What is a joule?

A joule is a measurement of energy. The definition of a joule can be slightly different depending on whether you’re measuring electricity or natural gas use, but at the end of the day, it’s all energy. For example, you can use joules to measure the energy needed to power a lightbulb or heat a litre of water.

When you look at your energy bill, you’ll see gigajoules (GJ) for measuring your natural gas use. Your electricity will be measured in kilowatt-hours (kWh); those kWhs are determined by joules and used to report electricity use. We will break that down further in this article.

What is a gigajoule?

The prefix “giga” means “billion,” so a gigajoule (GJ) equals one billion joules. This may seem like an insane figure, but a single joule is actually a tiny measurement of energy! You’ll see GJs on your monthly gas bill since utility providers charge for natural gas per GJ used. 

One GJ of natural gas has the same amount of energy as:

• 39 litres of propane.
• 27 litres of fuel oil.
• 26 litres of gasoline.
• 277 kilowatt-hours of electricity.

As you can see, natural gas is an energy-packed resource! Read more about understanding energy consumption in GJ.

Joules and your electricity bill

Regarding your electricity bill, the definition of a joule is the energy needed to emit one watt of power for one second (watt-second). So, let’s say you have a one-watt lightbulb. The lightbulb would consume one joule of energy to stay lit for one second.

1 joule = 1-watt X 1 second

Measuring your electricity by the second will give you some crazy numbers, so electrical utilities will measure it by the hour, or kilowatt-hour (kWh). You’ll see the amount used during a billing period on your bill in kWh. You can learn more about kilowatts vs. kilowatt-hours.

To calculate how much you pay for the electricity you use, multiply your total kWh used by your electricity rate.

Joules and your natural gas bill

Regarding your natural gas bill, the definition of a joule applies to thermal (heat) energy. Your bill will show the energy you used to raise the temperature of your home’s air or water, for example.

Natural gas suppliers issue natural gas rates on a per-GJ basis. When considering which home appliances require the most natural gas, your furnace is probably the first that comes to mind. Natural Resources Canada says 63.6% of the average Canadian household’s energy goes to home heating.

What are BTUs?

If you look at your furnace, you might see that it measures energy using British Thermal Units (BTUs). BTUs per hour measure the heat output of your furnace, and one BTU equals 1055 joules.

So, if your furnace has a rating of 100,000 BTU/hr. and you run it for 100 hours per month; you can calculate your natural gas use in gigajoules like this:

1. 100,000 BTU/hour X 100 hours = 10,000,000 BTU
2. 10,000,000 BTU X 1055 joules/BTU = 10,550,000,000 joules
3. 10,550,000,000 joules 1,000,000,000 joules/GJ = 10.55 GJ per month

From there, you can estimate your monthly natural gas costs by multiplying your monthly GJ use by your providers natural gas rate.

Read more about how much natural gas Canadians are using.