Who is Prepared for a Week Long Power Outage

[Dr. Spa™ Ret.]

The other sides of the tub are insulated enough that the residual heat in the main body of the water will keep them warm enough to not freeze.

[rin-spa-aic]

I'm ready. The first thing I would do is nothing for about 3 days.  I do have a generator that I would carry enough capacity for me to run the tub heater for a few hours at a time.

I would also personally go with this approach suggested by wmccall.  You don't need the tub fully functional, just keep the water warm enough for a few days.

I've seen the space heater suggestion previously either here or other forums. IMHO you could run into issues with this on a fully foamed tub. If it's thermopane it could work out fine for you though.

Another suggestion is a submersible pond heater laid at bottom of tub (not floating type). Not sure if the convection would apply, but I think it could keep it warm enough throughout to prevent heating since the tub shell does radiate heat.

[SerjicalStrike]

I've seen the space heater suggestion previously either here or other forums. IMHO you could run into issues with this on a fully foamed tub. If it's thermopane it could work out fine for you though.

Another suggestion is a submersible pond heater laid at bottom of tub (not floating type). Not sure if the convection would apply, but I think it could keep it warm enough throughout to prevent heating since the tub shell does radiate heat.

What issues would you run into in a fully foamed spa?  We have been doing it for years now with 0 issues.  You are only trying to keep the equipment bay warm, and they have a thermostat on the heaters, so they don't get too hot.  Thermopane would be harder to keep warm as you would need to warm the entire cavity between the skirt and the shell. 

Keeping the water in the main body of the tub warm isn't the issue in freezing weather.  It is the exposed pipes.  With no circulation, exposed pipes would freeze even with the main body of water warm. 

[rosewoodsteel]

Imo, with a thermopane tub, you would ensure that all of the plumbing is kept warm with a space heater.  If it is insulated properly, there should be little effort in keeping the cavity warm.   The space heater should work, as well, with a full foamed tub, imo.