"Art Todesco" wrote

http://www.nordyne.com/literature/8569%20Compr.pdf
The best would be to compare the electric meter with the unit running and
not running. This way you will have actual watt-hours and you don't have
to think of things like power factor, etc. But, it will change depending
on temperature. And the run time will vary when the temperatures are
cooler than when it is just plain hot outside.

Many electric bills give a 13 month use history. Mine even gives the
average temperature for the month. Look at months like April and October
when neither heat or AC is used much and compare to July and August and you
get a pretty good idea. Aside from that, the only accurate method is a
recording ammeter over time.