To calculate the capacity of the solar power system you need, first find out how much electricity your household uses. Pull out your last electric bill and see how much kWh you use.
What is your electricity requirements?
Let’s say that this number is 880 kWh per month. Now divide your average monthly usage by 30 to get your average daily kWh usage. In this case, it would be about 29.33kWh per day.
How many Sun hours you expect in your area?
System capacity also depends on how many hours of sunlight you get in your area and the climate conditions. In the United States, sunshine duration differs from state to state.
For example, if you live in CA you can expect to have a 5 hours of sunlight per day. Now divide your daily kWh requirement by the number of daily peak sunlight hours. This gives you the capacity you need (29.33/5) = 5.8Kw.
How much capacity I need?
BUT NOW we need to take into account some losses occurs like weather conditions, loss occurs when converting from DC to AC, losses occurs as the electricity flows along the wires etc. So add a 18% compensation to your daily average to offset such inefficiencies. This will make the system capacity (5.8 X 1.18) = 6.8Kw.
In short, to meet 880 kWh/month you need 7Kw solar system.
Keep in mind that this is a rough estimate. Actual value may differ based on a site location. Contact a solar installer to figure out how many panels you need.