This is how to interpret a British Gas gas bill.
Your gas meter measures cubic metres of gas flowing through it. (The old ones used to measure cubic feet; but the gas companies switched to proper measurements longer ago than the lifetime of a gas meter, so they should all be cubic metres by now.) But your bill is calculated on the number of kilowatt-hours of energy liberated from the gas.
On your bill, there will be a calculation such as this:
metric units used 239 * calorific value 39.3572 * volume correction 1.0226400 / convert to kWh 3.6 = gas used in kWh 2448.43
So what do the figures mean?
- metric units used is the number of cubic metres of gas measured by your meter.
- calorific value is the number of megajoules of energy in a cubic metre of gas, at some specified combination of temperature and pressure.
- volume correction is a “fiddle factor” to account for seasonal variations in air pressure and temperature. When it’s cold, the molecules are more densely packed, so you get more energy in the same space. When the atmospheric pressure is low, the gas molecules are free to jiggle about more, so you get less energy in the same space. Each molecule contains a certain amount of potential energy stored in the bonds between atoms. So the calorific value depends on pressure and temperature, which is why this calculation is required.
- conversion to kWh is because there are 3.6 megajoules in a kilowatt-hour. (A watt is one joule per second; there are 3600 seconds in an hour. Kilo means * 1 000 and mega means * 1 000 000.)
Given these figures, you can work out how many kWh are in one cubic metre of gas, simply by doing the same calculation for 1 metric unit used. According to the above, it is 11.23 kWh per m³; but this figure may well be different for you, as the volume correction is seasonally-dependent.
But that’s not the whole story. The price per kWh depends on how many units you have used, with the first few kWh each day being more expensive. There will be a section like this:
2448.43 kWh used over 103 days
First 756.00 kWh @ 6.846p = £51.76
Next 1692.43 kWh @ 3.275p = £55.43
Total cost of gas used = £107.19
The higher rate applies to 756.00 kWh over 103 days, and adds an extra cost of 3.571p per kWh. As long as we actually use up all the “expensive” kWh, we will end up paying a fixed extra amount each week. If we are allowed 756.00 kWh in 103 days, then that is equivalent to 756.00 * 7 / 103 = 51.38 kWh in 7 days; which will cost us an extra 51.38 * 3.571p = £1.83 what they would have cost if we had only been paying 3.275p for them.
Meanwhile, one cubic metre of gas on the meter gives us 11.23 kWh and so costs us 36.78p. So, our weekly bill will be equal to £1.83 plus 37p for each m³ of gas used.
If you ask your gas supplier nicely they will send you a payment card, which can be used at local shops to make payments towards your gas bill. All you need to do is to work out the cost of the gas you have used each week (basing it on the figures from your last bill, which probably won’t be the same as mine), and round it up or down to the nearest whole pound.