I have old PTAC units with an 8.6 EER rating. If l purchase a modern one with a 13.3 rating, could the same mathematical anology be made as comparing a car that gets 8.6 MPGs to a car that gets 13.3 MPGs? I'm trying to determine if it is worthwhile to junk three old units and replace them with energy efficient models. I estimate that the heating and cooling component of my electric bill is about $1000 a year based on comparisons in electric bills for months l don't run the AC or heat such as May. I can get 3 brand new PTACs shipped for about $2200-2300 if l install them myself. We had an unusually cold December in the Northeast and my usual $70 to 80 dollar electric bill for December skyrocketed to $300. My inclination is that new units will pay for themselves in under 10 years, but l need to determine if my math is correct.