Energy Units

Joule

The joule (J) is an SI derived unit for measuring energy, and the related mechanical work and heat [1]. The joule in SI base units is 1 J = 1 kg⋅m²⋅s^-2 which is dimensionally equivalent to the newton-meter (N⋅m) and the watt-second (W⋅s) [1].

A good way to visualize an amount of energy is to look at it in terms of gravitational and kinetic energy.

• Convert energy to height in Earth's gravity using h = E/(mg) and a sample mass like 1 g, 1 kg, or 1 000 kg.
• Convert energy to speed using v = sqrt(2E/m) and a sample mass.

For example, 1 joule is not a lot, as it is only enough to lift 100 grams by about 1 meter, or the kinetic energy of a 100 gram object moving at 6 m/s [1]. Similarly, 1 kilojoule is enough to lift a 100 kg weight by 1 meter, and to launch a 5 g object at 632.5 m/s.

Heating water is very energy intensive due to its high specific heat capacity. For reference, raising the temperature of 1 L of water by 1 degree Celsius takes 4 184 J, bringing 1 L of water from room temperature (22°C) to boiling (100°C) takes about 1.46 MJ, and fully evaporating that litre would take an additional 2.26 MJ due to latent heat of vaporization [2].

The main unit of energy for measuring electricity consumption is the kilowatt-hour (kWh), which is equivalent to 3.6 MJ. In Ontario, current electricity prices vary between 8 to 18 cents per kWh [3]. The average household in Ontario consumes around 600 to 800 kWh per month or 2.16 to 2.88 GJ per month [4]. In terms of batteries, which convert chemical energy to electrical energy, an alkaline AA battery stores about 9 kJ of energy, a large smartphone battery of 5000 mAh at a nominal voltage of 3.7 V translates to 66.6 kJ, and the average electric car battery stores about 60.4 kWh or 217.4 MJ [2][5].