I would attack this from a different angle. The spa already has a system that controls power to the heater and pump(s). You can intercept the low voltage signals that control those, or you might look at upgrading the control, I'd expect there are models available that can be programmed to run during certain hours. Mine has an "eco mode" that only heats during the scheduled filter cycle and that can be programmed to run at a specific time and it's pushing up on 30 years old so nothing new there.
James_s - Thanks, I gave that some thought. All of the relays are mounted on a circuit board and there are four different clean, heat, filter programs. Just way to complicated. Far easier to just install a high current relay after the GFCI and interrupt the mains power to the spa. The spa's circuits have memory so the temp and other settings will all remain the same. (I had another spa where the settings were not retained on power loss.)
In looking at posts in other forums getting others are doing exactly the same thing I am doing for AC units, electric heaters, pool pumps, electric hot water heaters etc.
Not sure if you read my other post, but the power company will change the amount of money they charge for a kWhr 5 times today for $0.12 to $0.54.
This $10 relay will save me over $1,000 in just 1 year.