Solar Panel Calculator

How to use this tool

Enter your average monthly electricity use in kWh, which is on your bill; the average of twelve months is best, because heating and cooling make some months much bigger than others. Pick the nearest city, or choose your own peak sun hours if you know them, and the panel size you are looking at. The panels needed, the system size and the output a year appear at the top.

Change the share of use to cover if you want a smaller system, and adjust the system efficiency if you know the roof is shaded or faces east or west. Add the installed cost per watt and your electricity rate to see what the output is worth and a simple payback. The chart shows each month’s output against your use, and the drawing lays the panels out in rows.

How the number of panels is worked out

Solar output depends on how much sunshine reaches the panels, measured in peak sun hours: the day’s sunlight expressed as hours of full sun at 1,000 watts per square meter. A panel rated 400 W makes 400 W in that full sun, so each kW of panels makes about one kWh per peak sun hour before losses. The inverter, wiring, heat, dirt and small amounts of shade take a share, and the page allows 80 percent for the system efficiency.

So each kW of panels makes the sun hours times 365 days times 0.8 each year, worked out month by month from each city’s monthly sunshine. Your yearly use divided by that is the kW needed, and the kW divided by the panel size, rounded up to whole panels, is the panel count.

Peak sun hours and output by city

NASA POWER 2001 to 2020 average daily sunshine on a south facing surface tilted 15 degrees below the latitude; output at 80 percent system efficiency; panels of 400 W.

A worked example you can check on paper

A home in Kansas City uses 900 kWh a month, 10,800 kWh a year. Kansas City averages 4.72 peak sun hours a day, so each kW of panels makes 4.72 times 365 times 0.8, about 1,379 kWh a year. The system needs 10,800 divided by 1,379, which is 7.83 kW, and with 400 W panels that is 7,830 divided by 400, 19.6, so 20 panels.

Twenty panels make 8 kW, about 11,000 kWh a year, 102 percent of the use. At about 21 sq ft each they cover 420 sq ft. At, say, $2.80 a watt installed the system costs $22,400, and if power costs $0.16 a kWh, the 10,800 kWh it replaces are worth about $1,730 a year, a simple payback of about 13 years before any incentives.

What changes the answer

Direction and pitch. A south facing roof tilted near the latitude catches the most sun. East and west faces make roughly 10 to 20 percent less, and a flat or very steep roof also loses some; lower the system efficiency to allow for it.

Shade. Trees, chimneys and neighboring buildings cost more than their size suggests, because a shaded cell holds back the rest of its string unless the panels have optimizers or microinverters.

How surplus is paid for. With full net metering, summer surplus credits winter bills, so sizing to 100 percent of use works. Where exported power earns less than you pay for power, a smaller system that matches daytime use can pay back faster.

Future use. An electric car or a heat pump can add thousands of kWh a year. Adding panels later costs more per watt, so include planned loads now if the roof has space.

Mistakes that catch people out

Using one month’s bill. Summer or winter alone can be half or double the average. Use twelve months.

Confusing kW and kWh. kW is the size of the system; kWh is the energy it makes. A quote in kW needs the local sunshine to turn into kWh.

Counting the whole roof. Setbacks, vents, skylights and shaded areas cut the usable space, often by a third or more.

Forgetting the roof’s age. Panels last 25 years or more; removing and refitting them to reroof later costs extra.

Popular calculations

Each opens the calculator with its own address, and every value can be changed once it is open.

By monthly use, Kansas City

1,000 kWh a month by city

Bigger homes in sunny places

Panel size and share of use

Canada

How this was checked

The engine was tested in Node against the hand calculation above: 4.72 sun hours give 1,379 kWh a year per kW and 20 panels for 900 kWh a month. It was then compared with NREL’s PVWatts, the standard US estimator, for ten cities with the same panel direction and tilt. With 80 percent system efficiency the page came within about 11 percent of PVWatts in every case, a little higher in the Southwest and a little lower around the Great Lakes, where the two sources’ sunshine data differ most.

Every address in the lists above was then opened and checked for a distinct title, the right answer, charts that render, and clean CSV, image and print output containing only that calculation, at desktop, tablet and phone widths.

Common questions

How many solar panels do I need for 1,000 kWh a month?

It depends on where you live. With 400 W panels and 80 percent system efficiency, about 17 in Phoenix or Los Angeles, 22 in Atlanta or Kansas City, 25 in Chicago and 27 in Seattle. The sunnier the place, the more each panel makes.

What are peak sun hours?

The day’s sunshine expressed as hours of full sun at 1,000 watts per square meter. A place with 5 peak sun hours receives 5 kWh of sunlight per square meter a day, however long the day actually is. Most of the lower 48 states get between about 4 and 6.5.

How much electricity does 1 kW of solar panels produce?

Roughly 900 to 1,900 kWh a year in the US and Canada, from about 885 in Anchorage to about 1,880 in Tucson at 80 percent system efficiency. The table on this page lists 73 cities.

How much roof space do solar panels need?

A 400 W panel is about 21 sq ft, so 20 panels cover roughly 420 sq ft. Fire codes keep panels back from ridges, hips and edges, so plan on more roof than the panel area alone, and on south, east or west facing slopes.

Should I size solar to cover all of my electricity use?

Only if your utility credits surplus power at a fair rate. With full net metering, covering around 100 percent usually makes sense; where exports earn less than you pay for power, a smaller system that matches daytime use can pay back faster.

Do bigger panels mean fewer panels?

Yes, the system size in kW stays the same and the panel count falls: 8 kW is 20 panels of 400 W or 16 of 500 W. Bigger panels are also physically larger, so the roof area changes little.

Related Calculators and Reference

• Electricity Cost Calculator: what each appliance adds to the bill.
• Energy Converter: kWh, BTU, therms and joules.
• Watts to Amps Calculator: current for a given load and voltage.
• Generator Size Calculator: backup power for an outage.
• Roof Squares Calculator: the area of a pitched roof.
• EV Charging Time and Cost Calculator: the kWh a car draws over a year.
• Solar Panel Tilt Angle Calculator: set the tilt that suits your latitude before sizing the array.
• NREL PVWatts Calculator: detailed solar output for any address.