kW is power
A kilowatt describes a rate of producing or using energy. Residential solar systems are commonly labeled by DC system size in kW, such as 6 kW, 8 kW or 10 kW.
kWh is energy over time
A kilowatt-hour measures an amount of energy. Utility bills show household use in kWh, and solar production estimates describe how many kWh the array may generate each month or year.
The same kW system can produce different annual kWh
Two 8 kW systems can have the same nameplate capacity but different annual production because location, roof direction, tilt, shade, weather and system losses differ.
Use kW to compare system size and kWh to compare output
When reviewing quotes, compare DC kW, annual modeled kWh and the assumptions behind that production estimate. A larger kW number is not automatically a better proposal if production assumptions are weak or the price is much higher.
Batteries use both terms too
Battery kWh describes stored energy, while battery kW describes how much power can be delivered at once. Both numbers matter when evaluating backup capability.