It’s easy to look at your monthly energy bill and feel completely lost. What are all these numbers and labels and how do they relate to your day-to-day gas and electric usage in your home? Even more basic, how is energy even measured and calculated?

In short, it all comes down to the joule. But just what is a joule, anyway? And more specifically, what is a joule equal to in terms of your energy usage? It’s all fairly easy to understand once you break it down—and we’re here to do just that. Let’s explore the world of joules, gigajoules and energy to find out.

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 the energy needed to heat a litre of water.

It’s important to note that when it comes to what you see on your energy bill, you will only see joules (and gigajoules) to measure your natural gas use. Your electricity provider will use kilowatts and kilowatt-hours, which are based on joules, to report on your electricity use. Let’s get into the finer details of what a joule is in the context of the electricity and natural gas to better understand how it all works.

As far as your electricity bill goes, the definition of a joule is the energy needed to emit one watt of power for one second (watt-second). To put it in practical terms, let’s say you have a one-watt lightbulb. The lightbulb would consume one joule of energy to stay lit for one second, and 60 joules to stay lit for one minute.

The equation for a joule is as follows:

1 joule = 1-watt X 1 second

Which means:

1 watt = 1 joule/second (one joule per second)

As mentioned above, joules are part of the equation for calculating your electricity use but are not the main unit of measurement you’ll see on your electricity bill. We’ll go over how joules factor into the kilowatt-hours that show up on your monthly energy bill a bit later.

When it comes to your natural gas bill, the definition of a joule applies to thermal (heat) energy. In other words, how much energy it takes to raise the temperature of your home’s air or water, for example.

For context, it takes 4200 joules to increase the temperature of one litre of water by one degree Celsius. When your water heater, for example, burns natural gas, it’s transferring joules of energy from the natural gas to your water in the form of heat.

So, what is a gigajoule? The prefix “giga” means “billion,” so a gigajoule (GJ) equals one billion joules. This may seem like an astronomical figure, but a single joule is actually a very small energy amount. You’ll see GJs on your monthly gas bill since utility providers charge for natural gas on a per-GJ basis.

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!

The average Alberta home uses about 600 kilowatt-hours of electricity per month. So, how many gigajoules is that equal to? The equation for converting kWh to GJ is simple:

1 GJ = 0.0036 X kWh

Using that equation, we can figure out the average Alberta home uses 2.16 GJ of electrical energy per month. However, you’ll see your electricity use measured in watts, kilowatts or kilowatt-hours, not joules.

Watts measure power, which is the rate at which electrical devices use energy. A kilowatt (kW), which you’ve likely seen on your utility bill, is equal to 1000 watts, or 1000 joules per second. You may have also seen kilowatt-hours (kWh) on your electric bill, which are units that help your utility company measure the amount of power you use on an hourly basis.

When comparing kW vs. kWh with your electric appliances in mind, remember that kW stays constant and kWh changes depending on your use. For example, your vacuum will always require 1 kW of power to function, but you may use 3 kWh of energy in one month—or 10 kWh if your floors are really dirty. The kWh amount takes individual use into account since you are charged for electricity usage according to price-per-kWh.

As stated above, 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. In fact, 61% of the average Canadian household’s energy goes to heating the home.

But when you look at your furnace and other heating equipment, you might see a different unit of measurement: British Thermal Units (BTUs) per hour. BTUs per hour measure the heat output of your furnace. One BTU is equal to the energy required to raise the temperature of one pound of water by one degree Fahrenheit. For the purposes of calculating your natural gas use in joules, 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 by doing the following calculations:

100,000 BTU/hour X 100 hours = 10,000,000 BTU

10,000,000 BTU X 1055 joules/BTU = 10,550,000,000 joules

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 gigajoule consumption (e.g. 10.55 GJ) by the regulated natural gas rate.

Another way to think about a joule’s measurement is to examine it through a broader lens. The average Canadian household that uses natural gas consumes about 88 GJ per year. Interestingly, of all the provinces, Alberta uses the most. On average, our customers consume about 119 GJ of natural gas annually.