Solar Panel and System Size Calculator
Turn annual household electricity use and a location-specific annual solar yield into a first-pass DC system size and whole-panel count.
Compare up to three scenarios
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View all saved project estimatesWhat does the solar panel calculator calculate?
Enter annual electricity use, the share you want solar to offset, local annual production per installed kilowatt, and panel wattage. The calculator estimates the required DC system size and rounds up to whole panels. It is an early planning result; site design, shade, losses, roof constraints, and utility rules still matter.
Solar size and panel count from annual yield
Annual usage sets the load, while target offset chooses how much of that annual energy the array should nominally produce. A user-supplied local annual yield converts energy into DC capacity. Dividing capacity by module wattage and rounding up gives whole panels; it does not perform a site-specific photovoltaic simulation.
How to measure for an accurate estimate
- Collect twelve consecutive utility billing periods and add the kWh values, adjusting for unusual vacancies or temporary loads.
- Choose a target offset that reflects utility export rules, planned electrification, budget, and realistic available installation area.
- Obtain annual kWh per installed kW from a location-specific model using the proposed tilt, direction, shade, and system-loss assumptions.
- Use the exact module nameplate wattage being considered and confirm its physical dimensions separately during layout.
Full annual offset with 400-watt panels
A household uses 12,000 kWh per year and targets a 100% annual offset. A location-specific estimate indicates 1,400 kWh per installed kW each year, and the proposed panels are rated 400 watts.
The target requires about 8.57 kW DC. Rounding 21.43 modules up gives 22 panels, a rated 8.8 kW DC array producing about 12,320 kWh at the entered yield.Before you purchase material
- Compare proposals using the same annual usage, production assumptions, DC system size, estimated first-year kWh, and degradation terms.
- Ask installers to document shade, orientation, equipment, warranties, roof work, permits, interconnection, and production-model inputs.
- Review current utility rates, export compensation, fixed charges, incentives, and eligibility rules before calculating financial savings.
- Have roof condition, structural suitability, electrical capacity, fire setbacks, and local requirements evaluated by qualified professionals.
Common questions
Where should I get the local annual solar yield?
Use a location-specific production model such as the National Laboratory of the Rockies’ PVWatts with realistic direction, tilt, shade, equipment, and loss assumptions. The needed output is annual kilowatt-hours per installed DC kilowatt. Ask bidders to disclose model inputs so estimates can be compared.
Should my target solar offset always be 100 percent?
Not necessarily. Available roof or ground area, shade, budget, export compensation, interconnection limits, fixed utility charges, and future electricity use can make another target more practical. Model several offsets and compare current utility rules before treating annual net energy as bill savings.
Why does the calculator round the solar panel count up?
Panels are purchased as whole modules, so a fractional result cannot meet the calculated target. Rounding up slightly increases rated DC capacity and estimated annual production. A final layout may instead use fewer panels because of roof geometry, setbacks, shading, string design, or interconnection constraints.
Does this calculator replace a solar site assessment?
No. It converts user-entered annual consumption and energy yield into a preliminary size. A site assessment must address usable area, orientation, shade, structure, roof condition, electrical equipment, fire access, local code, utility interconnection, equipment compatibility, production losses, and qualified installation.
Sources and reference standards
- National Laboratory of the Rockies — PVWatts CalculatorU.S. national-laboratory model for location-specific photovoltaic energy estimates using system, orientation, loss, and weather assumptions.
- U.S. Department of Energy — Homeowner's Guide to SolarFederal homeowner guidance covering electricity use, solar resource, roof condition, bids, interconnection, and local requirements.
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