Units of power: why I heart the gigawatt

Posted by – 2010/05/31

“Why are you using those units for power? Why don’t you use something else?”

Is that what you’re asking? Is it?

On most pages here, you can choose from a wide number of units: when the option is available, there will be a box for you to choose the power unit of your choice. It should be over on the right, right now. A headline that says Switch Units, and underneath it, a selector that allows you to, well, switch the units. Go on, try it now:

One gigawatt = 1 GW (go on, switch units, this line will change)

If the option is missing from a particular page, and you think it should be there, just let us know.

I like gigawatts, but you can use whatever you feel comfortable with – switch to your preferred unit now!

But now, maybe you’re thinking, ok, I can display this blog in my preferred unit of power, and this is the only place I can do that, and thank heavens for not having to do lots of mental arithmetic somersaults every time someone switches from one unit of power to another, but something’s missing from my day. How about a detailed explanation of why gigawatts just rock, and all the others just get on my tit. And if that is what you are thinking, then (1) you might be a bit odd, but (2) you’re in luck:

Why gigawatts rock

It’s part of the standardised SI system of units understood around the world, and it enables numbers from different countries and different times to be compared.

Standardising on one unit of power makes a lot of sense. That is what the SI system does. Switching between mtoe/y, mboe/d, TWh/y, EJ/y, quads/y, kWh/d is a pain – I guess we all agree on that.

Take a 1GW power plant running at 90% load factor. How much energy does it generate if it were running in Britain today? 0.9GW. In Sweden? 0.9GW. In China? 0.9GW. In Britain in 2050? 0.9GW. What’s the average power over a day? 0.9GW. Over a year? 0.9GW. Gigawatts don’t change their value, depending on the length of time, or the size of the population you’re talking about.

If you want to understand what a gigawatt is at the personal level for every individual in Britain, it’s the equivalent of every person having a 16W fluorescent lamp switched on. Or one unit on your electricity meter for every person in your household, every two and a half days. A 1-bar electric fire for every single person in the country would be 60GW, or 24kWh/d, or one unit per person on your electricity meter every hour.

As context, British final energy demand in 2008 was 205GW (Digest of UK Energy Statistics 2009). If you need it in terms of power per person, that’s 82kWh per person per day, or 3.4kW per person: the equivalent of a 3-bar electric fire, and four (outlawed) 100W incandescent light bulbs, for every man, woman and child.

For average rate of energy consumption over the year, people use different units to distinguish between electrical power (TWh/y), thermal power (Quads/y), and chemical power (Mtoe/y). Those different types of power (electrical, thermal, chemical) are not directly convertible at an exchange rate of 1:1, and so there is an argument for using different units for each one. But that’s just the sort of thinking that created the mess of power units we have today.

It does make sense to pick one unit and stick to it, because that still gives you numbers that transparently make sense when you do calculations with them. For example, take a particular portfolio of installed capacity 20GWp (the subscript “p” denotes installed peak capacity, here), and a guaranteed availability of 90%, how much of peak demand can you serve? Easy: 18GW [90% of 20GWp] .

If there’s a load factor (including availability) of 70% on that 20GWp, what’s the mean power over the year? Easy: 14GW [70% of 20GWp].

Now, yes, we could convert that 14GW mean power into TWh/y by multiplying by 8.76. Or into kWh/d in Britain in 2010, by dividing by 2.5 (remember, with kWh/d, you always need to state place and time, because you’re factoring by population size). You could convert it into Quads/y, or indeed any type of energy per unit time: cream buns per squirrel per lunar month, if you like. But by keeping to one unit, you keep the calculations clear and transparent.
70% load factor x 20GWp = 14GW. Easy.
Same calculation, different units:
70% load factor x 20GWp = 5.6kWh/d or 123 TWh/y. Much less transparent. Gigawatts rock. You know it. I know it. But hey, the unit switcher is up there. It’s not yet programmed for cream buns per squirrel per lunar month, but it will let you switch between the most commonly used units of power.

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