Rain & Sun Harvest Calculator
Calculate combined rainwater and solar energy harvest from your roof
sq ft
in/yr
hrs
%
Rainwater per Year (gal)
22,877
Solar Production (kWh/yr)25,529
Solar Roof Area720 sq ft
Rainwater Collection Area1,080 sq ft
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How to Use the Rain & Sun Harvest Calculator
Solar roofing is one of the best long-term investments for homeowners in sun-rich climates. The Rain & Sun Harvest Calculator estimates how much of your roof area is usable and what energy output you can expect.
- Enter your total roof area and the orientation (south-facing is best in the Northern Hemisphere).
- Select your climate/sun zone to get peak sun hours.
- Enter the panel wattage you are considering (typical residential panels are 380–420 W).
- Review usable area after setbacks and obstructions.
- Multiply panel count by wattage to get system size in kW.
Formula & Methodology
The Rain & Sun Harvest Calculator uses the following established formulas:
System Size (kW) = Panel Count × Panel Wattage / 1000
Annual Output (kWh) = System Size × Peak Sun Hours × 365 × 0.80
Usable Roof Area = Total Area × 0.65–0.75 (after setbacks)
Payback Period = System Cost / Annual Savings
The 0.80 derate factor accounts for inverter losses, wiring losses, and real-world temperature effects.
Frequently Asked Questions
A standard 400 W residential panel measures about 17.5 sq ft. After accounting for setbacks and obstructions, most homes can fit 20–40 panels. Our Rain & Sun Harvest Calculator calculates your specific roof's capacity based on usable area.
South-facing roofs at 30°–40° tilt produce the most energy in the Northern Hemisphere. East or west-facing roofs generate about 80–85 % of south-facing output. North-facing roofs are generally not suitable for solar unless you are in the Southern Hemisphere.
The average payback period in the US is 6–10 years, after federal and state incentives. With the 30 % federal ITC (Investment Tax Credit) available through 2032, most homeowners see payback in 7–8 years and panels last 25–30 years.