Solar Panel Calculator
System size, panel count and payback period.
Enter your values
ⓘ Live results
Results
- System size needed
- 8.21 kW
- Number of panels
- 21
- Estimated installed cost
- $22996
- Annual bill offset
- $1728
- Simple payback
- 13.3 years
Quick answer
System size in kW equals daily kWh use divided by peak sun hours divided by system efficiency, usually around 0.8.
kW = daily kWh ÷ sun hours ÷ 0.8
How to use the Solar Panel Calculator
- 1Take monthly kWh from your utility bill.
- 2Look up peak sun hours for your area.
- 3Read the system size and payback period.
Worked example
Using the values the calculator loads with:
Inputs
- Monthly electricity use900 kWh
- Peak sun hours per day4.5 hrs
- Panel rating400 W
- Installed cost per watt2.80 $
- Electricity rate0.16 $/kWh
Results
- System size needed8.21 kW
- Number of panels21
- Estimated installed cost$22996
- Annual bill offset$1728
- Simple payback13.3 years
Frequently asked questions
What are peak sun hours?
The equivalent hours of full 1,000 W/m² sunlight per day — typically 3.5 in the northern US and 5.5–6 in the southwest.
Does payback include incentives?
No. The 30% US federal tax credit and state rebates typically cut the payback period by around a third.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.