Rain & Sun Harvest Calculator

Calculate combined rainwater and solar energy harvest from your roof

Quick Estimate

Roof Area

sq ft

Annual Rainfall

in/yr

Peak Sun Hours per Day

hrs

Roof Used for Solar?

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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Need more accuracy?

Extended mode adds scenarios, charts, and detailed breakdowns

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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.

Tip: Start with the Quick calculator for a fast estimate. Switch to Extended mode for scenario comparison and charts, or Professional mode for complete parameter control.

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

How many solar panels fit on my roof?

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.

Does my roof need to face south for solar?

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.

What is the payback period for solar panels?

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.

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Sources & References

The formulas, code requirements, and reference data used in this calculator are sourced from the following authoritative organizations and publications. Local codes vary — always verify with your jurisdiction's building department.

International Residential Code (IRC) 2024 — Chapter 9: Roof Assemblies — ICC
NRCA Roofing Manual (Steep-Slope & Low-Slope) — National Roofing Contractors Association
Asphalt Shingle Installation Manual — ARMA (Asphalt Roofing Manufacturers Association)
NREL PVWatts® Calculator — National Renewable Energy Laboratory
IRC R324 / IBC 1510 — Solar Photovoltaic Systems — ICC
SEIA Solar Installation Best Practices — Solar Energy Industries Association

Disclaimer: Calculator results are estimates intended for planning and educational purposes only. They do not replace professional engineering analysis, building permit review, or contractor estimates. Always consult a licensed roofing contractor or structural engineer for final specifications.