*time:*

volume = ﬂow × time.

We say that a * ﬂow* is a * rate* at which * volume* is delivered. If you know the

volume delivered in a particular time, you get the ﬂow by dividing the

volume by the time:

Here’s the connection to energy and power. * Energy* is like water * volume*:

*power* is like water *ﬂow*. For example, whenever a toaster is switched on, it

starts to consume * power* at a rate of one kilowatt. It continues to consume

one kilowatt until it is switched off. To put it another way, the toaster (if

it’s left on permanently) consumes one kilowatt-hour (kWh) of energy per

hour; it also consumes 24 kilowatt-hours per day.

The longer the toaster is on, the more energy it uses. You can work out

the energy used by a particular activity by multiplying the power by the

duration:

energy= power × time.

The joule is the standard international unit of energy, but sadly it’s far

too small to work with. The kilowatt-hour is equal to 3.6 million joules (3.6

megajoules).

Powers are so useful and important, they have something that water

ﬂows don’t have: they have their own special units. When we talk of a

ﬂow, we might measure it in “litres per minute,” “gallons per hour,” or

“cubic-metres per second;” these units’ names make clear that the ﬂow is

“a volume per unit time.” A power of * one joule per second* is called * one watt*.

1000 joules per second is called one kilowatt. Let’s get the terminology

straight: the toaster uses one kilowatt. It doesn’t use “one kilowatt per sec-

ond.” The “per second” is already built in to the deﬁnition of the kilowatt:

one kilowatt means “one kilojoule per second.” Similarly we say “a nuclear

power station generates one gigawatt.” One gigawatt, by the way, is one

billion watts, one million kilowatts, or 1000 megawatts. So one gigawatt

is a million toasters. And the “g”s in gigawatt are pronounced hard, the

same as in “giggle.” And, while I’m tapping the blackboard, we capital-

ize the “g” and “w” in “gigawatt” only when we write the abbreviation

“GW.”

Please, never, ever say “one kilowatt per second,” “one kilowatt per

hour,” or “one kilowatt per day;” none of these is a valid measure of power.

The urge that people have to say “per something” when talking about their

toasters is one of the reasons I decided to use the “kilowatt-hour per day”

as my unit of power. I’m sorry that it’s a bit cumbersome to say and to

write.

Here’s one last thing to make clear: if I say “someone used a gigawatt-

hour of energy,” I am simply telling you* how much* energy they used, not

*how fast* they used it. Talking about a gigawatt-hour *doesn’t* imply the