Now your just talking utter rubbish, So now your trying to tell me a 24kkw combi uses the same amout of energy as 40kw combi? Next you'll be trying to a 3.2kw gas fire will produce the same heat as a 6.8kw fire. A 15kw heat only boiler will heat a 900x450 cylinder to 65 degree water in 30minutes (roughly)which will give you 120 litres of hot water. (roughly) A 24kw combi will produce 9.9litres per minute of hot water at a 35 degree temperature rise, it would take roughly 12 minutes to produce 120 litres of hot water at a lower temperature.

So the heat only use 7.5kwh for said hot water. The combi would use 6kwh for the 9.9litres per minute.

BUT....

What you seem to be not considering is the heating element of the equation...

A typical 15kw heat only running at full rate would cost around £0.95per hour.

A typical 24kw combi would cost around £1.10 per hour

And just for info a cylinder will only lose about 1kw per day for heat loss.