Hi force9BOAT,
I like your idea of putting your extra energy into an RE heater to your tank, but I'm concerned that what you are thinking might cause your fish more harm than good. Skipping all the AE aspects of this thread, I think you need a much bigger heater.
(Momentary digression: Ok, in deference to the previous discussion you can also put on lots of insulation. Personally, though, I think saltwater tanks are far too gorgeous to hide in insulation. Either you worry about energy or you shell out the big bucks to outfit a 100 gallon saltwater tank. Doing both reminds me of people who pay lots of money for cars that are so fancy that they become afraid to drive them. Ok, the flameproof suit is now on and yes, I'm sure someone out there has done a pretty job of insulating their tank. I just haven't seen one yet. ;-)
Saltwater fish, the corals, and all those other beautiful invertebrates are notoriously finicky about the temperature needing to be constant. Unless the room your tank is in is consistently very warm -- such as 75-78 degrees -- a 100w heater is far below the recommendation for a saltwater tank of your size. Even if it was a freshwater tank, where the fish don't mind temperature fluctuations as much, the normal recommendations would have you using about a 200-250w heater. (I have a 75w heater on my comparatively small 29 gallon freshwater.) Since salts are more finicky, most recommendations are for near double that power, 2.5 to 5w per real gallon, to make sure that the temperature doesn't fluctuate. Presuming that you have around 90 real gallons of water after the gravel volume is subtracted out, a heater in the 250 to 450w range would keep the tank at the nice consistent temperature needed to have a healthy saltwater setup. A heater of this size won't be on 7x24, and yours being on 7x24 shows that it is too small, but it will have the ability to keep the water temp at a nice steady temperature.
Ok, so back to my original statement that adding an RE heater set a few degrees higher could be problematic for your fish. Having a heater setup designed to allow the tank to vary from 77 to 80 degrees and back depending upon the current wind is a temperature variance far beyond what I've ever seen recommended for a saltwater setup. Besides adding a more powerful heater on your grid-based side, what I'd suggest is that you set the RE heater just a tiny bit (half degree?) above the grid-heater setting. As long as you have a very good filtration flow in the tank to distribute the heat from the RE-heater, even with that little temperature gap I'd venture that the grid-heater would rarely turn on while the wind is blowing. This will, admittedly, not absorb as much RE as a 3-degree swing would.
One more comment -- be careful about the notion of the heaters never needing to turn on during the summer months. "Heater never turning on" = "water above the target temperature" so make sure that you aren't accidentally cooking the fish. One family I know that has a beatiful saltwater tank in their bedroom (100 or 120 gallon) has a window AC that is on 12 months out of the year to make sure that the room never gets warm enough to cause the tank to heat up. (The AC doesn't cycle many times during a New Hampshire winter, but it is on just in case a sunny day overheats the room.) They want the tank's heater to occasionally come on to hold it at the right temperature. That might be a little bit of overkill (and potential heresy on this board ;-), but it does show how concerned some saltwater owners are about the temperature.
Craig