Solar Panel Calculator
System size, panel count and payback period.
Enter your values
Enter the monthly electricity use in kWh.
Enter the peak sun hours per day in hrs.
Enter the panel rating in W.
Enter the installed cost per watt in $.
Enter the electricity rate in $/kWh.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- System size needed
- 8.21 kW
- Number of panels
- 21
- Estimated installed cost
- $22996
- Annual bill offset
- $1728
- Simple payback
- 13.3 years
Quick answer
System size in kW equals daily kWh use divided by peak sun hours divided by system efficiency, usually around 0.8.
kW = daily kWh ÷ sun hours ÷ 0.8
At a glance
| What it does | System size, panel count and payback period. |
|---|---|
| Category | Home & DIY |
| Inputs needed | Monthly electricity use, Peak sun hours per day, Panel rating, Installed cost per watt, Electricity rate |
| Main output | System size needed |
| Formula | kW = daily kWh ÷ sun hours ÷ 0.8 |
| Cost | Free — no sign-up, no download |
How to use the Solar Panel Calculator
- 1Take monthly kWh from your utility bill.
- 2Look up peak sun hours for your area.
- 3Read the system size and payback period.
Inputs explained
- Monthly electricity use(kWh)
- Enter the monthly electricity use in kWh.
- Peak sun hours per day(hrs)
- Enter the peak sun hours per day in hrs.
- Panel rating(W)
- Enter the panel rating in W.
- Installed cost per watt($)
- Enter the installed cost per watt in $.
- Electricity rate($/kWh)
- Enter the electricity rate in $/kWh.
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.
What do I need to enter into the Solar Panel Calculator?
Just 5 values: monthly electricity use, peak sun hours per day, panel rating, installed cost per watt, electricity rate. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Solar Panel Calculator work out the answer?
System size in kW equals daily kWh use divided by peak sun hours divided by system efficiency, usually around 0.8. It applies the formula kW = daily kWh ÷ sun hours ÷ 0.8 and shows the working so you can check each step by hand.
Is the Solar Panel Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.