This may sound stupid, but it sounds like you should run less efficient pumps. I use 2 Mag 18's from Danner / Supreme externally on my 240 acrylic. They are not very efficient. I keep my house around 70 in the winter and 76 - 78 in the summer. I don't need a heater!. In the winter my tank stays at 79 and in the summer I have a fan running over my sump trying to keep it below 86. They are not very expensive pumps so it might be worth getting an inefficient pump. I am building a 700 and bought Laguna 4200's (2) that are very efficient. I am going to heat it off of my gas hot water tank. That will have about an 18 month payback for me.
In the summer time your pumps are overheating your 240G, so yes, that is a waste of electricity. The fact that you have to run a fan to cool your tank down adds to your electric consumption.
Running 2 Mag 18's is like having a 290W heater on 24/7. It sounds like in the summer your heaters wouldn't even need to come on at all; you are essentially wasting ~209KWHrs per month in the summer months by running those inefficient Mag 18 pumps.
If you lived in an area that was always cold what you are doing might make sense, but if your summers are warm it's a waste of electricity; run efficient pumps and an appropriately sized heater(s) and you will be better off. By doing it this way, you are only adding the energy required to maintain your desired temps (no more, no less) which will always be more efficient than trying to deal with pumps overheating your system.