We measure energy in different ways, though you can convert one to another. Electricity and gas are measured in kilowatt-hours, for example, while scientists prefer measure energy in Joules. But since there are 3,600,000 joules to a kilowatt hour, you can see why we use different units. Another unit in common use when applied to food and sweets is the calorie. A scientific calorie is 4.2 Joules, which is a tiny amount, so when your Mum or Dad talk about food or dietary calories, they really mean kilo-calories. A kilo-calorie is 1000 calories.
The next time you have a bar of chocolate, which if you’re well-behaved may be quite soon, you can read the small label to find out how many kilo-calories it contains. A full size Mars Bar, for example, holds 260 kCals of energy, and that is equivalent to 300 watt hours. Since that’s a useful amount of energy, I propose that we use the Mars Bar as our universal unit of energy, and then we can compare the different amounts of energy we need to do different things.
Here are a few measurements for comparison:
1. An i-phone battery holds about 6 watt-hours – which is a fiftieth of a Mars bar
2. A normal car uses 1 kWh to travel a mile, 3 Mars bars per mile, or a third of a mile per Mars Bar.
3. A school bus uses 10 Mars bars worth a mile, or a tenth of a mile per Mars Bar. If that bus were carrying 40 cubs, that’s 4 cub-miles per Mars Bar.
4. A Twizy does nearly 3 miles to the Mars Bar.
5. Riding a bike takes about a Mars bar of energy for every 15 miles.
6. A grown-up can walk about 3 miles on a Mars Bar.
7. An electric kettle takes just over a third of a Mars Bar of energy to boil a litre of water (about 4 cups of tea).
8. A 7 year-old cub consumes the equivalent of 6 Mars Bars a day of energy just to live. A Twizy can do 18 miles on that amount of energy, which is also enough to pedal a bike 90 miles. However, if you plan to cycle 90 miles tomorrow be sure to eat the equivalent of an additional 6 Mars bars to avoid getting seriously tired.
[The Talybont Energy Turbine, when going flat out, can theoretically generate energy at a rate of 36kW. So in a second it produces 36,000 Watt-seconds of electricity. That’s 10 watt hours in a second, or 600 Watt-hours in a minute. So it would be generating about 2 Mars Bars of energy a minute, or 2,880 Mars bars a day].