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

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

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

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

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

is measured in
is measured in
kWh per day
W (watts)
MW (megawatts)
GW (gigawatts)
TW (terawatts)