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Solar Panel Calculator

Estimate a solar system’s size, upfront cost, expected bill savings and simple payback using your home’s electricity use.

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Result

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Verify before you act: review these results with a licensed contractor, engineer or the local building department before making any decision based on them.

What is a solar panel calculator?

A solar panel calculator estimates how much solar capacity your home may need, how many panels that could require, and how the estimated cost compares with potential electricity bill savings. This solar calculator uses your electricity use, peak sun hours, system losses, panel wattage, installed cost per watt and electricity rate.

The result is a planning estimate, not a system design or a guaranteed quote. Actual production and savings depend on your roof, shade, location, utility rules and how much of your solar electricity you use. The simple payback estimate also does not account for financing costs, incentives or changes in electricity prices.

Why a solar savings calculator matters

Solar is a major home purchase. An estimate can help you compare a proposed system’s size and price with your current electricity use before you request quotes or choose how to pay.

The calculation also helps you spot assumptions worth checking. A system sized from annual use may not match your home’s energy use in every season, and your utility may not credit exported electricity at the same rate you pay to buy it. Ask installers how their production estimate and utility compensation assumptions compare with this solar cost calculator’s simple estimate.

How to use this solar panel calculator

  1. Enter your monthly electricity use in kilowatt-hours. Use your bill or an annual total divided by 12 months. If your use changes seasonally, check several bills rather than relying on a single month.
  2. Enter peak sun hours per day using an estimate suited to your location and roof. This represents usable solar exposure, not simply the hours of daylight.
  3. Enter system losses as a percentage. Losses account for electricity the panels do not deliver because of factors such as heat, wiring and inverter conversion.
  4. Enter the panel wattage you are considering. A quote can help you use the proposed panel’s rating.
  5. Enter installed cost per watt and your electricity rate per kilowatt-hour. Check your bill for the rate you actually pay, and treat the installed price as an estimate until you have a written quote.
  6. Review system size, panel count, installed cost, yearly savings and simple payback. Change an input to see how a different roof estimate, panel choice or quote affects the result.

Solar calculator example

Worked example

With monthly use of 1,100 kWh, 5 peak sun hours per day, 14% system losses, 400 W panels, an installed cost of $2.85 per watt and an electricity rate of $0.15 per kWh, the estimate is an 8.41 kW system with 22 panels. Yearly use is 13,200 kWh. Estimated installed cost is $23,969, yearly bill savings are $1,980, and simple payback is about 12.1 years. The size follows yearly kWh divided by peak sun hours, 365 days and the share remaining after losses. Payback is cost divided by yearly savings.

How to read your solar calculator results

System size is the estimated generating capacity in kilowatts. Panel count translates that capacity into the number of panels at the wattage you entered; the calculator rounds to a whole panel.

Installed cost is the estimated upfront price based on system size and cost per watt. Yearly bill savings are estimated by multiplying yearly electricity use by your electricity rate. Simple payback divides the estimated cost by yearly savings. It does not include financing, maintenance, incentives or future changes in rates, so it is not the same as lifetime savings or a complete investment return.

How to improve your solar estimate

  1. 01

    Use your own electricity records

    Check bills across the year and account for planned changes, such as an electric vehicle or a heat pump. A more representative usage figure makes the size estimate more useful.

  2. 02

    Check roof conditions

    Ask an installer to assess roof direction, shade and usable area. Compare that assessment with the peak sun hours and system losses you entered.

  3. 03

    Compare written quotes

    Review each quote’s system size, panel wattage, installed price and production assumptions. Ask what is included in the price and how the installer estimated your savings.

  4. 04

    Check utility and tax assumptions

    Confirm how your utility credits exported power and whether the estimate accounts for your rate plan. The calculator does not include a tax credit or rebate. For tax-credit eligibility, confirm current IRS rules with a qualified tax professional.

Common solar calculator mistakes

  • Using a rate that does not match your bill

    A flat rate may not reflect time-based prices, fixed charges or different export credits. Ask the utility or installer how those details affect the savings estimate.

  • Treating payback as a promise

    Simple payback depends on the inputs and assumptions. It does not guarantee that your bill savings will stay level or that the system will produce the estimated amount.

  • Comparing quotes with different assumptions

    A lower price may come with a different system size or production estimate. Compare the equipment, expected output and pricing basis before deciding.

The solar panels formula

System size (kW) = yearly kWh / (sun hours x 365 x (1 - losses)). Payback = cost / yearly savings.

Key terms

Peak sun hours
A way to express the solar exposure available for estimating daily panel production.
System losses
The share of potential solar electricity not delivered for use because of factors such as heat, wiring and equipment conversion.
Simple payback
Estimated upfront cost divided by estimated yearly bill savings. It does not include financing or future changes in costs and savings.

Solar panels questions

Is there still a federal solar tax credit?

The 25D residential credit does not apply to systems placed in service after December 31, 2025.

Does the estimate include batteries or financing?

No. The calculator estimates system size, installed cost, bill savings and simple payback from the inputs shown. It does not model battery costs, loan payments or financing charges.

Why might my actual savings differ?

Your production may differ from the estimate because of roof conditions, shade, weather, equipment performance and utility rules. Your electricity use and rate can also change.

Should I use this estimate to choose a system?

Use it to prepare for quotes, not as a final design. Ask installers to assess your roof and provide production and savings assumptions you can compare.

Sources

Before you act on this result. Cordwell calculators and guides are educational estimates, not legal, tax, investment, lending or other professional advice. They rely on the numbers you enter and on simplified rules, and they can't see your full situation. Before you file, sign, borrow, invest, hire, change your withholding or form a business based on anything here, meet with a licensed contractor, engineer or the local building department who can review your specific facts and confirm the numbers. Rules change, and official agency guidance always controls.

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