What’s a kWh?

I recently attended an excellent IET lecture by Michael Podesta on the pre-history of climate change.  What stunned me was a question that came up in the discussion at the end, which was the simple one – “What’s a kiloWattHour?”  Despite having used the unit for decades, including a fair amount of time working in the energy sector, I’d never really thought about it.  It was evident I wasn’t alone from the audience reaction.  For almost everyone, it was just a unit of energy that appeared on our gas and electricity bills.

Many people know what it is in that context.  It’s what an electric fire uses in around half an hour.  It’s what an old 100W filament bulb uses to light a room for 10 hours (a new LED bulb will light the same room for 100 hours using the same 1 kWh).  It’s what your washing machine uses for a low power wash, your dishwasher for an ECO cycle, and what your oven uses to roast a small chicken.  Depending on where you live, that kWh of electricity costs between 1p and 50p.  But what does it mean?  That’s an important question, as almost every discussion about energy usage, net zero and energy security ends up talking about kWhs.  So, unless we can relate those kiloWattHours to some sort of physical experience, it’s difficult to engage with the discussions we need to have about energy, which is why they become academic arguments that few relate to.  It started me thinking about how to solve that and bring the kWh to life.  The first step is go back to where it started and look at James Watt.

When I heard that question “What is a kWh”, my mind flashed back to an obscure piece of information I remembered from school, which is that the unit of horsepower is equivalent to 746 Watts.  Which in turn, took me back to James Watt.  James has been called the father of the industrial revolution and equally deserves to be called the father of tech startups.  He was born in 1736 and became interested in early steam engines.  Steam engines, in the form of water pumps had been developed by Thomas Newcomen in 1712 and were used in Cornish tin mines.  Local engineers and manufacturers shared innovations and improvements, much like today’s open source software communities.  That all changed in 1776 when Watt patented a novel design which doubled the efficiency of the existing Newcomen engines.  He used those patents to stop other copying his design, initially running through some innovative licence schemes, before the partnered with Matthew Boulton to form the successful manufacturing company of Boulton and Watt.

Watt’s next step was to try to expand the use of steam engines.  Although his engines were more efficient, their main use continued to be pumping water out of mines.  To make them more universally applicable, he developed a method of converting the linear pumping system to provide rotary motion, but was faced with the problem of how to convince factory and mill owners that it would work for them.  Although industrialisation was advancing apace in the late 18th century, it was still a Milibandish renewables era, with machinery powered by water.  If you weren’t near a river, the alternative was horses, or occasionally wind.  Watt’s next great innovation was to find a way to market the benefit of his coal-burning steam engines over the prevailing renewable options.

At that point in time, there was no agreed way of measuring power, or even talking about it.  There were no commonly used units for it, all of which made it difficult for Watt to sell the benefits of his improved steam engine to anyone other than the owners of leaky Cornish tin mines.  To solve the problem of promoting his “better  mousetrap”, he invented the horsepower.  Although driven by a marketing need, it was possibly a wider reaching invention than his condensing cylinder which had revolutionised steam pumps. 

In many factories, horses were harnessed to 24 foot diameter wheels, pulling them round at about 150 revolutions per hour.  Watt calculated that the total work done by each horse was 33,930 foot-pounds per minute.  Using this calculation, he codified and publicised his new “horsepower”, derating it slightly, so that he could always be sure that his steam engines outperformed the equivalent horse, ensuring that they exceeded his customers’ expectations.

It was a brilliant way of explaining the relationship between what a horse could do compared with his steam engines.  It let factory owners compare costs by equating the cost of feeding a team of horses (you typically needed six teams to let them rest and sleep) with the cost of coal.  It was a calculation that was easy to explain and Boulton and Watt became rich.  Coal’s calorific value won out time after time. 

While history lauds Watt’s inventions, Boulton needed Watt’s knowledge and application of patents , as well as his marketing genius and marketing genius to make the business take off.  Watt told the story of coal-fired power in a way that others could understand and gave us a unit of power that is still used today.  When you go to buy your new Ferrari, hp (horsepower) is the only non-SI unit that you’ll find in its specifications.  More fittingly, Watt’s name has been taken as the SI unit for power.  That is an immense honour.  Only 19 people have had their names elevated to become SI units.  In contrast, the 230 people who have received a Nobel prize for physics make that award seem almost commonplace.

Watt is a hero.  He revolutionised the design of steam engines, developed the power of IP licensing and understood how to tell a story that convinced users that they needed his technology.  Those are lessons for every fledgling tech startup.  He also did much to start the industrial revolution and popularise the widespread use of fossil fuels, which has got us into the problem we now have.

Which brings us back to today, where we’ve mostly forgotten Watt’s message about how to measure and compare energy, probably because we’ve lost our link to other physical sources of energy.  While a few people know that an average UK home uses around 13kWh per day, I doubt they realise that equates equates to about three horses per home.  They probably don’t even have a stable.  A 12 storey, 1960’s block of flats with six flats per floor would need over 200 horses.  That’s before you look at the hundreds of millions of horses that would be needed to replace car journeys, where each hour of driving on fossil fuels burns around 50kWh.  We’d be into tens of millions of horses just for the school run.  Those are silly comparisons, but they illustrate how we’ve lost touch with what energy means, and take it for granted.  It’s not surprising that we’re profligate with energy, because it is cheap, available and convenient.  We don’t even know how to put it into perspective.

Take another example.  A cleaner will expend about 1kWh a day, as a human is only about as powerful as a sixth of a horse.  If you priced that 1kWh in electricity terms, it would come to around 40p, equivalent to paying the cleaner 5p per hour.  It’s not a living wage, but it demonstrates how we have lost touch with the value of energy when we can get it so cheaply from fossil fuels.  As we divorce ourselves from the physical realty of things, it’s difficult to comprehend why everyday decisions and choices matter.

If we want to reduce CO2 emissions, we should all start thinking about why we’re using energy, not just how to generate cleaner energy, as that doesn’t address the primary problem, which is that it’s too easy to flip a switch.

I’m pleased to say that Michael de Podesta will be reprising his lecture at the Turing Theatre at the IET in Central London on 29th January 2027.  It’s one of the most inspiring talks I’ve attended in recent years and I’d encourage everyone to come.  Registration is free at https://events.theiet.org/events/the-pre-history-of-climate-science/