I should have let this go, but I remembered being told by one of teachers of the heat pumps at the Festival of Britain. Two ex-Spitfire engines powered the heating system in the Royal Festival Hall. A wee bit about it here.
“Heating systems
A ground source heat pump was used to heat the building in the winter and cool the building in the summer. Water was extracted from the River Thames below Hungerford Bridge using a centrifugal pump. Heat was extracted from the river water using a heat pump. The compressors were driven by two Rolls-Royce Merlin engines, adapted to run on town gas. It was highly successful, providing both heating and cooling for the Hall, but over-sized, and was sold off after the Festival of Britain.”
placeandsee.com/wiki/royal-festival-hall
I expect Mr CF to work out how much energy the 2 engines used, and whether simply burning that amount of gas would overheat the hall as the heat pumps did.
(Hint…it wouldn`t!)
Summat For Nothing? Nope.
And for the TNUK audience 
:
A Gardner 6LXB or even a Rover V8 ticking over will easily power a house or more. Burning the same amount of petrol or diesel they would use, wouldn`t.