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Solar Panel Calculator for Your Home

By Nick Major · Founder, Calculator Campus Last updated June 5, 2026

A solar panel calculator turns electricity use into a first-pass home solar size. Enter your energy use, local peak sun hours, panel wattage, and loss factor, and the tool returns required system size, whole-panel count, and roof footprint. Detail rows add simple savings, installed cost, and payback using editable assumptions. The result is transparent planning math for a grid-tied home array, not a permit-ready design or a utility bill guarantee.

Primary output 7.43 kW · 19 panels

Enter your yearly electricity use, average peak sun hours, and panel wattage to size a basic home solar array. Open Advanced for monthly bills, loss/cost assumptions, or an existing-system generation estimate.

What are you starting with?

Start with annual electricity use to size a new system, or switch to estimate output from an existing system.

Enter your electricity use for the selected period. A recent 12-month utility total is best.
Average daily peak sun hours for your location. Many U.S. homes fall between about 3.5 and 5.5 hours/day.
Nameplate DC wattage of one panel.
Advanced options
The calculator normalizes daily or monthly entries to a yearly kWh total before sizing the array.
Use the DC nameplate size of an array you already have or are evaluating.
The share of your annual electricity use you want solar to cover.
Combined real-world losses. NREL PVWatts commonly uses 14% as a default loss setting.
Physical footprint of one panel, used to estimate total roof area.
Your blended retail electricity rate. The 2024 U.S. residential average was 16.48 cents/kWh.
All-in installed price before incentives. Used for the cost and simple-payback outputs.
Answer 7.43 kW · 19 panels
System size 7.43 kW
Number of panels 19 panels
Required roof area 37.05 m² (399 ft²)
Estimated annual generation 10,500 kWh/yr
Show calculation details
Estimated annual savings $1,730.40/yr
Estimated installed cost $22,300.10
Simple payback period 12.9 years

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Reading your solar sizing result

Use the result as a planning range, not an installer drawing. The system size and panel count are useful only if the energy, sun, wattage, and loss inputs describe your actual house.

Annual kWh

A monthly bill entered as annual usage makes the system about one-twelfth of the needed size. Use a full year or select the correct period.

Peak sun

Peak sun hours are full-strength-sun equivalents, not total daylight hours.

Panel watts

Use the panel nameplate wattage from a spec sheet. Higher-watt panels reduce count, not the total kW target.

Loss factor

Leave 14% unless you have a better site-specific estimate. Shading, heat, and non-ideal orientation can justify more loss.

Example: 10,500 kWh per year with 400 W panels

A home uses 10,500 kWh per year, gets 4.5 peak sun hours per day, wants a 100% bill offset, and uses 400 W panels with the default 14% loss factor. Effective yearly sun is 4.5 x 365 x 0.86 = 1,412.55 hours. Required system size is 10,500 / 1,412.55 = 7.43 kW. Panel count is 7,433 / 400 = 18.58, so the calculator rounds up to 19 panels. With the 1.95 m2 panel-area default, that is about 37.05 m2 of panel footprint before setbacks, walkways, vents, or row spacing. At a $0.1648/kWh rate (the EIA 2024 U.S. average) and $3/W installed price (within NREL's 2024 residential band), the same inputs imply about $1,730 in first-year offset value and about $22,300 installed before any incentives or financing effects.

Annual kWh is the foundation

The safest input is a full year of electricity use from bills or your utility portal. A year smooths out air-conditioning, heating, vacations, and seasonal daylight. Monthly and daily entries can work, but only when the period selector matches the number you typed.

Peak sun hours are not daylight hours

Peak sun hours measure solar intensity, not clock time. A long summer day can still average far fewer full-strength sun hours than daylight hours. Because system size divides by this number, overstating peak sun quietly undersizes the array.

Panel count and roof footprint are different questions

The calculator rounds panel count up because partial panels do not exist. The roof area output is only module footprint: panel count times area per panel. It does not include fire-code setbacks, access paths, row spacing, roof pitch, vents, chimneys, shade zones, or whether the roof plane can physically fit the layout.

Savings, cost, and payback are simple planning math

The detail view assumes each generated kWh offsets one kWh at the rate you enter, and cost equals system watts times the price per watt you enter. That is useful for comparing rough quotes, but it does not model net metering, time-of-use rates, fixed charges, financing, incentives, tax credits, degradation, or utility-specific bill rules.

When to take the estimate further

Once the rough size makes sense, run a deeper model or get a site-specific design. Tools such as NREL PVWatts can model tilt, azimuth, weather, and location more closely, and an installer still needs to check roof condition, shading, electrical constraints, setbacks, and the actual panel layout.

References

  1. PVWatts Version 5 Manual (NREL/TP-6A20-62641)

    National Renewable Energy Laboratory (NREL) · accessed 2026-07-03

  2. U.S. Solar Photovoltaic System Cost Benchmarks (Q1 2024)

    U.S. Department of Energy / NREL · accessed 2026-07-03

  3. Electric Sales, Revenue, and Average Price

    U.S. Energy Information Administration (EIA) · accessed 2026-07-03

  4. National Solar Radiation Database (NSRDB) and Solar Resource Data

    National Renewable Energy Laboratory (NREL) · accessed 2026-07-03

  5. How much electricity does an American home use?

    U.S. Energy Information Administration (EIA) · accessed 2026-07-03

Frequently asked questions

What size solar panel system do I need for my home?
A first-pass size comes from annual kWh, peak sun hours, and loss-adjusted output. Divide yearly kWh by peak sun hours per day times 365 times (1 - loss factor), then divide the resulting kW by panel wattage for a panel count. This calculator runs those steps and rounds up to whole panels.
How do I use a solar panel calculator for my home?
Start with your electricity use, choose the right usage period, enter local peak sun hours, and use the wattage of the panels you are considering. The calculator returns system size, panel count, roof footprint, and optional savings and cost details.
What does a solar size calculator estimate?
It estimates system size in kW and the number of panels needed for a target energy offset. It can also estimate annual generation from a known system size. The result is a planning estimate, not a permit layout or a guarantee of your utility bill outcome.
How do I estimate my solar system size from my electricity use?
Use a full year of kWh when possible. Divide annual use by loss-adjusted yearly peak sun hours to get required system kW, then divide by panel wattage and round up for whole panels. If you only know monthly use, select monthly or multiply by 12 first.
What panel wattage should I use in a solar panel calculator?
Use the rated wattage from the panel spec sheet, such as 400 W. Higher-watt panels reduce the number of modules needed for the same system size, but they do not change the kW target set by your energy use and sun hours.

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