NOW, WHERE DID I PUT THAT ELECTRIC BILL?
Electricity consumption is measured in kilowatt-hours (kWh) and varies by household and season. First, add up all of the kWh and divide it by 12 months/year to get your average monthly consumption. This will give you your kWh Per Month.
HOW MANY HOURS IS THE SUN OUT ANYWAY?
One ‘sun hour’ is the amount of solar radiation that your city gets in one hour of direct sunlight at sea level and depends on the weather, the length of the day, changes in the angle of the sun and your altitude. Multiply the daily sun hours by 30 days to get your average sun hours per month.The map above will help you figure out the daily sun hours that you receive on average. If you don’t live in the US, search the web for “solar insolation”.
DO I HAVE TO ACCOUNT FOR ELECTRICITY LOSSES IN THE SYSTEM?
Yes. The National Renewable Energy Laboratory (NREL) uses a ‘derate factor’ to account for electrical losses in converting Direct Current (DC) that comes from solar panels to Alternating Current (AC) that goes through the inverter for safe use in your home. NREL uses the derate factor of 0.77 to account for electricity losses in the solar system and gives you a better estimate of the power necessary to actually offset your electricity bill.
AND THE SYSTEM SIZE THAT YOU NEED IS…
Divide the kWh that you want to offset every month with the number of sun hours that the system will get every month, and then by the derate factor. The result will be your solar system size in kilowatts (kW).
[kWh Per Month] / [Sun Hours Per Month] / [Derate Factor] = Solar System Size in kW
500 kWh per month / 105 Sun Hours Per Month / 0.77 Derate Factor = 6.2 kW
My system size is 6.2 kW. I will need around a 6200 watts (W) system to offset my entire electricity bill.
Important Note: This equation calculates the total kW of a solar system necessary to offset an entire electricity bill. Dividing the 6200 watt system by the number of 250 W solar panels will yield the number of solar panels necessary to offset your entire electricity bill (~25 solar panels). However, your roof may not hold 25 panels, so the next step is to figure out how many panels will actually fit on your roof.