Many people are not familiar with the difference between a kW (kilowatt) and a kWh (kilowatt hour). These are important measures in the electrical world. As battery storage becomes more important, I think it is beneficial to explain what they are since all battery systems have ratings in both kW and kWh. To start, remember that ‘kilo’ means a thousand, so kW means a thousand watts.

A kW is a measure of power or in a battery system, how much power can be discharged at one time. A microwave with a 1,000-watt label requires 1,000 watts of power (or 1 kW) to work. In the case of a battery, the kW rating determines what all can be on at the same time during a power outage in a backup system design. Battery systems also have a surge rating in kW which is important when sizing a system to handle well or booster pumps and other items that require a surge of power to get started. This is an increased level of power for small amounts of time (seconds, milliseconds).

A kWh is the measure of energy usage over time. A 100-watt lightbulb that is on for 10 hours would be using 1 kWh. The capacity of the battery is measured in kWh which is the duration a battery can discharge or last during a grid outage. This duration is affected by what all is on and how long it is on. Your utility bill is based on kWh or how much energy you use over the billing period.

All battery systems have both ratings, but home storage battery naming is usually based on kWh. The new Enphase 5P battery has 5 kWh of capacity while a Sonnen Eco 20 has 20 kWh of capacity. Most battery systems can be stacked to increase the capacity of the system to meet the homeowners needs. The Enphase 5P batteries can be stacked up to a capacity of 80 kWh. In some systems such as the Enphase system the power of the system will also increase when increasing the capacity.

A good analogy of both using a car is that kW is like the horsepower of the engine and the kWh is like the size of the fuel tank (or for an electric vehicle how many miles you can drive on a charge).