kWh is for energy stored in this example, the "size of the tank". kWh is analogous to toe (tonne of oil equivalent) or gallons of gas. kW is for power, that is the flow of energy (energy/s), another non SI unit is horsepower. The SI unit for energy is Joule (J), the SI unit for power is J/s. 1 Watt is 1 Joule per second.