Kilowatt-hours

When an electric device is running, we usually say that it is "using electricity". To use electricity is to convert electrical energy into other forms of energy - such as heat, or light. The higher the power a device has, the faster the energy is converted. What appears on your electric bill, and what you pay for, is the amount of electrical energy that's been converted during a month. How is that related to the power of your electrical devices?

And is there a simple way to calculate the electricity consumption for a specific device? Let's go back to the units used for energy and power! The most common unit for energy is Joule. When we say how fast the energy is used - how many Joules are converted every second - we've stated how many Watts of power that device has. The unit 'Watt' represents how fast energy is being converted, while the unit 'Joule' tells us how much energy has been converted, in total.

When we know the power of a device - the number of Watts - we can calculate how much energy it uses during a specific period of time. Let's look at a couple of examples. Here's an electric radiator, with a power of 1000 W, and two lamps: an older type of light bulb with a power of 100 W, and a newer type of "low energy" light bulb, that only uses 10 W. If these three are running for an entire day, 24 hours, how much energy will they each use? The number of Watts tells us how many Joules of electrical energy is converted every second.

24 hours is 86,400 seconds. How many Joules will they use, if they are left running the entire time? The radiator uses 1000 J every second for 86,400 seconds, that is... ... about 86 million Joules. The older light bulb uses 100 J every second, and the newer one uses 10 J every second.

In 86,400 seconds, those amount to about 8.6 million Joules for the older light bulb, and about 860,000 Joules for the newer one. We reach large numbers pretty quickly when calculating the energy consumption in Joules. There is another unit for energy, that's commonly used for measuring electricity consumption. Now, we'll measure the power in thousands of Watts - in kilo-watts. Then, the radiator has a power of one kilowatt, the older lightbulb 0.1 kilowatts and the newer one 0.01 kilowatts.

Then we'll multiply each by the time in hours instead of seconds. And the time was 24 hours. The consumption of the radiator is one kilowatt times 24 hours - 24 kilowatt-hours. The light bulbs use: 0.1 kW... and 0.01 kW...

for 24 hours. Their electricity consumption will be: 2.4 kilowatt-hours... and 0.24 kilowatt-hours. We take the power of the device in kilowatts... ... times the time in hours, to get the electricity consumption in kilowatt-hours.

The words: "kilowatt" and "kilowatt-hour" sound similar... but they represent different things. The unit kilowatt tells us how fast the energy is converted. It is a unit of power. Kilowatt-hour is a specific amount of energy.

If we add together all the electrical energy that an average family uses, except for heating... ... it amounts to between 10 and 15 kilowatt-hours per day. Devices that have low power can run for a long time without reaching a high total consumption. Such as the low-energy light bulb, a cell phone charger, or a radio. These can run for several days without reaching even one kilowatt-hour.

A device with high power on the other hand, will quickly run up a huge electricity bill! Luckily, devices that have a power of around a kilowatt or more - appliances like ovens, electric kettles, or vacuum cleaners - usually only run for a few minutes at a time. And both ovens and radiators have thermostats that switch the current on and off. The number of kilowatt-hours an appliance uses is calculated by multiplying its power in kilowatts - thousands of watts - by the number of hours you run it for.