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Solar Panel Calculator

Estimate the solar system size and number of panels you need to cover your electricity use, based on your monthly kWh, local sun hours and system losses.

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ItemValue

System size by sun hours

Peak sun hoursSystem sizePanels
A rough sizing estimate. The 14% system loss and 96% inverter efficiency defaults follow NREL’s PVWatts model. Roof angle, direction, shade and local weather change real output, so use NREL’s PVWatts tool or a solar installer’s site survey before you buy. Incentives and net metering rules are not included.

Quick Answer

Solar system size (kW) = yearly use (kWh) ÷ (peak sun hours × 365 × efficiency). A home using 900 kWh a month with 4.5 peak sun hours and about 82.6% overall efficiency needs about 7.96 kW, or 20 panels of 400 watts.

Last updated: 2 November 2026 ·How we calculate this: the solar sizing formula with PVWatts default losses

Table of Contents

How This Solar Panel Calculator Works

Enter your average monthly electricity use from your bills, the peak sun hours where you live, and your panel size. The calculator works out how many kilowatts of solar you need to cover the share of use you choose, after system losses and inverter efficiency. It rounds up to whole panels and shows the expected yearly production.

Quick explanation: more sun means fewer panels for the same energy.

  • Peak sun hours: the hours of full-strength sunlight per day, on average.
  • System losses: shading, dirt, wiring and other losses.
  • Panel size: many home panels are about 350 to 450 watts.

How to Use the Solar Panel Calculator

  1. Add up 12 months of electricity bills and divide by 12 to get your monthly kWh.
  2. Enter your local peak sun hours.
  3. Enter the panel size you plan to use.
  4. Choose how much of your use to cover, and keep or change the losses.
  5. Press Calculate and read the system size, panel count and production.

Core Formulas

  • Yearly use: monthly kWh × 12 × share to cover
  • Efficiency: (1 − system losses) × inverter efficiency
  • System size (kW): yearly use ÷ (sun hours × 365 × efficiency)
  • Panels: system size × 1,000 ÷ panel watts, rounded up
  • Yearly production: kW × sun hours × 365 × efficiency

Key Terms

  • Peak sun hours: the number of hours per day when sunlight averages 1,000 watts per square meter.
  • System losses: energy lost to shading, dirt, wiring, mismatch and other factors.
  • Inverter: a device that turns the panels’ DC power into AC power for your home.
  • Offset: the share of your electricity use covered by solar.
  • Net metering: a billing setup that credits you for extra solar power sent to the grid.

Worked Example

A home uses 900 kWh a month in an area with 4.5 peak sun hours.

  • Yearly use = 900 × 12 = 10,800 kWh
  • Efficiency = 0.86 × 0.96 = 0.8256
  • Size = 10,800 ÷ (4.5 × 365 × 0.8256) = 7.96 kW, or 20 panels of 400 W

Real-Life Examples

  • A sunny area: 900 kWh a month with 6 sun hours needs about 5.97 kW.
  • A cloudier area: the same use with 3.5 sun hours needs about 10.24 kW.
  • Partial offset: covering half of a 1,200 kWh bill with 5 sun hours needs about 4.78 kW.
  • Budgeting: at $3 per watt, the 900 kWh example costs about $24,000 before incentives.

kW vs kWh

  • kW (kilowatts): the size or power rating of the solar system.
  • kWh (kilowatt-hours): the energy it produces or you use over time.
  • Link: a 1 kW system running at full power for 1 hour makes 1 kWh.

What Changes Solar Output

  • Location: sunnier places get more peak sun hours.
  • Roof direction and tilt: in the US, south-facing roofs usually produce the most.
  • Shade: trees and buildings can cut output a lot.
  • Temperature: panels produce slightly less when very hot.

Common Mistakes to Avoid

  • Using one month’s bill. Use a 12-month average, since use changes by season.
  • Guessing sun hours. Use a local estimate, such as from NREL’s PVWatts tool.
  • Ignoring shade. Shade can make a big difference to output.
  • Forgetting roof space. Check that the panels will fit.
  • Skipping incentives and rules. Net metering and tax credits change the payback.

Frequently Asked Questions

Divide your yearly kWh use by (sun hours × 365 × efficiency) to get kW, then divide by panel size in kW. This calculator does it for you.

The average hours per day of full-strength sunlight. They vary a lot by location and season, so use a local estimate.

It is the default in NREL’s PVWatts model, which it says suits most typical systems.

A 400 W panel with 4.5 sun hours and typical losses makes roughly 1.5 kWh a day, or about 540 kWh a year.

It depends on use and sunshine. Enter your own numbers for the best estimate.

Not always. Net metering rules, roof space and budget may make a smaller system better.

No. Batteries store energy but do not change how many panels you need to cover your use.

Check the dimensions on the panel data sheet and multiply by the number of panels. Leave space for setbacks that fire codes may require.

Yes, but output drops. Peak sun hours already average this over the year.

Use NREL’s PVWatts calculator or ask a local installer.

Yes, slowly. Many panels lose a small amount each year, as listed in their warranty.

No. It is a rough estimate. Get quotes and a site survey from installers.

References & Sources

SourceUsed ForLink
NREL, PVWatts Version 5 Manual Default 14% system losses and how they are applied. https://pvwatts.nrel.gov/downloads/pvwattsv5.pdf
Wikipedia, Photovoltaic system How home solar systems work. https://en.wikipedia.org/wiki/Photovoltaic_system