Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
System size (kW)
8.68
Panels needed
22
Gross installed cost
$26,027
Net cost after incentives
$18,219
Annual electricity savings
$1,938
Payback period (years)
9.4
20-year net benefit
$20,541
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter monthly electricity use (kWh).
- 2Enter peak sun hours per day.
- 3Enter panel wattage (W).
- 4Enter installed cost per watt ($).
- 5Enter electricity rate ($/kWh).
- 6Enter tax credit and rebates (%).
- 7Read your system size (kw) on the right — it updates as you type.
- 8Hit Share to keep the scenario or send it to someone.
About this calculator
System size follows your annual usage divided by local sun hours and system losses. Payback depends far more on your utility rate than on panel efficiency — high-rate states pay back in six years, cheap-power states in fifteen.
Worked example
Using the values the calculator loads with:
Inputs
- Monthly electricity use: 950 kWh
- Peak sun hours per day: 4.5
- Panel wattage: 410 W
- Installed cost per watt: 3 $
- Electricity rate: 0.17 $/kWh
- Tax credit and rebates: 30 %
Results
- System size (kW): 8.68
- Panels needed: 22
- Gross installed cost: $26,027.40
- Net cost after incentives: $18,219.18
- Annual electricity savings: $1,938.00
- Payback period (years): 9.4
- 20-year net benefit: $20,540.82
What each field means
Inputs
- Monthly electricity use (kWh)
- The monthly electricity use used in the calculation, measured in kWh. Starts at 950 kWh so you have a working example on load.
- Peak sun hours per day
- The peak sun hours per day used in the calculation. Starts at 4.5 so you have a working example on load. Accepted range: 1–8.
- Panel wattage (W)
- The panel wattage used in the calculation, measured in W. Starts at 410 W so you have a working example on load.
- Installed cost per watt ($)
- The installed cost per watt used in the calculation, measured in $. Starts at 3 $ so you have a working example on load.
- Electricity rate ($/kWh)
- The electricity rate used in the calculation, measured in $/kWh. Starts at 0.17 $/kWh so you have a working example on load.
- Tax credit and rebates (%)
- The tax credit and rebates used in the calculation, measured in %. Starts at 30 % so you have a working example on load. Accepted range: 0–70 %.
Results
- System size (kW)
- Returned as a decimal number and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Panels needed
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Gross installed cost
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Net cost after incentives
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Annual electricity savings
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Payback period (years)
- Returned as a decimal number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- 20-year net benefit
- Returned as a money amount in US dollars. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
How does the solar panel calculator work?
System size follows your annual usage divided by local sun hours and system losses. Payback depends far more on your utility rate than on panel efficiency — high-rate states pay back in six years, cheap-power states in fifteen. The underlying maths is: kW needed = Annual kWh ÷ (Sun hours × 365 × Efficiency factor). Payback = Net cost ÷ Annual savings.
What do I need to enter?
6 values: monthly electricity use, peak sun hours per day, panel wattage, installed cost per watt, electricity rate, and tax credit and rebates. Each field starts with a sensible default, so you can change one number at a time and watch the result move.
What does the system size (kw) result mean?
It is returned as a decimal number and updates live as you edit the inputs, so you can compare two or three versions of a scenario in a few seconds.
Is this calculator free, and do I need an account?
Yes, it's free, and no account is required. Nothing you type is stored on our servers — the maths runs entirely in your browser.
How accurate is the result?
It applies the standard formula exactly, so the arithmetic is precise. Estimates assume standard, average conditions — local rules, pricing, and materials vary.
Who is this tool for?
It's built for anyone doing everyday planning maths — a quick estimate that would otherwise need four browser tabs and a calculator app.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Solar Panel Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/solar-panel
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/solar-panel" target="_blank" rel="noopener">Solar Panel Calculator — RevenueLab</a> (2026).</p>
Source: [Solar Panel Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/solar-panel) (2026).