kW | 13 |
| 900 kWh | 7.6 kW | 20 |
| 1,200 kWh | 10.2 kW | 26 |
| 1,500 kWh | 12.7 kW | 32 |
Sunnier regions need fewer panels: at 6.5 peak sun hours the same 900 kWh home needs about 14 panels, while at 3 hours it needs about 29.
Frequently Asked Questions
How many solar panels do I need to power my house?
A typical US home using 900 kWh per month needs about 20 panels rated 400 W each, or roughly 8 kW, assuming 4.5 peak sun hours and 14% system losses. Homes in sunnier states need fewer panels, and larger or electric-heated homes need more.
How do I calculate solar panel size from my electric bill?
Find the kWh used, not the dollar total, on your bill. Multiply monthly kWh by 12 and divide by 365 to get daily kWh, then divide by peak sun hours times 0.86 to get the system size in kW. Divide that by panel wattage to get the panel count.
What are peak sun hours and why do they matter?
Peak sun hours are the number of hours per day that sunlight averages 1,000 watts per square metre at your location. NREL publishes values for every US location, and a site with 6 hours needs about one third fewer panels than one with 4.
How accurate is this solar panel size calculator?
It gives a reliable planning estimate using average sun hours and a standard loss factor. Roof tilt, orientation, shading, panel degradation of about 0.5% per year, and local weather can change real output, so confirm the final design with an installer or NREL's PVWatts.