How much solar power you get in each region of the United States

The same solar panel gives very different amounts of energy depending on where it sits and the time of year. This page shows what one 100 watt panel gives on an average day in each month, in the 7 regions our quiz uses, worked out from many years of hourly weather records for one city in each region.

The short version

• A 100 W panel's "100 W" is its output in ideal test light. Over a whole day it gives energy measured in watt-hours (Wh): how much electricity it collects, like water collected in a bucket.
• Across these regions, a 100 W panel gives roughly 120 to 580 Wh on an average day, depending on the place and the month.
• The darkest month matters most for year-round use: in the Northwest it gives about 120 Wh a day, in the Southwest about 400 Wh.
• Averages hide bad days: in every region, one day in ten in the darkest month gives much less than the average (the figures below show how much).
• That is why our quiz sizes systems by how often you would run short over many real years, not by an average.

How these numbers were worked out

The figures come from PVGIS, the European Commission's free solar calculator, which holds hourly sunshine and temperature records from satellites. For each region we took one city, the weather records for its area (about 5 km across) and every day of the years shown. We assumed one 100 W panel tilted 30 degrees and facing south, standing free so air cools it, and the PVGIS standard allowance of 14 % for losses in wiring, dirt and ageing. The result is the energy that reaches your charge controller (the box that charges the battery), before any battery losses. This is our own calculation from the PVGIS records, with the same method our sizing engine uses (see how we size systems).

A larger panel scales in proportion: a 400 W panel gives about four times these figures, if it is set up the same way. Panels lying flat, shaded, or on a vehicle roof give less, especially in winter.

Northwest (Seattle, WA)

• Best month: July, about 500 Wh a day.
• Darkest month: December, about 120 Wh a day; on one day in ten it gives less than 10 Wh.
• Over a year: about 116 kWh (kilowatt-hours, thousands of watt-hours).
• The best month gives about 4.1 times as much as the darkest one.
• Month by month, average Wh a day: Jan 130, Feb 230, Mar 300, Apr 410, May 450, Jun 450, Jul 500, Aug 460, Sep 370, Oct 240, Nov 160, Dec 120.
• Weather records: 2005-2015, PVGIS-NSRDB.

California (Sacramento, CA)

• Best month: July, about 570 Wh a day.
• Darkest month: December, about 260 Wh a day; on one day in ten it gives less than 50 Wh.
• Over a year: about 165 kWh (kilowatt-hours, thousands of watt-hours).
• The best month gives about 2.2 times as much as the darkest one.
• Month by month, average Wh a day: Jan 290, Feb 370, Mar 440, Apr 520, May 550, Jun 560, Jul 570, Aug 570, Sep 510, Oct 450, Nov 340, Dec 260.
• Weather records: 2005-2015, PVGIS-NSRDB.

Southwest (Phoenix, AZ)

• Best month: April, about 580 Wh a day.
• Darkest month: December, about 400 Wh a day; on one day in ten it gives less than 170 Wh.
• Over a year: about 182 kWh (kilowatt-hours, thousands of watt-hours).
• The best month gives about 1.5 times as much as the darkest one.
• Month by month, average Wh a day: Jan 430, Feb 480, Mar 540, Apr 580, May 570, Jun 540, Jul 490, Aug 490, Sep 490, Oct 500, Nov 450, Dec 400.
• Weather records: 2005-2015, PVGIS-NSRDB.

Mountain West (Denver, CO)

• Best month: June, about 530 Wh a day.
• Darkest month: December, about 330 Wh a day; on one day in ten it gives less than 110 Wh.
• Over a year: about 164 kWh (kilowatt-hours, thousands of watt-hours).
• The best month gives about 1.6 times as much as the darkest one.
• Month by month, average Wh a day: Jan 350, Feb 420, Mar 490, Apr 500, May 500, Jun 530, Jul 490, Aug 490, Sep 470, Oct 430, Nov 380, Dec 330.
• Weather records: 2005-2015, PVGIS-NSRDB.

Midwest (Chicago, IL)

• Best month: July, about 490 Wh a day.
• Darkest month: December, about 170 Wh a day; on one day in ten it gives less than 20 Wh.
• Over a year: about 133 kWh (kilowatt-hours, thousands of watt-hours).
• The best month gives about 2.9 times as much as the darkest one.
• Month by month, average Wh a day: Jan 210, Feb 280, Mar 380, Apr 440, May 460, Jun 480, Jul 490, Aug 460, Sep 400, Oct 330, Nov 260, Dec 170.
• Weather records: 2005-2015, PVGIS-NSRDB.

South (Atlanta, GA)

• Best month: April, about 480 Wh a day.
• Darkest month: December, about 300 Wh a day; on one day in ten it gives less than 50 Wh.
• Over a year: about 151 kWh (kilowatt-hours, thousands of watt-hours).
• The best month gives about 1.6 times as much as the darkest one.
• Month by month, average Wh a day: Jan 350, Feb 380, Mar 420, Apr 480, May 470, Jun 470, Jul 430, Aug 430, Sep 410, Oct 420, Nov 380, Dec 300.
• Weather records: 2005-2015, PVGIS-NSRDB.

Northeast (Boston, MA)

• Best month: July, about 460 Wh a day.
• Darkest month: December, about 220 Wh a day; on one day in ten it gives less than 30 Wh.
• Over a year: about 136 kWh (kilowatt-hours, thousands of watt-hours).
• The best month gives about 2.1 times as much as the darkest one.
• Month by month, average Wh a day: Jan 280, Feb 350, Mar 400, Apr 450, May 440, Jun 430, Jul 460, Aug 450, Sep 410, Oct 330, Nov 280, Dec 220.
• Weather records: 2005-2015, PVGIS-NSRDB.

What this means for your system

If you only use solar in summer, size for the summer months: the figures above show how much less panel power you need than for year-round use. If you need power all year, the darkest month decides the size, and the gap between summer and winter is large in the northern regions. Past a point, adding panels for the darkest weeks costs more than it gives back, and a backup source (a generator or a grid connection) for those weeks can be the sensible choice.

Your own place may differ from the city shown for your region: hills, coast, altitude and local shade all change the figures. Our quiz uses the weather records for your own area, not the regional city.

What can go wrong

• Using the yearly average to size a system for winter: the battery runs flat in the darkest weeks.
• Forgetting shade: a tree or a wall that shades the panel in winter, when the sun is low, can cut the output far below these figures.
• Cold and heat: panels give a little more in cold sunshine and less when hot; the figures include that, but a hot roof in the Southwest can still be warmer than assumed.
• Snow: a panel covered by snow gives almost nothing until it slides off; our quiz adds a snow allowance for your area.

Size a system for your place

Tell the quiz where and when you will use solar, and it works out panels and battery from the weather records for your own area, then checks that the parts work together. Terms used here are explained in the glossary.

Our results and guides are estimates and do not replace advice from a qualified electrician.