What are "peak sun-hours"?

It's a daily average of how many hours your location receives 1,000 W/m² of solar irradiance — the standard test condition for solar panels. Phoenix gets ~6.5; Seattle gets ~3.5.

Why isn't 100% efficiency used?

PV systems lose energy to inverters (3%), wiring (2%), soiling/dust (3%), and heat-driven panel derate (5%+). NREL's PVWatts default is 0.86; we use 0.80 as a conservative real-world figure.

Does this include tax credits or net metering?

No. The payback figure is simple — installed cost divided by annual bill savings at your current rate. The federal Investment Tax Credit (30% as of 2024) can reduce payback by 3–4 years. Check your state's incentive database.

Why does panel wattage matter?

It affects the number of panels and roof area required, but not the system's output (which is set by total kW). Modern 400+ W panels mean fewer panels and less roof needed for the same kW.

What if my roof can't fit the calculated area?

Most roofs only have 60–70% usable area after shading, vents, and setbacks. If the tool says 400 sq ft and your usable south-facing roof is 250 sq ft, scale the system down to match — you'll cover a smaller fraction of your bill.

Solar Panel Calculator

Size a rooftop solar system in kW, panels, and roof area from your monthly bill or kWh usage.

Written by Golam Rabbani, Founder & Lead Engineer

This interactive tool requires JavaScript. Read the formula and worked example below; you can compute the result by hand.

How to use this solar panel calculator

Pick imperial (sq ft) or metric (m²) for roof area, and choose Bill or kWh input mode.
Enter either your monthly bill ($) and your $/kWh rate, or directly enter monthly kWh.
Select your region (NREL peak sun-hours) or use Custom and enter your local value.
Set panel rating (default 400 W), system efficiency (default 0.80), and installed cost ($/W).
Press Calculate to see system size in kW, panel count, roof area, annual production, and simple payback.

About this solar panel calculator

The solar panel calculator sizes a rooftop PV system to match your annual electricity use. It first converts your bill or kWh input into annual kWh, then uses the formula annual kWh = system kW × peak sun-hours × 365 × system efficiency to back-solve the system size. The panel count is the system watts divided by panel rating, and roof area is panels × ~21 sq ft each. Payback is installed cost ÷ annual savings at your $/kWh rate.

Example: $150/month bill at $0.16/kWh = 937.5 kWh/month = 11,250 kWh/year. In a Southern US region (5.2 sun-hours/day) with 0.80 efficiency: system = 11,250 / (5.2 × 365 × 0.80) = 7.41 kW. At 400 W panels: 7,410 / 400 = 19 panels. Roof area = 19 × 21 = 399 sq ft. At $3/W installed: 7,410 × 3 = $22,230. Annual savings = 11,250 × 0.16 = $1,800. Payback = 22,230 / 1,800 ≈ 12.4 years.

Peak sun-hours come from NREL's National Solar Radiation Database. The 0.80 system efficiency default accounts for inverter losses (~3%), DC wiring (~2%), soiling (~3%), and temperature derate (~5%) typical of fixed-tilt rooftop arrays. Real quotes vary — get three bids before signing.

FAQ

What are "peak sun-hours"?: It's a daily average of how many hours your location receives 1,000 W/m² of solar irradiance — the standard test condition for solar panels. Phoenix gets ~6.5; Seattle gets ~3.5.
Why isn't 100% efficiency used?: PV systems lose energy to inverters (3%), wiring (2%), soiling/dust (3%), and heat-driven panel derate (5%+). NREL's PVWatts default is 0.86; we use 0.80 as a conservative real-world figure.
Does this include tax credits or net metering?: No. The payback figure is simple — installed cost divided by annual bill savings at your current rate. The federal Investment Tax Credit (30% as of 2024) can reduce payback by 3–4 years. Check your state's incentive database.
Why does panel wattage matter?: It affects the number of panels and roof area required, but not the system's output (which is set by total kW). Modern 400+ W panels mean fewer panels and less roof needed for the same kW.
What if my roof can't fit the calculated area?: Most roofs only have 60–70% usable area after shading, vents, and setbacks. If the tool says 400 sq ft and your usable south-facing roof is 250 sq ft, scale the system down to match — you'll cover a smaller fraction of your bill.