Calculations
Just thinking....
18.7 million kwH is about 6.7 x 10^13 joules
The heat capacity of the atmosphere is about 1000 joules/kg/degree K
The total mass of the Earth's atmosphere is about 5 x 10^19 kg
It would therefore take 5 x 10^22 joules to raise the whole atmosphere one degree K
18.7 million kwH would raise the temperature of the whole atmosphere by about 1.3 x 10^-9 degrees - around one one billionth of a degree.
Don't think I'll lose any sleep over that one.