kWh vs kW: capacity vs power, explained

This is the one to get right, because almost every other home-battery number depends on it. kWh and kW look alike and sound alike, but they measure two different things, and confusing them is how people end up disappointed by a battery that "should have lasted longer."

The one-line version

• kWh is capacity. It is the size of the tank: how much energy is stored.
• kW is power. It is the size of the pipe: how much energy can flow out at any instant.

A battery has both a capacity rating and a power rating, and you need both to know what it will do for you.

The water-tank way to think about it

Picture a water tank. The kWh rating is how many gallons the tank holds. The kW rating is how fast water can leave through the outlet.

A huge tank with a narrow outlet stores a lot but can only trickle. A small tank with a wide outlet empties fast but cannot run anything for long. A good home battery needs enough of both: enough capacity to last, and enough power to actually start your appliances.

Doing the math

The relationship is simple division. Capacity divided by load equals runtime:

• A 13.5 kWh battery running a steady 1 kW load lasts about 13.5 hours.
• The same battery running a 2 kW load lasts about 6.75 hours.
• The same battery running a 4.5 kW load lasts about 3 hours.

So "how long does the battery last?" has no single answer. It depends entirely on how much you are pulling from it. A fridge, some lights, and a phone charger might draw a few hundred watts and run for a day or more. Add an electric oven or a central air conditioner and the same battery drains in hours.

Why the power rating bites in an outage

Capacity tells you how long. Power tells you whether something turns on at all. If a battery is rated for, say, 5 kW continuous, it cannot simultaneously run loads that add up to more than that, no matter how much capacity is left in the tank. A well-stocked tank with a narrow pipe will still stall when you ask for too much at once.

This is why sizing a backup system is really two questions, not one: what do you want to run at the same time (that sets the kW you need), and for how long (that sets the kWh you need).

Where people go wrong

The common mistake is shopping on capacity alone. A 13.5 kWh battery sounds like plenty until you realize you wanted to keep the AC running through a summer outage, at which point both its power rating and its capacity get tested hard.

The honest move is to start from your actual loads, not the headline number. Our Worth It calculator works from your usage so the backup-hours figure reflects your home, not a best case. And remember that the capacity you can actually use is not always the rated number, which is exactly what depth of discharge covers next.