Welcome to our forum.
I don't live in the hot south, but wouldn't your electrical costs be drastically lower in the hot summer anyway? I mean if it's 90+ degrees all the time, your heater sure won't be working very much, and all you'd be doing is running the filters. That can't cost all that much, could it? I suppose it would cut down on the routine of testing the water, but then again, not using the spa very much would lower water maintenance anyway. And the spa would be there if you did want to soak.
Some folks have a hard time getting the water temp down that low. A Fully insulated tub, sittting in the hot sun gets pretty warm. A lot of folks have recomneded propping the cover open to let some of the heat escape.
One of you math wizards do a calculation: How many pounds of ice are required to lower 350 gallons of water 20 degrees?