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 at 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 6 kW+ air conditioner on, the homeowner needs to add another 5 kW, doubling his charge and discharge speed. It’s a power problem, and it needs a power fix.
The salesman’s proposed extra 10 kWh of battery energy capacity is as useful as teats on a bull in this context.
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.
About Finn Peacock
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A timely reminder of basic knowledge in this age of home batteries. Those ads pairing undersized inverters with big sized batteries (like a 5kW inverter with 40kWh sized battery) needs some serious addressing by authorities.
I blame Watt. It’s a rate, and people are more used to seeing rates as expressions of basic units eg kilometres per hour, litres per second.
Hence we often see kW/h — the second derivative as in acceleration, because people like to see a ‘per’ in there somewhere.
Joules and Joules per second might be a more familar assembly but of course less conventional.
It’s also hard because both energy and power are relatively complex when broken down into the most basic SI units.
It’s very simple to visualise kilograms, metres and seconds. Understanding those three, people can pretty easily latch onto quantities that are one step away from those (e.g. velocity in metres per second).
However energy is a dog’s breakfast: kilograms times metres squared per second squared. Breaking it down one step gets force times displacement… force is also not easy to visualise. That’s mass times acceleration… OK so acceleration is… um…
Power is then a dog’s breakfast per second (or kilograms times metres squared per second cubed). Who is visualisng a cubic second???
Argh!
I guess the hoses and buckets analogy is probably as good as it gets.
About 55 years ago, the basic physics involved was explained by examples such as raising a 10 kg mass 5m. Equating the work done, force x distance, with the resulting potential energy, force x height was easy to visualise. Adding the time element for power was no stretch.
Admittedly, that the downward force exerted by that mass is F = ma = mg = 10 x 9.81 m/s² here on Earth, expressed in Newtons, might be why we had both humanities and applied science academic streams. Not everyone wants to know. An invisible qualitative difference is harder to discern when abstract thought is more than arm’s length from daily experience.
Maybe horsepower-hours? Four horses x 8 hours = 32 horse-hours of hauling up a hill? And a kW is just 1.3 horses. Remind people that a human can output more than 1 hp for some seconds, but not for an hour, so takes longer to accumulate a horse-hour, might work? And 1.3 horse-hours = 1 kWh.
I guess it’s a matter of what clicks.
Yes raising a weight against gravity is probably the other most tangible example, but then you have to explain why the gravitational acceleration is 9.81 m/s2 (which we could just accept as essentially a constant assuming we do all of our business here on Earth, but is actually a result of a more complex equation involving the mass of the planet and the distance from its core, conveniently ignoring every other mass in the universe also acting on the object in question…).
Work required to raise a mass of 1 kg by 1 metre, against a force that makes it accelerate downwards at (sigh) 9.81 m/s2 = 9.81 kg.m2/s2 = 9.81 J.
Doing that work in 1 second would require power of 9.81 J/s = 9.81 kg.m2/s3 = 9.81 W.
It’s fine. I just wish it was neater. Maybe if the Earth was a little denser it could get up to 10 J which is a nice round number but really no easier to explain.
Anyway… solar good, shonks bad. Carry on.
A useful analogy is a garden hose and bucket. Water goes through the hose at a certain volume per unit if time, litres per second. For electricity this is joules per second or Watts.
The hose fills the bucket which has a volume measured in litres. The battery is the bucket with its volume measured in Watt hours.
At the risk of taking the analogy too far, put your finger over the end of the hose and you feel the pressure, measured in pascals or old school psi (pounds per square inch). The “electrical pressure” is measured in Volts. But don’t put your finger on the end of the electrical pipe, you won’t enjoy feeling the volts!
There’s so many different units used for energy, kWh, Joules, or even Calories if you jump on a rower. All the same, just multiply them by a constant to convert.
You should explain that kWh is kW x hours, so it’s how long a battery can deliver kWs.
Example, your 13 kWh battery can deliver 1kW for 13 hours or if you had a big enough inverter it could deliver 13kW for one hour or any other combination of kW times hours that makes 13.
You solar guys have it easy! In the caravan world, to power espresso machines, induction cooktops, fridges etc, there’s been a ten-year arms race in inverters, batteries and voltages. So manufacturers and commentators quote current (A), charge (Ah), energy (kWh), state-of-charge (%), voltage (V) and power (kW). There’s some fun reading to be had.
And Finn, thanks for a fabulous service. But I’m an ex-CSIRO guy (IT) too, and I think I can out-pedant you on quantities and units!
https://scholar.google.com/citations?hl=en&user=jPZcPiwAAAAJ&view_op=list_works&sortby=pubdate
Hi Marcus,
I refer to them as the cigarette lighter mafia, bloody caravanners use 12V everywhere and end up with batteries in parallel, 95mm² cables and 300 amp continuous current to cook breakfast. It’s nuts, but they just can’t help themselves.
Marcus,
A serious all-terrain long-range full-nerd caravanner pretty much has to go (fold-out) solar to avoid energy deprivation away from caravan parks, I figure. That leads to the 12/24/48V Victron menagerie on page one of available options. And LFP batteries are light enough that it’s 24V if you only want lights, TV, and water pump, then 48V for all else, especially a safe breakfast. If determined to charge from a 12V vehicle alternator while driving, there’s DC/DC converters for boost. It’s pretty easy, and lots less chance of smoke & flames.
Then a 300A 12V breakfast is only 75A at 48V, much gentler on the battery and the copper-buying wallet. My concern is the rupture capacity of the fusing used, not always HRC. Keep the fire extinguisher close, and stay out of the bush this summer. 😉
Apropos terminology, my most disdained groaner is using “mw” for megawatt (MW). The 9 orders of magnitude difference is hard to miss.
milliwatts! are we talking some seriously tiny LED’s?
Random, and completely off-topic question – does anyone know when the daily supply charges exploded? I was just randomly looking to see what current offerings are and noted that supply charges are up 50c/day!!! I’ve still a few months on my current plan so it won’t affect me for a bit, but I’m not looking forward to that sort of change!!!
In addition: my latest bill (after getting a battery upgrade in March that gives me a total of 42 kWh, told me I have to pay $50+!
My DSC is currently only $1.42. I still have some FiT. So yes, they are screwing us I think! I looked at some other providers and the Daily Supply Charges are shockingly high!
The change was the last price upgrab by the retailers (who still want the same money per month even when they are providing less product (more of us generate and store our own electricity).
At $1.80 per day supply charge that’s $650 a year for just having two wires connected to the house, and yet retailers are very happy to take our PV generation for almost nothing as well.
Hi John, The electricity retailers get busy each 1st July as they unleash their new pricings upon the ‘market’ but this year it seems they have excelled themselves and the ‘Solar Sharer’ thing has been cover for an explosion in DSC, at the same time FiTs continue their fall in value that sees the retailers buying your excess green electrons for 4 cents and selling those electrons at multiples the buying rate – a tidy profit being made for no trouble to them at all. This is all supposed to pass as the ‘market’ at work and competition delivering benefits to consumers. It all feels like a protection racket, where the government / authorities in charge greenlight the ongoing gouging of consumers. Going off-grid to escape exploding DSC is not an option for most consumers who remain at the mercy of electricty retailers and the government / authorities in charge.
I’m pretty sure I checked prices in July and didn’t see the jump, but maybe I’m misremembering?
The bonus FiT is dropping to perhaps a third of the daily supply charge, and state electricity subsidies cap out at maybe half the daily supply charge – obviously that depends on the state and eligibility. Those eligible for neither have to export several score kWh just to cover the cost of their connection, or pay $700+/year. Actual power is what, 10:1 exports:imports, or worse? Emphasis naturally on worse!
I haven’t yet done the new maths on the value of off-grid, but I suspect the new charge weighting means on-grid batteries simply aren’t worth the cost for most – the maths never worked for me in the past even when the maths was more favourable.
Remember when AEMO was thinking of doubling or tripling supply tariffs? They backed off but did increase them. July . This is a mask to pay for the ‘solar sharer’ scam. Everyone said its not free and point is proven. Finn has some good articles..Some supply charges rose 30c some 50c and some doubled. Check your plan. When it changed they had to advise you
To me all the confusion comes from standard convention having settled on Watts rather than Joules in describing these things.
It goes against intuition to have a time component in a unit that doesn’t describe a rate of delivery. If we all talked about MegaJoules for storage capacity and MJ/hr for the rate of delivery of an inverter there would be NO confusion!
I remember it like this. kW is how wide the pipe is. kWh is how much actually goes through it over time. One’s vertical in my head, the other horizontal. Same deal with bandwidth versus data usage.
KiloWatt-Hours has always been a silly unit. MegaJoules would be much better, avoid all the confusion, and 1kWh = 3.6MJ is not a difficult conversion factor.
I struggled with this concept most of my life, until I got rooftop solar and really thought hard about it.
In the end I visualized it thus: The trick is to imagine electricity as having dimensions, just like geometry.
1 dimension= current (Amps) A line of electrons flowing past
2 dimensions = power (Amps x Volts = Watts) A rectangle / area of power
3 dimensions = energy (Watts x time) = kWhs A box / volume of energy
Your inverter shows kW. Your accumulated production or usage is kWhs
I made an infographic but haven’t managed to add it in here.
I’ve been writing about this stuff, and correcting this and similar errors, for over 30 years. It’s inexcusable that sparkies and installers don’t know this stuff, but my experience has been than many simply don’t have the science education to actually understand it. They use the terms, often incorrectly, but never really understand what it means.
But that’s not uncommon in the general population, most people don’t understand technical terms even when it’s part of their job. It’s everywhere, and isn’t going to change, as the general level of science education continues to decrease as people rely more on AI search results and social media for their info.
Basically, the human race is becoming more ignorant and less intelligent in a visible timeframe.