Even though your gas meter records your gas consumption in hundreds of cubic feet, your energy company is required to bill you
in terms of kilowatt hours (kWh). This seems to be an uneccessary complication, that is, until you need to consider how you might
compare your gas charges with your electricity charges (which have always been billed in kWh). A Kilowatt-hour is an amount
of energy equal to 1 killowatt (1000 Watts) operating for 1 hour. So when you consider an electric fire that will consume, say, 2 Killowatts
in an hour, against a gas fire that may consume 1.5 kilowatts an hour you can see which has the lowest running cost by looking at what
you pay for a kWh of gas and a kWh of electricity where you live.
This is, of course a ficticious example, given to just illustrate the point and not meant to prove that gas fires are cheaper to run.
Accordingly, for the purpose of analysis both gas and electricity charges are using a common unit which is the kilowatt-hour.

I am confused even more now, so what is the calorific value of gas, where does that come into it! and why does it vary?
The calorific value is a measure of the heat contained in the gas, it represents how quickly the gas can heat up water to a formula which is
applied to the gas when it is tested periodically at various points throughout the national grid system. In simple terms gas delivered to homes
near the point where it comes ashore contains a different level of moisture than the gas which is used say 50 miles from the shore.
Moisture in the gas affects it's performance and so the calorific value element should ensure that a hundred cubic feet of gas used inland, costs
the same to the consumer as a hundred cubic feet of gas consumed on the coast - for doing the same amount of work.

Phew! I wish I hadn't asked; so what happened to the good old Therm? Nothing really the British Thermal Unit is still used in science,and
industry but it is no longer very relevant to domestic household bills.

So how do we convert hundreds of cubic feet of gas to kilowatt-hours?
For this example take the current reading of your gas meter and deduct the previous meter reading from it. If your previous figure was an
estimate this estimated reading may actually be higher, in which case deduct the previous reading from the current reading. This does not affect
the arithmetic it just means either you have more to pay, or your energy company owes you something back.
Either way the difference between the numbers is net hundreds cubic feet of gas.
Take this result and multiply it by 2.83, this converts hundreds of cubic feet to cubic meters.
Multiply this result by a conversion factor of 1.022640 (may vary slightly between shippers) and then by the calorific value shown on your
last gas bill. Finally divide the result by 3.6 to give kWh.

As a very rough check a hundred cubic foot of gas is beween 28 and 31 kWh depending upon the conversion factors which are used.
Obviously the consumer has no option but to rely upon the accuracy of the suppliers declared calorific value and conversion factors.
We have no evidence to suggest that there are any irregularities, but this is perhaps an area which one hopes is tightly monitored by the
regulator Ofgem and the consumer body Energywatch, simply because it is subject to some variation between geographical areas.

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