I'm looking at a problem where I have two different heat generators supplying thermal energy to a building. The first heat generator provides 80% of the building's heating load and has an efficiency of 95% (natural gas boiler) whilst the second heat generator provides 20% of the building's heating load and has an efficiency of 500% (this is an air source heat pump).

I want to calculate the overall efficiency and I have gone about it the following way:

Heat Generator 1: 0.95 x 0.80 = 0.76

Heat Generator 2: 5.00 x 0.20 = 1.00

Overall Efficiency = 1.76

I am not sure if this is the correct way of calculating the combined overall efficiency. Can someone please review and comment on whether this is the correct method.

Thanks