Calculate Solar Array Size and Cost

(Updated: April 27, 2015)

What size solar system do I need for my home? Though this is a complicated question, we have simplified it for you below. Follow the 4 steps to estimate your solar power system needs. Then calculate how much it might cost to install the solar system!

Step 1: Yearly Electricity (kWh) Usage

Look at your electricity bills and average out your usage to find your annual electricity usage. Look for ‘kWh’.

  • kWh = Kilowatt Hours; a measure of electricity
  • Your bill may tell you your kWh/month. If your bill is ~$100/month, you might use ~1,000 kWh/month. If your bill is ~$200, maybe closer to 1,800 kWh/month.

Step 2: Average Sun Hours at your house

*Updated: Choose the nearest city in the dropdown box in the calculator below. It will automatically input the average sun hours (solar irradiation) at that location. This number is important to calculate the expected production of solar electricity at your place.

State City Avg State City Avg
AK Fairbanks 3.99 MO Columbia 4.73
AK Matanuska 3.55 MO St. Louis 4.38
AL Montgomery 4.23 MS Meridian 4.43
AR Bethel 3.81 MT Glasgow 5.15
AR Little Rock 4.69 MT Great Falls 4.93
AZ Tuscon 6.57 MT Summit 3.99
AZ Page 6.36 NM Albuquerque 6.77
AZ Pheonix 6.58 NB Lincoln 4.79
CA Santa Maria 5.94 NB N. Omaha 4.90
CA Riverside 5.87 NC Cape Hatteras 5.31
CA Davis 5.10 NC Greensboro 4.71
CA Fresno 5.38 ND Bismark 5.01
CA Los Angeles 5.62 NJ Sea Brook 4.21
CA Soda Springs 5.60 NV Las Vegas 6.41
CA La Jolla 4.77 NV Ely 5.98
CA Inyokern 7.66 NY Binghampton 3.16
CO Grandby 5.69 NY Ithica 3.79
CO Grand Lake 5.08 NY Schenetady 3.55
CO Grand Junction 5.85 NY Rochester 3.31
CO Boulder 4.87 NY New York City 4.08
DC Washington 4.23 OH Columbus 4.15
FL Apalachicola 5.49 OH Cleveland 3.94
FL Belie Is. 4.99 OK Stillwater 4.99
FL Miami 5.62 OK Oklahoma City 5.59
FL Gainsville 5.27 OR Astoria 3.72
FL Tampa 5.67 OR Corvallis 4.03
GA Atlanta 4.74 OR Medford 4.51
GA Griffin 4.99 PA Pittsburg 3.28
HI Honolulu 6.02 PA State College 3.91
IA Ames 4.40 RI Newport 4.23
ID Boise 4.92 SC Charleston 5.06
ID Twin Falls 4.70 SD Rapid City 5.23
IL Chicago 3.14 TN Nashville 4.45
IN Indianapolis 4.21 TN Oak Ridge 4.37
KN Manhattan 4.57 TX San Antonio 5.30
KN Dodge City 5.79 TX Brownsville 4.92
KY Lexington 4.94 TX El Paso 6.72
LA Lake Charles 4.93 TX Midland 5.83
LA New Orleans 4.92 TX Fort Worth 5.43
LA Shreveport 4.63 UT Salt Lake City 5.26
MA E. Wareham 3.99 UT Flaming Gorge 5.83
MA Boston 3.84 VA Richmond 4.13
MA Blue Hill 4.05 WA Seattle 3.57
MA Natick 4.10 WA Richland 4.44
MA Lynn 3.79 WA Pullman 4.73
MD Silver Hill 4.47 WA Spokane 4.48
ME Caribou 4.19 WA Prosser 5.03
ME Portland 4.51 WI Madison 4.29
MI Sault Ste. Marie 4.20 WV Charleston 3.65
MI E. Lansing 4.00 WY Lander 6.06
MN St. Cloud 4.53

Click here to close the above table


Step 3: Input the numbers

Input the numbers in the calculator below. For those calculating by hand, the formula is:

Array Size (kW) = (Annual kWh usage) / (365 days/year) / (Solar Hours/day) / (0.75 derate factor)
  • Example: My electricity bill is ~$110/mo and I use about 900kWh/month. I live in SoCal near LA, so I get about 5.62 solar hours/day.
  • (10,800 kWh/year) / (365 days/year) / (5.62 kWh/day) / (0.75) = 7 kW Array
  • Due to real world efficiency losses (irradiance, dust, temperature, and wiring), you should expect your system power output (AC power) to be about 75% of the system (DC power) size. This is the 0.75 efficiency constant included.

Step 4: Calculate solar array size!

Disclaimer: Material presented on the SRoeCo Solar website is intended for information purposes only. It is not intended as professional advice and should not be construed as such. Any use of the information and calculators on SRoeCo Solar for any purposes other than for a general overview or estimated results is not endorsed or encouraged.

10 comments on “Calculate Solar Array Size and Cost

  1. About every two years, I do a cost calculation on solar panels to see if they have come down in price enough. The payback needs to be at 6 years or less to make financial sense. The total cost, including installation and less off setting tax credits is still three times too high.

    Anyone contemplating putting up solar panels would be much better off putting their money into the S&P 500.

      1. You’re mixing apples and oranges. What you’ve failed to grasp via your comments is that it will take a 15 to 18 year payback to break-even; that’s paying back the investment via reduces rates charges etc. Wherein at 20 years your panels are out of warranty and efficiency has dropped to 75% or less.
        Whatever you’re smoking I’d strongly suggest you stopped and quit using emotional knee jerk in your decision making

        1. Don, sounds like you know what you’re talking about. These solar companies are like used car salesman I don’t believe a word that comes out of their mouths. However, I hear of happy customers where I live in San Diego, so it makes me think maybe it is a good deal you just have to be EXTREMELY careful about which company you pick. Is this true? Or is solar just a rip off anyway you slice it???

          1. I gave a pragmatic, here’s the breakdown of the payback period. Solar is not a “rip off” as you say, but it isn’t cost effective based on a payback period and the degradation of the panels power output about the time you reach break-even

            As to the companies I can’t respond other than buy vs lease. If you lease most get your govt fed subsidies and you’re still on the hook if you move

            As to the “used car salesman” make that gender neutral, women are selling used cars now, but I’ll likely be told I’m creating another war on women 😉 . There are reputable companies out there. It is up to you to do the due diligence and verify what they say is what you get. If you go in jaundiced towards all, cynicism will prevail

            Closing: it sounds like you’re still making an emotional decision, which is fine, but recognize that. Do your homework: check out costs; payback periods to break-even; the efficiency of the panel ( some are lower efficiency so you need more panels which means higher costs; who gets the fed subsidy; company bonding and liability insurance whether they’re in business or gone out of business; BBB for reputation; lease vs buy responsibilities and liabilities

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