How to Use the Solar Panel Calculator
Use the calculator to estimate panel count, DC system size, roof area needed, annual production, battery daily coverage, costs, savings, and simple payback.
- Choose Annual (kWh/year) or Average monthly (kWh/month) under Usage entered as. Enter Electricity use and Target percent of use to offset.
- Choose Peak sun hour preset or edit Peak sun hours per day. Enter Panel wattage and System losses / derate.
- Enter Area per panel and, if available, Usable roof area available (optional).
- Optionally enter Usable battery capacity.
- Enter Installed price per watt, Incentives / rebates, and Electricity price you pay.
- Select Calculate. Select Reset to clear the inputs.
What the Result Shows
The headline uses the mid-value panel count and actual system size. Annual production and simple payback are shown as low-to-high ranges. Roof area needed comes from panel count and converted panel area. Battery output is the share of average daily use covered, capped at 100%.
Formula used appears separately from the two-column Component breakdown & assumptions table. The table lists converted use, target production, sun-hour values, losses, system size, panel count, roof area, battery, costs, savings, and payback values.
How the Estimate Is Calculated
Daily use = Annual use ÷ 365, orMonthly use × 12 ÷ 365.Target daily production = Daily use × Offset % ÷ 100.Required system size = Target daily production ÷ (Peak sun hours × (1 - System losses %)).- Panel count rounds up to a whole number; actual system size uses that rounded count.
Annual production = Actual system size × Peak sun hours × 365 × (1 - System losses %).Roof area needed = Panel count × Area per panel.Battery coverage % = MIN(100, Usable battery kWh ÷ Daily use × 100).- Gross and net cost use actual system watts, Installed price per watt, and Incentives / rebates.
- Annual savings use Electricity price you pay and are capped at annual electricity use.
Simple payback = Net system cost ÷ Annual savings.
Why Production and Payback Are Ranges
Only peak sun hours vary by minus or plus 10% for the low and high calculations. The entered system-loss percentage stays unchanged. The calculator reruns production, savings, and payback with those sun-hour values. This is not a measurement of shading, orientation, tilt, weather, equipment performance, or degradation.
Inputs, Units, and Validation
- Electricity use: 0.01 to 10,000,000; offset 1% to 200%.
- Peak sun hours per day: 1 to 9; panel wattage 50 to 800 W; system losses 0% to 50%.
- Area per panel: 1 to 1,000; panel and roof area accept ft² or m².
- Usable roof area available (optional): blank or 0 to 10,000,000. Usable battery capacity is blank or 0 to 10,000 kWh.
- Installed price per watt and Electricity price you pay accept 0 to 100. Incentives / rebates accept 0% to 100%.
Blank optional roof and battery fields are treated as no entry. A roof value of 0 is accepted but does not trigger a fit status; that status appears only when converted usable roof area is greater than zero. An invalid value stops calculation and displays a specific error.
Copy or Download the Result
Copy result copies the plain-text result to the clipboard. Download result (.txt) downloads the same type of result as solar-panel-calculator-result.txt.
Limits and Privacy Details
This estimate does not guarantee production, use live utility rates, or model net-metering and export compensation. It does not design wiring, string layout, conductor sizing, inverter placement, permitting, interconnection, or code compliance. It also does not perform a site survey, shading analysis, or structural roof assessment.
The calculator calculations run locally in the browser from entered or selected values. The page also loads Ahrefs and Google Analytics scripts. Its separate subscription form sends an email address to /assets/subscribe.php when submitted.
Frequently Asked Questions
How is the number of solar panels calculated?
The calculator divides your target daily production (daily electricity use x offset percentage) by peak sun hours and a system-loss derate factor to get a required DC system size in kW. That size is divided by your panel wattage and rounded up to a whole number of panels.
What are peak sun hours and why do they matter?
Peak sun hours are the equivalent number of hours per day sunlight arrives at an intensity of 1,000 W/m^2. They summarize a location's solar resource into one number used to convert system size into expected daily energy output. Higher peak sun hours mean fewer panels are needed for the same energy target, and vice versa.
Does this tool guarantee my system will produce this much power?
No. This is a simplified educational estimate using the peak sun hours, loss percentage, and prices you enter. Actual production depends on real shading, panel orientation and tilt, local weather patterns, equipment efficiency, and degradation over time - none of which this tool measures. It does not replace a site survey or a professional solar design.
What counts as a system loss or derate factor?
System losses combine inverter conversion loss, DC and AC wiring loss, soiling/dust, shading, panel mismatch, and temperature derating into one percentage that reduces theoretical production down to a realistic estimate. A common starting range is 14% to 23% total loss.
Does this calculator design the wiring or handle permitting?
No. It only estimates panel count, system size, roof area, production, battery contribution, and payback. It does not size conductors, breakers, or inverters, does not lay out strings, and does not address permitting, interconnection, or code compliance - those require a licensed solar installer or electrician.