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kW versus kWh: power versus energy, made simple

Mixing up kilowatts and kilowatt hours is the most common mistake in backup planning. One tells you how much can run at once, the other how long it can run.

Speed and distance

Think of a car. Kilowatts, kW, are like speed: how hard the system is working at a given moment. Kilowatt hours, kWh, are like distance: how much energy has been used over time. A 2 kW heater running for 3 hours uses 6 kWh. The same heater running for half an hour uses 1 kWh. The rate stayed the same, only the time changed.

What it would keep running

Kettle and toaster, high kW, short time
Fridge, low kW, all day
Heat pump, moderate kW, hours at a time
Modem, tiny kW, around the clock
Hot water cylinder, high kW, long reheat

What each number means for the system

The system's 15 kW continuous output is a kW figure. It tells you how many appliances can run at the same time. Switch on enough at once to go over that and something has to give. The 25 kW electrolyser rating is also a power figure, telling you how fast it can use electricity to make hydrogen.

The 30 to 40 kWh battery is an energy figure, telling you how much is stored. Hydrogen storage is also energy, though it has to pass through the fuel cell at 45 to 55% efficiency on the way back out. How long you can last depends on kWh stored divided by kWh used per day.

How it works →

The H2OME loop: an electrolyser splits water into hydrogen, the hydrogen is stored in a tank for weeks, a fuel cell turns it into electricity for your home, and the water returns to the loop.

Reading your appliances and bill

Appliance rating plates show power in watts or kilowatts, where 1,000 watts make 1 kW. Your power bill shows energy in kWh. To plan backup, use the kW ratings to check what can run together, and use your bill or smart meter data to see how many kWh a day you actually use. Both numbers matter, and they answer different questions.

Questions

Which matters more for backup, kW or kWh?

Both, for different reasons. kW decides whether your loads can run at the same time without overloading. kWh decides how long storage lasts. A system can have plenty of one and too little of the other.

Why is a fridge a small load if it runs all day?

Its compressor draws modest power and cycles on and off, so its energy use over a day is moderate. A kettle draws far more power but only for a few minutes, so its daily energy can end up smaller.

What does 15 kW continuous mean?

It is the power the inverter can supply steadily, not briefly. Short start-up bursts above that may be handled by the battery and inverter, but sustained loads should stay within it.

Elsewhere

Every use →

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