Solar Panel Calculator
Size a solar system from your electricity use and sun hours: system size, number of panels, rough cost, yearly savings and payback years. Start with your monthly kWh or your bill; the defaults follow the PVWatts model, and every result is an estimate, not a quote.
Your numbers
On your bill. A 12-month average is best.
On your bill, in dollars. Used for savings and to turn a bill into kWh.
PVWatts shows it for your address as solar radiation (kWh/m²/day).
From the panel's spec sheet.
14% is the PVWatts default (dirt, shade, wiring). An inverter at 96% is applied on top.
100% aims to make as much as you use in a year.
EnergySage's US average is $2.60 before incentives.
Any rebate you qualify for, as a percent of the cost.
System size
- Panels
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- Production a year
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- Share of your use
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- Cost before rebates
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- Savings a year
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- Simple payback
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An estimate, not a quote: roof angle, direction, shade and local prices change the real numbers.
Savings assume each kWh the panels make replaces one you buy (full net metering). Where the utility pays less for exported power, savings are lower.
No federal tax credit is included: the residential credit ended for systems placed in service after 2025.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How the calculator sizes a solar system
Start with what you use in a year: kWh a month × 12, or your bill ÷ the price per kWh × 12. Then work out what 1 kW of panels makes where you live: peak sun hours × 365 days × what is left after losses. Divide the first by the second and you have the system size; divide that by the panel size and round up for the number of panels.
Example: 900 kWh a month is 10,800 kWh a year. At 4.5 peak sun hours, with 14% losses and a 96% efficient inverter, 1 kW makes 4.5 × 365 × 0.86 × 0.96 = 1,356 kWh a year. 10,800 ÷ 1,356 = 7.96 kW, which is 20 panels of 400 W, an 8 kW system that makes about 10,850 kWh a year.
Peak sun hours and system losses
Peak sun hours are not hours of daylight. They are the day's sunlight packed into hours of full sun at 1,000 watts per square meter, so a place that gets 4.5 kWh of sunlight per square meter a day has 4.5 peak sun hours. The PVWatts calculator from the National Laboratory of the Rockies (formerly NREL) shows the yearly average for your address as solar radiation in kWh/m²/day; that figure goes straight into the calculator.
System losses cover what a real system loses before the inverter: dirt, shade, snow, wiring, mismatched panels, panels that rate a little under their label, and downtime. PVWatts uses 14% as its default and models the inverter separately at 96% efficiency; this calculator does the same. Raise the losses for a shaded roof, and remember a roof facing east or west makes less than one facing south.
What solar costs, and the end of the federal tax credit
Installers price home systems per watt. EnergySage's national average is about $2.60 per watt before incentives (data updated September 2026), so an 8 kW system comes to about $20,800. Some states are cheaper and some dearer, and the panels, inverter, roof and installer all move the price. Type the cost per watt from a quote when you have one.
The 30% federal Residential Clean Energy Credit (Section 25D) no longer helps homeowners buying now: the IRS says it is not available for property placed in service after December 31, 2025. That is why the calculator has no credit built in. If a state, city or utility rebate applies to you, type it as a percent in the rebates field.
Savings and payback
The calculator values each kWh the panels make at your price per kWh, up to what you use in a year. That is right under full net metering, where the utility credits exported power at the retail rate. Many utilities now credit exported power at less than retail, and then savings depend on how much of the power you use as it is made. Check your utility's rules.
Simple payback is the cost after rebates divided by the yearly savings. In the example, $20,800 ÷ (10,800 kWh × $0.18) = $20,800 ÷ $1,944, about 10.7 years. The figure leaves out panel wear, maintenance, inverter replacement, financing and changes in power prices, which push it in both directions.
For solar reps: the first estimate
A quick size and payback from the last bill is a good way to open a conversation at the door or on the phone. The real proposal needs the roof: its direction, pitch, shade and condition, and the household's plans, such as an electric car or a heat pump, which change the size.
Write down what the homeowner told you on the visit: the monthly bill, the months it peaks, who else decides, and what worried them. Those notes are what the follow-up, and the second visit, are built on.
Every result on this page is an estimate from the numbers you type, not a quote or a site survey. An installer's proposal uses your roof's direction, pitch and shade and local prices.
Sources, checked September 30, 2026:
- NLR (formerly NREL) PVWatts Calculator: sun hours for your address, 14% default system losses
- PVWatts v8 documentation: losses and inverter efficiency (96%)
- EnergySage: solar panel cost, $2.60 per watt national average (September 2026)
- IRS: Residential Clean Energy Credit, not available for property placed in service after December 31, 2025
- EIA Short-Term Energy Outlook: US residential electricity prices
More for solar and home-services reps: Solar sales: keeping track of every household, Door-to-door sales, Roofing sales training.
The estimate starts the conversation; the kitchen table closes it
For solar reps, this number opens the visit. What decides the sale is what the household says about their bills, their roof and their plans. ParrotNotes records the home visit (with permission) and drafts the follow-up with the numbers they gave you.
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