I would agree that the bigger unit would probably cost more to operate. But I'm not sure you're drawing the right conclusion about your new a/c. You said your 2 1/2 ton couldn't keep up on days above 85. Presumable your new 3 ton is keeping up. So the 3 ton is actually doing more work and producing more cooling that your 2 1/2 ton. So it's not an apples to apples comparison.
Originally posted by berad
It depends on the run time variable. I would have to say yes the bigger unit would still cost more to operate. I say this based on my own home. I bought it and found the first summer the a/c didn't keep up on days above 85. I made sure the duct system was ok and wnt through the unit and made sure everything was right, it was. I ran a load calculation, the nit was 1/2 ton to small. I Replaced everything, went from an 8 seer 2 1/2 ton to a 12 seer 3 ton and my operating usage is nearly identicle. If the new unit was an 8 seer like the old one my costs would be higher now.