Kilowatts vs Kilowatt-Hours: The Mix-Up That Never Dies

I first wrote about this back in 2012. Fourteen years later, I’m still writing about it. That should tell you something about how sticky this confusion is.

Kilowatts (kW) measure power: the rate at which electricity flows. kilowatt-hours (kWh) measure energy: the total amount you’ve either generated or consumed. A 5 kW inverter can push 5 kW at any given instant. If you charge a battery ar 5 kW for 2 hours, you’ll put 10 kWh of energy into it. Mixing them up is like confusing your car’s top speed with the size of its fuel tank.

Normally I’d leave it there and point you at the original article. But a post landed in my feed this week from a battery owners’ group that’s such a perfect specimen of the problem, I had to pull it apart.

The post

A Sydney homeowner, on a battery forum, laid out his situation: a 5 kWh inverter paired with a 40 kW battery, three phase, on a smart meter. His air con was dragging the system into grid purchases. He was on Amber, eyeing GloBird’s three free hours instead, but reckoned he’d need a 10 kWh inverter to make the switch worthwhile. His plan: add another 5 kWh inverter and maybe another 10 kW battery.

Read that again slowly and count the errors. His inverter isn’t 5 kWh, it’s 5 kW. His battery isn’t 40 kW, it’s 40 kWh. The “10 kWh inverter” he wants is a 10 kW inverter. Every single unit in that post is swapped.

To be fair to him, he’s not a sparky, just a bloke trying to keep his aircon running without drawing from the grid. Pretty much every battery post on social media confuses kW and kWh. That’s not what irked me.

The reply

What annoyed me was an alleged solar retailer jumping in with an answer about rebates and pricing for adding a “10 kW battery to his 40 kW battery”. This clown just picked up the customer’s wrong terminology and ran with it, offering him exactly what he didn’t need: an extra 10kWh on his already giant, but underpowered battery.

I’ve said for years that the fastest way to sort a good solar salesperson from a bad one is to ask them to explain the difference between kW and kWh. If they get it wrong, that tells you everything you need to know.

This isn’t just me being anal about a missing or extra ‘h’. This is a textbook example of kW/kWh confusion resulting in a homeowner being quoted thousands of dollars for an upgrade that will not solve his problem.

To stop pulling from the grid when he switches his 6kW+ air conditioner on, the homeowner needs to add another 5kW, doubling his charge and discharge speed. It’s a power problem, and it needs a power fix.

The salesman’s proposed extra 10kWh of battery is as useful as teats on a bull.

One rule to take away

Power is what you need in the moment. Energy is what gets you through the night. Ask any battery or solar salesperson to explain the difference in one sentence. If they can’t, or if they just parrot your own mixed-up units back at you like the retailer in that Facebook thread did, find someone else to install your system.

And if you don’t understand the difference yourself, watch the video at the top of this post. I’ve aged 10 years since I filmed it. So have my kids. kW and kWh haven’t.