The difference between a kilowatt (kW) and kilowatt-hour (kWh) is the most misunderstood piece of understanding electricity and a key point in understanding what you are buying with a solar PV system. kW represents the actual electrical size of the system and kWh is what that sized system will produce over a given time. To help make more sense of this let me use the light bulb as an example. A household light bulb is rated at 100 watts, which describes the power it draws instantaneously. If you leave that 100-watt bulb on for one hour it will have used 100 watts continuously for an hour or as we call it, 100 watt/hrs. Now if that light is on for 10 hours the bulb will use 1000 watt/hrs. (100 watt/hrs. x 10) or converted to kilo (meaning 1000) it used 1 kWh. So now if I build you a 10 kW sized system (about 40 solar PV panels) it is also called a 10,000 watt system and if it is directed south at an optimum angle in Colorado I can calculate that in one year that system will produce 14,333 kWh of electricity. Now the system size never changes, it is always a 10 kW system on this particular roof but as solar radiation varies from year to year and equipment ages the production or kWh will vary each year.