Power Rating and Energy Capacity

Power rating, in kW or MW, is how much a battery can deliver at one moment; energy capacity, in kWh or MWh, is how much it can deliver in total before it is empty.

The two are sized separately. Power decides how much load the battery can serve or how much solar it can absorb at once, and is set by the power conversion system. Energy decides how long it can keep doing so, and is set by the cells. A peak shaving battery needs enough power to cover the peak and enough energy to last the peak’s duration; a frequency regulation battery needs power more than energy.

Also called: kW vs kWh, MW vs MWh