← U.S. Residential Solar Fundamentals
Watts, kilowatts, and kilowatt-hours
By the end: Calculate energy from a stated average power and time, and distinguish DC rating from AC output.
Power is the rate of producing or using energy. A watt is a unit of power; 1,000 watts equals 1 kilowatt (kW). Energy accumulates over time and is commonly billed in kilowatt-hours (kWh). A device averaging 1 kW for 3 hours uses 3 kWh. For changing power, add the energy in each time interval.
A module nameplate is a rating under defined test conditions, not a promise of constant output. Array size is often quoted in kW DC; an inverter has an AC power rating. They describe different sides of the system. Weather, temperature, shade, conversion losses, and operating limits affect delivered energy.
- Ten fictional 400 W modules total 4,000 W DC, or 4 kW DC of nameplate capacity.
- If measured AC output averages 2.5 kW over 4 hours, delivered energy is 10 kWh.
- Do not multiply nameplate power by 24 hours and call that a daily forecast.
- A 10 kWh battery and a 5 kW inverter are not competing size descriptions: one is energy, the other power.
When comparing a proposal, underline every number and label its unit: kW DC, kW AC, kWh per year, or dollars. Ask whether an energy estimate is modeled or measured and which losses it includes. A unit mistake can make a plausible-looking comparison meaningless.
Quick check
A load averages 0.8 kW for 5 hours. How much energy is used?