You can measure this at the meter.
There are two main types of meter in use:
1) Imperial meters, where there’s a dial with a needle going round marked 1 cu ft
Measure the time it takes to do one full revolution, in seconds, call that “T”.
Work out 1092 divided by “T”.
That’s how much you’re using, in kW.
So if it takes 40 seconds, you’re burning at 27.3 kW
2) Metric meters. These have numeric displays, measuring in cubic metres.
One cubic metre would read
(0’s then) 1.000 on the display, the decimal point is always 3 digits from the right.
Measure the usage in 2 minutes, using the rightmost 3 digits. Remember it might go past zero. You’ll have an answer like
0.XYZ cubic metres.
Multiply that figure by 321
That’s how much you’re using, in kW.
So if you’ve used
0.085 cubic metres in 2 minutes, that’s 27.3 kW
Appliances use more gas than they produce energy, because they aren’t 100% efficient, so the figures above might be for a “24kW” boiler.
Be aware that if the boiler can’t get rid of the heat fast enough, it might well “modulate” which means turn itself down.
Pro’s will argue about the precise figures, becuase they do depend on the “calorific value” of the gas, which varies. It’ll be stated on your gas bill, in MJ/m³.
(Pics from Viper)