How to use a Solar Panel Calculator
Start with the usage figure on a recent bill or annual summary. The calculator converts that figure to annual and daily use, then applies your offset target, sun hours, panel wattage, and loss percentage. It keeps the entered assumptions visible beside the result.
- Select Annual (kWh/year) or Average monthly (kWh/month) under Usage entered as. Enter Electricity use and Target usage offset (%).
- Select a value under Peak sun hour preset, or enter Peak sun hours per day yourself. Add Panel wattage and System losses / derate (%).
- Enter Area per panel. Add Usable roof area available (optional) when you want the result to check whether the panels fit.
- Enter Usable battery capacity, or leave it blank to treat the battery capacity as zero.
- Enter Installed price per watt, Incentives / rebates (%), and Electricity price you pay.
- Select Calculate to generate the result. Select Reset to clear the fields and hide the previous result.
How to calculate solar panel count and system size
Average monthly use is multiplied by 12. The calculator divides annual use by 365 for average daily use, applies Target usage offset (%), and divides by Peak sun hours per day after system losses. It rounds the required panel count up to a whole panel, then calculates the actual DC size from that count.
10,000 kWh/year, 100% offset
4.5 peak sun hours/day, 400 W panels, 17% losses
Result: 19 panels, 7.60 kW DC systemRoof area needed is panel count multiplied by Area per panel. The calculator converts ft² and m² to compare the values in one unit.
What changes the solar system size estimate
Peak sun hour preset fills Peak sun hours per day with 3.5, 4.5, 5.5, or 6.5 hours. You can edit the filled value, which changes the preset to Custom. System losses / derate (%) lowers the production estimate by the percentage entered. Target usage offset (%) can size a partial system or a target above 100%.
Usable roof area available (optional) is skipped when it is blank or zero. A positive value triggers the fit check. Missing, nonnumeric, or out-of-range values stop the calculation and show an error for the related field.
How solar panel payback and battery coverage are estimated
Battery coverage compares Usable battery capacity with average daily use and caps the result at 100%. Gross system cost uses the actual system watts and Installed price per watt. Incentives / rebates (%) reduces that cost. Annual savings use Electricity price you pay and are capped at annual electricity use.
The production, savings, and simple payback ranges vary Peak sun hours per day by 10% below and above your entry. System losses stay unchanged. The calculation does not measure shading, roof orientation, weather, equipment performance, or degradation.
Where this comes up
- Enter annual use from a utility bill, try a partial Target usage offset (%), and compare the panel count with available roof space.
- Change Panel wattage, System losses / derate (%), or Installed price per watt to see how each assumption changes the estimate.
- Enter a battery capacity to see what share of average daily use it can cover.
When to use another construction calculator
Use Construction Calculators for material quantities, project dimensions, loads, or water volume. Choose Heating and Cooling Load Calculator for heating or cooling load in BTU/hr and watts. Use Roof Pitch Calculator for roof pitch, angle, and rafter length. For material quantities, use Construction Materials Calculator.
Frequently asked questions
How many solar panels do I need for my electricity use?
The calculator divides target daily production by Peak sun hours per day after System losses / derate (%), divides that size by Panel wattage, and rounds up to a whole panel.
How do I estimate solar system size in kW?
It uses average daily electricity use, Target usage offset (%), Peak sun hours per day, and System losses / derate (%). Actual kW uses the whole-panel count after rounding.
How is solar panel payback calculated?
Simple payback divides net system cost by estimated annual savings. Installed price per watt and Incentives / rebates (%) set net cost. Electricity price you pay and the capped production estimate set annual savings.
Does the Solar Panel Calculator check roof area and battery coverage?
Yes. It multiplies panel count by Area per panel, compares that result with a positive Usable roof area available (optional), and compares Usable battery capacity with average daily use.
Does this solar panel calculator design wiring or handle permitting?
No. It estimates panel count, system size, roof area, production, battery coverage, cost, savings, and payback. It does not design wiring or address permitting.