The most important metric of a renewables system is the amount of power it produces per year. This number can be actual or estimated. Annual production for systems producing electricity like wind or solar are measured in kilowatt hours (kWh) per year. A kWh is like a measure of the distance traveled per hour for a car – how far did it go over a certain period of time. Annual production is influenced by the size of the system, the conditions (How shady are the trees? How many cloudy days?, and the installation. Sellers typically have access to software that provides historical production totals.