System sizing

Solar panel calculator

Enter your monthly electricity usage, the panel wattage you're considering, and your local peak sun hours. Get a panel count and system size in seconds.

Your numbers

1 Project details Optional
Shown on your exported report. Leave blank to skip.
2 Your usage
Find this on your utility bill, usually labeled kWh used.
National average is roughly 4 to 5.5. Check NREL's PVWatts for your exact address.
3 Your system
Most residential panels sold today are 350 to 440 watts.
Accounts for inverter loss, wiring loss, dust, and temperature derate combined.

How it works

Three steps to a panel count

Step 1

Find your monthly usage

Pull your kWh usage from a recent utility bill. If your usage swings a lot by season, use your annual usage tool instead so summer and winter both get counted fairly.

Step 2

Set your panel wattage and sun hours

Higher-wattage panels need fewer of them for the same output. If you don't know your local peak sun hours, the PVWatts tool gives an exact number for your address in a few seconds.

Step 3

Check the result against your roof

Once you have a panel count, run the usable roof area calculator to confirm your roof actually has room, especially if you have a complex roof shape or heavy shading.

Understanding your panel count

Why this number is an estimate, not a quote

This calculator works from three inputs: how much electricity you use, how much power each panel produces, and how much sun your location gets. Real installations also depend on roof shading, panel placement, and inverter losses that vary site to site. Use this number to compare against an installer's quote, not as a replacement for one. If you want a second view on the same numbers, the monthly usage tool and square footage estimator approach the same question from different angles. The Department of Energy's homeowner's guide to solar explains what a real quote should include, so you can tell an estimate from a firm price.

How peak sun hours change the answer

Peak sun hours measure the sun's actual intensity, not how many hours it's light outside. A cloudy northern location might get 15 hours of daylight in summer but only 3.5 peak sun hours, while a desert location gets fewer daylight hours but higher intensity throughout. This is the single biggest factor separating a 15-panel system from a 25-panel system for the same usage. If you're near a coastline or in a region with frequent overcast, the cloud cover output reduction tool models that effect specifically.

Why panel wattage matters more than most people expect

A 400-watt panel and a 350-watt panel are often close to the same physical size, but the higher-wattage panel produces meaningfully more energy per square foot of roof. If your roof space is tight, paying more for higher-wattage panels can mean the difference between fitting your full system and needing a smaller one. Panel choice also interacts with inverter sizing, which is worth checking before you finalize the equipment list.

Sizing for what's coming, not just what's here now

If an EV or a heat pump is likely within the next few years, building slightly larger now is usually cheaper than adding a second array later, since a second installation means a second round of labor, permitting, and interconnection paperwork. The EV oversizing calculator and battery oversizing calculator both start from this same panel count and adjust for planned future load.

What to do with this number next

Take your panel count to the payback period calculator to see what a system this size might save you over time, and check what incentive programs apply to a system of this size before you get quotes.

Questions

Frequently asked

Most US homes need between 15 and 25 panels, depending on electricity usage, panel wattage, and local peak sun hours. A home using 900 kWh a month with 400-watt panels in a location with 5 peak sun hours a day needs around 19 panels.

Peak sun hours measure how many hours of full-intensity sunlight your location gets on an average day, not daylight hours. Sunny states like Arizona average around 6.5, while cloudier northern regions average closer to 3.5 to 4. NREL's PVWatts tool gives an exact figure for any US address.

Yes. A 400-watt panel produces roughly 33% more energy than a 300-watt panel of the same size, so a system built with higher-wattage panels needs fewer of them to hit the same total output.

If you're planning to add an EV or a heat pump within the next few years, it's usually cheaper to size the system slightly larger now than to install a second array later. Add 20 to 30% to your current usage figure if a major electric appliance is on the horizon.

Related tools

Keep sizing your system

Roof space vs output estimator System size by monthly kWh usage Usable roof area calculator kW to panel count converter Oversizing for a future EV Panel spacing and setback calculator Array tilt angle optimizer All system sizing tools