What’s a kWh? (And other money-saving tips)

When I was at school, they taught us how electricity works only as part of science lessons. It was something future engineers might need, yet we all use electricity at home every day.

The problem with electricity is we’re a little bit separated from its cost. With cars, we fill up the car with fuel and pay for it right there and then. With electricity, we use many different appliances which all add up to an eye-watering bill at the end of the month.

This is my guide to what everyone needs to know about electricity.

Introducing the kWh.

Electricity is sold in units of “kWh”. We’ll come to exactly what those three letters mean later on but for now, imagine your electricity is being delivered to you in barrels, each one a standard size called the “kWh”. Think about your local electricity station and imagine one of these “kWh” barrels of electricity being hooked up to the wires that lead to your home. When a barrel empties, someone comes along and replaces it with a new full barrel.

The “kWh” has a scientific definition that all electricity suppliers agree on. It is so ubiquitous that if any supplier decides to use a different unit, they’re most likely up to something dodgy.

How much is a single kWh barrel of electricity? Check your electric bill. Here’s mine…

The 45¾p per day standing charge is fixed. It doesn’t matter how much or how little I use; I still have to pay that 45¾p every single day and there’s little I can do about that other than maybe switch providers.

More interesting is the 33p per kWh. At the end of each month, they count up all the empty barrels of electricity I’ve gone through and bill me 33p for each one. I’ll use that figure in my examples but do look up your own rate and replace it with however much your kWh costs.

Also note that it doesn’t matter how quickly I go through each barrel of electricity. If I go away for a few days leaving everything except the fridge switched off, it will take a lot longer to finish that barrel than when I’m home and everything is switched on. Either way, they still charge me 33p once that barrel is empty.

We’ll now pull apart those three letters, but always keep in mind that metaphor of barrels of electricity hooked up to the wires leading to your house.

Little barrels on the hillside.
Little barrels full of ‘tricity…

What Watt?

The W is short for the “Watt”, named after James Watt who invented them. If you’ve seen a capital W or “Watts” or “Wattage”, they all mean the same thing. The number of Watts any electrical appliance has is a measure of the rate of consumption of electricity over time. If you like, think of it as the speed that something eats electricity coming out of the outlet on the wall.

"High power fan heater. 3000 Watt. 2 heat settings, 1500W/3000W. Adjustable thermostat with overheat cut out protection."

This heater consumes electricity at a rate of 3000 Watts, or 1500 Watts if you use the low setting. Because one Wattage figure is twice as much as the other, you can safely assume that the high setting consumes electricity exactly twice as fast as the low setting.

Lightbulb in packaging. "15 year warrantee. 13.5W. 100W replacement. 1527 Lumens."

This lightbulb consumes electricity at a rate of 13.5 Watts, yet it shines as brightly as an old-fashioned 100-Watt filament lightbulb. Quite the improvement!

A quick exercise: Find an electrical item in your home and look up its Wattage figure. It might be on a label or written on the original packaging. If you can’t find it written down, try using a search engine.

Ooh kay!

1 kW (or one kiloWatt) means exactly the same thing as 1000 W. Adding “k” to “W” to make “kW” means the amount is multiplied by one thousand. The heater above could have “3 kW” printed on the box instead of “3000 W”. It would mean exactly the same thing.

Devices that draw a small amount of electricity like lightbulbs or phone chargers are usually rated in Watts, while larger devices that eat a lot of electricity like ovens or electric car chargers are typically rated in kW. They mean the same thing underneath.

Whoever makes your electrical appliances might have a personal preference for small numbers in “kW” or big numbers in “W”. The manufacturer of that heater probably wants to emphasise how well it heats, so they prefer to use the bigger number of “3000 W” instead of “3 kW”. More W equals more heat.

Our hours

The last letter is “h”, which is short for an “hour”, named after its inventor Sir Claudius Hour. (At least that’s what a man at the pub told me. He might have been joking.)

You know what an hour is, don’t you? It’s the time it takes to watch a normal episode of Star Trek with ads. It’s how long it takes me to walk all the way around my local country park if I don’t stop. It’s the time it takes to walk my sister’s dog before she (the dog) gets tired.

“And I would walk 500 miles and I would walk 500 miles more.”

All together now!

Now we know what each letter of “kWh” stands for, let’s bring them all together. A “kWh” is the amount of electricity consumed by a 1000 W appliance if it is left on for an hour.

Find an appliance that’s rated at 1 kW. Plug it in and switch it on for an hour and then switch it off. You’ll have used exactly one kWh and your electricity bill will have gone up by 33p. (Or whatever your supplier charges.)

Let’s work out a practical example. Recall that 3000W heater from earlier. How much do you think it costs to run that heater for five hours on the high setting? We’ll ignore practical realities like the built-in thermostat and assume it goes for five hours straight with no gaps.

3000W is the same as 3 kW and we want to run it for 5 hours, paying 33p for each kWh. Multiply those numbers together:

3 kW × 5 h × 33 p/kWH = 495p (or roughly £5.)

Try this calculation yourself. Pick an electrical appliance in your home and find its rated wattage. Think about how long you switch it on for and work out how much it costs to use it for that amount of time.

Applying the knowledge

It can be tempting to look at how much some appliances like heaters or ovens cost and conclude the only way to save money is to be cold and not eat. I hope that’s not the conclusion you draw. The benefit of knowing how much something costs to use is that you can make informed choices.

Will buying an air fryer save you money when your kitchen already has an oven? Work out how much it costs to cook your favourite meal in the oven then do the same for an air fryer. If you know both in actual pennies, you can make an informed decision to make that purchase or not.

While the Wattage figure tells you the rate it consumes electricity, it may be that the higher Wattage appliance gets the job done faster. Say you have a choice of two kettles, one runs at 1 kW and the other at 3 kW, it may seem at first blush that the 1 kW kettle will cost less. However, if the 3 kW kettle gets the water boiled in a third of the time as the 1 kW kettle, they will cost the same to use.

Does your supplier offer a different service with more expensive electricity during the day and cheaper electricity overnight? Which appliances would you use overnight when the kWh barrels are cheaper? Would that save you money overall?

Many thanks to my wife and my brother Andrew for their helpful feedback. Thanks also to my local B&M store for the pictures of lightbulbs and heaters I took while shopping there.

Creative Commons Picture Credits:
📸 “saturday recycle” by Andrea de Poda.
📸 “sad kilo” by “p med”.

My Crazy Software Engineer Tattoo (that I didn’t get)

I had an idea for a nerdy tattoo a few years ago. It would represent myself as a software engineer and I thought it was quite clever. I seriously considered having it done but decided against it in the end, despite its cleverness.


This is my idea, the “end comment” symbol in many programming languages:


In C, and other languages that can trace their lineage to C, comments start with a /* and end with a */. Anything inside is ignored by the language, allowing the programmer to describe what’s going on. This is tremendously useful when reading other people’s code or even your own code from the past.

    /* This is a comment. */

    /* This is another comment. */

Another way of looking at it is that these /* and */ symbols mark the change of state between comments and code. /* says “After this is comment” while */ says “After this is code.”

Or to put it another way, */ means “Enough talk, time for action.”

(This is where you exclaim to yourself how clever I am to have thought of that.)

I didn’t have the tattoo done in the end. Describing what it meant would have taken too much explanation. Even if a fellow programmer recognized the symbol, they would probably first think it looked like I’ve been “commented out”, as they wonder if I had the /* on the other side.

Also, rotated a little, it looks a bit like a squinting cyclops.


Falco T310 – Unleashed

1993. Computers were desktop PCs running MS-DOS and the Internet was unheard of. My school had a number of PCs with Borland Pascal installed which my friends and I happily learnt. Along the way, we wrote a clever variation of the Minesweeper game. Life was good.

That would all change when I started my Computer Science degree course at university that year. Instead of many single-user machines running MS-DOS, we’d all be sharing a multi-user machine running UNIX.

Terminal Illness

To use this multi-user machine, we’d need to log-in from a terminal. If you were fortunate enough to find a vacant PC, you could use the terminal emulator program to connect. This had the very useful feature of being able to switch between screens so you could operate many sessions at once. I would usually have one with the email program running so I could switch to it occasionally to see if any new messages arrived, while a second session would run EMACS for whatever I was writing. A last one would compile and run stuff.

If I wasn’t quite so fortunate to find a vacant PC, I’d have to use one of the Falco T310 terminals. These were serious old-school terminals that connected to that machine over a serial port. Actual RS232 connecting to a multiplexing box in the corner. The university had maybe a hundred of them. Because they did only have a serial connection, you could only have one session per terminal. No fast switching between sessions for you – if you wanted to check your mail you had to shut down whatever you were doing and start up the mail reader.

These terminals weren’t all bad. It understood the standard ANSI codes to move the cursor about, so there wasn’t too much friction moving between the two. We coped and got on with the job.

Loss of control (characters)

One day, I intended to review a source code file, so I typed a “cat” command to show the listing, except I had accidentally run cat for the compiled binary executable instead. Oops! The screen filled with noise punctuated with beeping noises. Efforts to stop the onslaught were in vain as the buffers filled up with unintelligible bytes.

Then something unexpected happened. The screen changed mode and lines were drawn mixed in with the text. Not the box drawing characters I was used to but proper lines, drawn at funky angles spanning across most of the screen. These terminals supported some sort of control codes for vector line drawing, and my executable code just happened to randomly contain those codes. I must find them!

Living the student life, I wasn’t getting much of a chance to exercise my artistic muscles. Back at school, I knew how to program graphics in Borland Pascal and I’d come up with simple games and create animated art. Even dull homework projects would have a bit of a flourish thanks to creative use of the 640x480x16 mode. On UNIX in contrast, I was back in the 80s with an 80×24 character display, yet here was an elusive graphical mode I hadn’t seen in months.

grep -v “\a”

Actually finding what those magic control codes were was easier said than done. Once I had accidentally entered this graphical mode, I found I couldn’t type commands anymore. The only way I knew to get back to normal was to power cycle the terminal and login again. My attempts to split the file in half and display one of the halves would be accompanied with incessant loud beeping from all the BEL/7 bytes, which greatly disturbed the other people in the room. That amount of beeping could only mean I was up to no good!

After spending a day trying to extract the codes I needed, I had to give up. I was unfamiliar with working with Unix beyond dealing with plain text files. I knew how to open files in binary mode back on Borland Pascal, but not on any language I had access to in Unix. There was no StackOverflow to ask so I was stuck impotently banging rocks against this monolith. This was software development in those dark ages.

Next: Checking in at The Motel. BBS Systems, Fidonet and reinventing the remote-desktop.

Picture Credit: VT100 in the flesh, by Dana Sibera. (CC licensed.)
(I couldn’t find a picture of the Falco T310, so I used this picture of a VT100 instead. Sorry about that.)