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Solar Panel Calculator for Homes

By Nick Major · Founder, Calculator Campus Last updated May 18, 2026

This solar panel calculator turns your energy use and local sun hours into system size, panel count, roof area, annual generation, annual savings, installed cost, and simple payback period. You can also switch modes to enter an existing system size and estimate yearly output. Enter your power bill data and a few panel specs — the calculator handles the sizing, savings, and payback math.

Calculation mode

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.
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.
Average daily peak sun hours for your location. Many U.S. homes fall between about 3.5 and 5.5 hours/day.
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.
Nameplate DC wattage of one panel.
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
Estimated annual savings $1,730.40/yr
Estimated installed cost $22,300.10
Simple payback period 12.9 years

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How to check the math

Required system size

Divide the share of annual electricity consumption to be offset by the effective peak sun hours per year, adjusting for system losses, to find required system size.

Required System kW = (Annual Electricity kWh × (Bill Offset Percentage / 100)) / (Solar Hours Per Day × 365 × (1 − Loss Factor Percent / 100))
Panels needed

Convert required system size to watts, divide by each panel's power rating, then round up to the next whole number of panels.

Panels Needed = ceil((Required System kW × 1000) / Panel Power Watts)
Required roof area

Multiply the number of panels by the footprint of a single panel to find the total roof area needed.

Required Roof Area = Panels Needed × Panel Area
Annual generation

Multiply system size by daily peak sun hours and 365 days, then reduce by the loss factor to find annual energy output.

Annual Generation kWh = System kW × Solar Hours Per Day × 365 × (1 − Loss Factor Percent / 100)
Annual savings

Multiply annual energy generated by the retail electricity rate to find yearly dollar savings.

Annual Savings = Annual Generation kWh × Electricity Rate Per kWh
System cost

Multiply required system size in watts by the installed cost per watt to find total system cost before incentives.

System Cost = Required System kW × 1000 × Cost Per Watt
Simple payback period

Divide total installed cost by first-year savings to find how many years until the system pays for itself.

Payback Years = System Cost / (Required System kW × Solar Hours Per Day × 365 × (1 − Loss Factor Percent / 100) × Electricity Rate Per kWh)
Methodology

How the answer is computed

The calculator first normalizes your electricity use to annual kWh. It applies your target bill offset, then divides by effective annual production per kW using peak sun hours and the loss factor. That kW figure sets the panel count from panel wattage and roof area from panel footprint. The same production estimate feeds annual savings; installed cost divided by first-year savings gives the simple payback period.

Worked examples

See the math step by step

Phoenix household sizing a full-offset solar system

The Garza family in Phoenix runs their home on about 12,000 kWh of electricity each year. They want a solar system that covers their full bill — 100% offset. Their roof gets 6.5 peak sun hours per day. They factor in a 14% loss from heat, wiring, and inefficiency.

The required system size works out to 12,000 ÷ (6.5 × 365 × 0.86) = 5.88 kW. With 400-watt panels, they need ceil(5,880 ÷ 400) = 15 panels. Each panel covers 1.95 m² of roof, so 15 × 1.95 = 29.25 m² of roof space is needed.

A 5.88 kW system in Phoenix produces 5.88 × 6.5 × 365 × 0.86 ≈ 12,000 kWh per year. At $0.15 per kWh, that cuts the electric bill by 12,000 × $0.15 = $1,800 per year.

At $3.00 per watt installed, the total upfront cost is 5,880 × $3.00 = $17,640.

Colorado cabin sizing a 75% offset solar system

The Hendersons own a cabin outside Durango, Colorado. Their cabin uses 6,000 kWh per year, and they want solar to cover 75% of that use. Colorado averages 5.8 peak sun hours per day, with a 16% loss from temperature swings and wiring resistance.

To offset 4,500 kWh (6,000 × 0.75), they need 4,500 ÷ (5.8 × 365 × 0.84) = 2.53 kW of capacity. At 320 watts per panel, they need ceil(2,530 ÷ 320) = 8 panels. Eight panels at 1.65 m² each take up 8 × 1.65 = 13.20 m² of roof.

The system generates 2.53 × 5.8 × 365 × 0.84 = 4,500 kWh per year. At $0.11 per kWh, annual savings come to 4,500 × $0.11 = $495.

At $2.80 per watt installed, the upfront cost is 2,530 × $2.80 = $7,084.

When to use this calculator

Use this tool right after you get a power bill that surprises you. That's the best moment to see how much solar could cut your yearly costs. It also fits the early planning stage, when you want a rough system size before asking for installer quotes. Once you have installer quotes, switch to a tool that factors in tax credits and net metering rules.

How Peak Sun Hours Affect Your System Size

Peak sun hours measure the daily average solar energy reaching your roof. A home in Phoenix might see 6 per day; a home in Seattle might see only 3.5. More peak sun hours mean you need fewer panels to hit your energy target. Entering the right figure for your location keeps system size and cost estimates on track.

What the Loss Factor Means in Practice

No solar system turns all incoming sunlight into usable power. Heat, wiring resistance, inverter conversion, and dust all trim output below its peak. The loss factor captures these real-world drops — most home systems run at about 80 percent efficiency. A higher loss figure means the calculator sizes the array up to compensate.

Assumptions

What we assume

  • The formula uses a flat electricity rate all year, with no change for time-of-use pricing.
  • The result treats daily peak-sun hours as constant across every month of the year.
  • The calculator treats all inputs as rated values measured at standard test conditions.
  • The formula applies the loss factor uniformly across the whole system rather than modeling each component separately.
Limitations

What this skips

  • Does not account for shading from trees, chimneys, or buildings near the roof.
  • Excludes roof tilt angle and compass direction, both of which shift real energy output.
  • Ignores net metering rules, which vary by utility and can cut the payback period by years.
  • Does not include battery storage costs or the value of energy stored for nighttime use.
  • Excludes permit fees, inspection costs, and installer labor rates from the cost estimate.
  • Ignores panel output decline over time, which averages about 0.5 percent each year.
Common mistakes

What people miss

  • You enter one month of electric bills instead of a 12-month average, which skews the panel count.
  • Mixing up system size in kilowatts with individual panel wattage leads to a far-off panel count.
  • You choose peak-sun hours for a sunny climate when your area gets fewer clear days each year.
  • Forgetting to adjust the loss factor leaves you short on panels. Older wiring or a hot roof pushes losses above the default.
  • You treat the panel's rated watts as actual daily output, but real performance runs 10 to 20 percent lower.
References

References

  1. CDFS 4102 — ohioline.osu.edu

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  2. Sizing solar systems — gogreensolar.com

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  3. Solar calculator — sunwatts.com

    sunwatts.com · accessed 2026-05-18

  4. Articles2 12j en — leonics.com

    leonics.com · accessed 2026-05-18

  5. Ultimate guide to solar panel calculation — jackery.com

    jackery.com · accessed 2026-05-18

  6. Solar panel calculation — ankersolix.com

    ankersolix.com · accessed 2026-05-18

  7. How to size solar system — unboundsolar.com

    unboundsolar.com · accessed 2026-05-18

  8. Solar power calculation formula — bluettipower.com

    bluettipower.com · accessed 2026-05-18

  9. How to calculate solar panel and battery requirements — solarunoffgrid.com

    solarunoffgrid.com · accessed 2026-05-18

  10. Solar panel calculator — energy.ecoflow.com

    energy.ecoflow.com · accessed 2026-05-18

  11. San diego solar calculator — solartechonline.com

    solartechonline.com · accessed 2026-05-18

  12. Solar calculator — 8msolar.com

    8msolar.com · accessed 2026-05-18

  13. Calculator — energysage.com

    energysage.com · accessed 2026-05-18

  14. Balancing battery bank and solar array size — morganscloud.com

    morganscloud.com · accessed 2026-05-18

  15. Solar panel square footage calculator — shopsolarkits.com

    shopsolarkits.com · accessed 2026-05-18

  16. PVWatts Calculator — pvwatts.nrel.gov

    pvwatts.nrel.gov · accessed 2026-05-18

  17. Solar rooftop potential — energy.gov

    energy.gov · accessed 2026-05-18

Frequently asked questions

How many solar panels do I need for my home?
Divide your required system size in watts by the wattage of one panel to find the count. Choosing a higher-wattage panel — say 450 watts instead of 400 watts — means fewer panels reach the same output. Fewer panels also means less roof space, which matters on smaller or shaded rooftops.
How do I size a solar system for my electricity use?
Start with your yearly kilowatt-hour use from your utility bill, then divide by your local peak sun hours to get a raw system size. A 14 percent loss factor accounts for heat, shading, wiring, and inverter drops on a south-facing roof. That adjustment brings your paper number much closer to what the system will actually produce.
Do I need battery storage with my solar system?
Adding a battery lets you store daytime solar power and use it at night, but it does not remove your utility's fixed service fees. Even with full solar and battery coverage, most utilities keep a monthly connection charge on your bill. Going solar cuts your energy charges sharply, but a zero electricity bill is rare for grid-tied homes.
How much electricity can my solar panels produce?
Your panels produce power based on how many peak sun hours your location gets each day, and that number varies widely by region and season. The National Renewable Energy Laboratory publishes free solar resource maps by ZIP code at pvwatts.nrel.gov — look up your local peak sun hours there, then plug that number into this calculator for a more accurate sizing result.
Will a solar panel calculator work for my home?
A solar calculator gives you an estimated system cost, not a firm quote. The estimate uses average prices and typical installer margins, which vary by region and equipment choice. Use the output to set your planning budget, then collect multiple bids from local installers before you commit.

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