To calculate the home solar system you need, first gather the electricity bills from the last year and write down the kilowatt hours used each cycle. Add all 12 numbers together and divide the total by 12. Divide that average by the average number of monthly hours of sunlight in your area. Divide that number by the amount of kilowatts the panel you want to purchase produces to figure out how many panels you need to meet your needs.
If you have a paperless billing cycles, simply contact your electric company to send duplicates or find them online. Each month, you typically use hundreds of kilowatt hours, so expect a big number once you add each month's usage together. Use an online solar calculator to find the average number of daylight hours your house typically gets per month. Divide the average number of hours of kilowatts by the average number of hours of daylight. For example, if you use 350 kilowatt hours and the average daylight hours is 150, your solar system needs to generate 2.4 kilowatts per hour.
Different solar panels produce different watts and are different sizes. Choose a system that fits on your roof easily. Get the calculations above and measure your roof before choosing a panel system to ensure it fits. The more watts a panel produces, the larger the panel, but the fewer panels needed. If you use a panel system that produces 500 watts per hour per panel, you need five panels to produce 2.4 kilowatts per hour.