Calculate how much power you need with these solar calculators to estimate the size and the cost of the solar panel array needed for your home energy usage. Ideally, you will know your total kilo-watt hours (kWh) used for the past 12 months or the average monthly kWh used each month. Check your electric bills or contact your local utility for your kWh used.The cost and size of a solar panel system depends on your current and future energy consumption, your location, the types of solar panels and components to be installed, and the availability of rebates and incentives.