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Step 1: Look at your electricity bills and average out your usage to find “Your kWh/year”.
- kWh/year = Average Kilowatt Hours of electricity that you use in a typical year.
- 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: Look at the US Solar Map below to get the number of hours per day the Sun shines on your house.??
Step 3: What percent of your total electricity bill do you want to come from the Sun?
- If your budget is strong, 75- 100% is a good bet, but just realize that you’re still “on grid” which means you’ll likely still use the utilities electricity a little bit.
- If your budget is limited, start with 50% and talk to your solar installer about leaving room for expansion later.
Step 4: Array Size in kW = (Your kWh/year) / (365 days/year) / (Solar Hours/day) * (Percent you want to cover) / (75% efficiency constant)
- Example: My electricity bill is ~$110/mo and I use about 900kWh/month. I live in SoCal so I get about 5.5 solar hours/day and I want to cover 75% of my bill!
- (10,800 kWh/year) / (365 days/year) / (5.5 kWh/day) * (75%) / (.75%) = 5.38 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.
Step 5: Calculate solar array size!
