Calculate Solar Array Size and Cost

(Updated: March 13, 2016)

Image of a calculator and a pencil.What size solar system do I need for my home? Though this is a complicated question, we have simplified it for you below. It’s actually pretty easy to calculate solar. 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

Look at the chart below to get the number of hours per day the Sun shines on your house.

 Just find the closest big city.

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

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) / (0.82 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) / (0.82) = 6.4 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 82% of the system (DC power) size. This is the 0.82 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.