My 15 year old son is rather excited at the prospect of Adelaide’s first Krispy Kreme factory. This factory will be pumping out 5,000 donuts an hour.
Now a donut contains about 1000 kJ of energy. This is chemical energy, in the form of fat and sugars and carbohydrates. Our bodies are designed to run by “burning” this chemical energy. If we don’t need more energy when we consume the donut then some of the excess will be stored as fat.
Now energy comes in different forms, for example as electricity, mechanical, solar, thermal, or potential energy. It’s possible to convert between one form and another using a machine, for example a petrol motor converts chemical into kinetic energy. A solar panel converts the energy in the suns radiation to electricity.
Energy is measured in Joules (J), lots of energy in kilojoules (kJ), or megajoules (MJ). Power is the rate we use (or produce) energy. If I use 1 J/s in my LED torch, that is 1 Watt (W). My electric car uses 5 kW when I cruise along at 60 km/hr. So 5,000 J/s is moving from my batteries to the electric motor of the car.
The average human uses 8700 kJ per day. That means we need to injest roughly 8700 kJ of energy, and our body uses about the same amount of energy. This energy runs our body, and gives us some energy for moving about. There are 24(60)(60) seconds in a day. So the power consumption of the average human (energy/second) is 8,700,000J/(24(60)(60))=100W. About the same as a large incandescent light bulb.
So as we know the energy in a donut, and the rate at which the donuts are produced, we can measure the Power Output of the Krispy Creme factory. Then compare that to all sorts of other power producers and users in our lives.
Here are a few energy equivalents (spreadsheet):
A related analysis is Fuel Consumption of a Pedestrian Crossing.