The difference between a kilowatt (kW) and kilowatt-hour (kWh) is the most misunderstood topic while trying to understanding electricity and how we measure power and consumption/production. Both concepts are referenced when talking about a photovoltaic solar system and important to understand before buying a solar PV system.

A kW measure power, it represents the actual electrical size of an electrical component, and a kWh measures production of an electrical component such as a photovoltaic system or consumption of a component such as a light bulbs when on.

kWh is what a specific sized photovoltaic system will produce over a given amount of time. To help make more sense of this, let me use the light bulb as an example:

  • If a household light bulb is rated at 100 watts, which describes the power it draws instantaneously when turned on, and you leave that 100- watt bulb on for one hour it will have consumed 100 watts continuously for an hour or as it is called, 100 watt/hr.
  • Now if that same light is on for 10 hours the bulb will use 1000 watt/ hrs. (100 watt/hr. x 10) or converted to kilo (meaning 1000) it used 1 kWh.
  • So now if you buy a 10 kW sized solar system (about 25 solar PV panels) it is also called a 10,000 watt system or also called a 10 kW system (just move the decimal point left 3 spaces when adding the kilo). If it is built facing directly south at an optimum angle to the sun in Colorado I can calculate that in one year that system will produce 14,333 kWh of electricity. Now the system size (kW) never changes, it is always a 10 kW system on this particular roof, but as solar radiation varies from year to year and the equipment ages, the production or kWh will vary each year due to many variables.