Human power and energy
I like to start my analysis of power and energy with human power since it is something we can relate directly to. Let’s start with some definitions.
Energy is the ability to carry out work. It is measured in Joules (SI units).
Power is the rate of doing work. It is measured in Watts (SI units).
When Sir Chris Hoy was in his prime, on his track bike apparently he had a max power output of 2300W, but he can only sustain this for 40 seconds, how much energy does he consume in that time?
Well let’s assume that he has a constant power output over this 40 seconds (or that 2300 W is the average over this time).
If this is the case, then if:
Power (Watts) = Energy (Joules) / time (seconds)
then rearranging this gives:
Energy = Power x time, so
Energy = 2300 x 40 = 92,000 Joules, or 92 kJ
Here is a graph of human power output capabilities over a range of time durations. A typical adult with a reasonable fitness level can typically average between 50 and 150 watts for an hour of vigorous exercise.
Food and energy
To put that into context, a Mars bar has 1004 kJ (240 kcal or 1 MJ) stored in it…more than 10 times as much! Check out other food calories data here or the NHS calorie page here.
To note, the energy burned by someone running a marathon is about 11,000 kJ (or 11 MJ, or 2600 kcal), which is about 11 Mars bars! It’s also interesting to note that this about the same as the typical recommended calorie intake consumed by a person in one day!
here’s a useful converter to convert from kcal (food) to kJ (standard international units for everything else).
Energy, power and appliances
OK, back to appliances….
understanding the power requirements of various appliances around the house is useful. Since I’m involved with the ‘usable’ design project, I’ve compiled a list of appliances around the house that heat. They are in a random order, have a think about how much power you think they draw from the mains and rank them from 1 (drawing the most power) to 11 (drawing the least power), then scroll further down this page for the answer.
| Appliance | Power (Watts) | Rank |
| Breadmaker | ||
| Kettle | ||
| Toaster | ||
| Toasted sandwich maker | ||
| Electric blanket | ||
| Milk warmer | ||
| Small oil filled radiator | ||
| Fan heater | ||
| Electric hob | ||
| Electric oven TOTAL | ||
| Electric oven |
Boiling water to make tea
I measure the power output of my Morphy Richards kettle (using a meter similar to this) when boiling 500 mL of water, it’s 2664 W (that’s the average, on the sticker it is rated as between 2500-3000 W), and it takes 74 seconds to boil the water.
So in this time, let’s work out how much energy it uses to boil that water:
Energy = Power x time, so
Energy = 2664 x 74 = 197,136 Joules, or approximately 197 kJ
Appliance power output
OK, so back to our appliance list, here’s the rank order with the power requirements…
| Appliance | Power (Watts) | Rank |
| Electric oven TOTAL | 11,000 | 1 |
| Electric oven | 5,000 | 2 |
| Fan heater | 3,000 | 3 |
| Kettle | 2,500-3,000 | 4 |
| Electric hob | 1,700 | 5 |
| Small oil filled radiator | 1,500 | 6 |
| Toaster | 1,200 | 7 |
| Toasted sandwich maker | 700 | 8 |
| Breadmaker | 600 | 9 |
| Electric blanket | 100 | 10 |
| Milk warmer | 60 | 11 |
Clearly, how much energy these things use depends on how long we run them for. An electric blanket does not draw much power (100 W) when compared with a toaster (1200 W), but we will keep the electric blanket on for many hours, while the toaster will only be toasting for a few minutes each day. Ok, the electric blanket may not be actually heating for the entire night, but you get the point.
