Solar Panel Calculator
Daily and monthly solar energy from system size, sun hours, losses, and power rate.
What it does
Solar panel output depends on system size, how much sun hits the array, and real world losses from inverters, wiring, dirt, and shade. The tool above turns those inputs into daily and monthly kilowatt hours.
Enter your system size in kilowatts, your local peak sun hours per day, an expected loss percentage, and your electricity rate. The page shows daily energy, monthly energy, and what that energy is worth at your rate.
Peak sun hours are not daylight hours. They are the equivalent hours of full 1000 watts per square meter sun your site receives, and they vary by geography, season, and roof tilt.
Losses commonly run 10 to 20 percent for a well sited array. Higher losses mean less energy reaches your appliances or the grid, so enter a number that matches your equipment and shading.
How to use
- Enter the solar system size in kilowatts.
- Enter peak sun hours per day for your location.
- Enter expected losses as a percent (14 percent is a planning default).
- Enter your electricity rate in dollars per kilowatt hour.
- Read daily energy first, then monthly energy and monthly savings.
Formula
- system kW
- Solar array nameplate size in kilowatts.
- sun hours
- Equivalent full sun hours per day.
- loss percent
- Inverter, wiring, soiling, and shade losses.
- rate
- Your electricity price in dollars per kWh.
This is a planning estimate. Real output shifts with panel orientation, temperature, weather, shading, and degradation over years. Use a site survey and installer model before buying.
Worked example
Example: 6 kW system, 5 sun hours, 14 percent losses, $0.15 per kWh
System size 6 kW, peak sun hours 5, losses 14%, rate $0.15/kWh.
- Gross daily: 6 x 5 = 30 kWh.
- After losses: 30 x (1 - 0.14) = 25.8 kWh per day.
- Monthly: 25.8 x 30 = 774 kWh per month.
- Savings: 774 x $0.15 = $116.10 per month.
- Tool shows daily 25.8 kWh, monthly 774 kWh, savings $116.10/mo.
Reference table
| System size | Daily kWh | Monthly kWh |
|---|---|---|
| 4 kW | 17.2 kWh | 516 kWh |
| 6 kW | 25.8 kWh | 774 kWh |
| 8 kW | 34.4 kWh | 1,032 kWh |
| 10 kW | 43.0 kWh | 1,290 kWh |
Limits
- Output is a planning estimate, not a production guarantee.
- Real energy shifts with orientation, tilt, temperature, weather, and shading.
- Panel degradation lowers output a fraction of a percent each year.
- Savings assume you use or export every kWh at your full rate.
Common mistakes
- Confusing peak sun hours with total daylight hours.
- Leaving losses at zero, which overstates real production.
- Using nameplate size alone without derating for heat and wiring.
- Assuming all solar energy offsets your bill at the full rate.
- Sizing from summer sun and expecting winter to match.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 kW, 5 sun hours, 14% loss, daily kWh?
17.2 kWh per day (4 x 5 x 0.86).
6 kW, 5 sun hours, 14% loss, monthly kWh?
774 kWh per month.
8 kW, 5 sun hours, 14% loss, $0.12 rate, monthly savings?
1,032 x 0.12 = $123.84 per month.
Questions
What are peak sun hours?
Equivalent hours of full sun per day, not all daylight hours. A site with 5 peak sun hours gets 5 hours worth of 1000 W per square meter energy.
Why subtract losses?
Inverters, wires, dirt, and shade all take a cut before energy reaches your home or the grid.
Is monthly kWh simply daily times 30?
Yes, this tool uses daily times 30 for a planning month. Real months vary by days and sun.
Does this include battery storage?
No. This sizes daily and monthly production only; batteries need a separate calculation.
What loss percent should I use?
10 to 20 percent is typical for a well sited array. Shadier or hotter sites run higher.
Can I size my system from this alone?
Use it as a starting point. A site survey and installer model account for your exact roof and local rules.