On energy: Units and measurements

On Energy is a series discussing and dissecting the energy sector in the UK.


Most of the world’s energy is produced from fossil fuels. The purpose of this series is to investigate whether we can live without fossil fuels. Can we conceivably live sustainably?

To answer this question, we’ll tot up the power requirements of an average British person and compare that with the total sustainable energy capacity of the UK. Following that, we will gauge how much of that capacity would be economically feasible to harness.

To facilitate comparisons across sectors and for my sanity, I have chosen the kilowatt-hour (kWh) as my main unit for measuring energy. 1 kWh is colloquially known as one unit on electricity bills.

Power, i.e. the rate at which energy is produced or consumed, will be reported in kilowatt-hour per day (kWh/d) to keep things simple.

It is important to appreciate the difference between energy and power. When one says they used 2 kWh of energy, they are stating how much energy they used, but not how fast they used it. The hour in kilowatt-hour doesn’t imply the said energy was used in one hour. You’ll use the same amount of energy by switching on ten 1000 W toasters for one hour than you would with if one toaster stays on for 10 hours.

10 units × 1000 W  × 1 hr =  10 kWh
1 units × 1000 W  × 10 hr =  10 kWh

Furthermore, numbers at global or national scale can be mindbogglingly large and incomprehensible. To make them more relatable, I’ll present powers in kilowatt-hour per day per person (kWh/d per person). As we have seen in previous chapters, per person statistics can shed light on insights that get lost in aggregated data. 

Finally, I have chosen to favour simplicity over absolute precision in my calculations as many of us tend to get turned off by long decimal calculations. So, while I know that 40W × 24 hr =  0.96 kWh, I will nevertheless round the result up to 1 kWh to keep things neat and tidy. You are, of course, welcome to run your own calculations to the 10th decimal place.

Let’s get on with it.

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