We are rapidly approaching the point where we have retrofitted all lighting to be LED and all motors to be driven by VFDs, eventually we will reach a point where we’ve maxed out efficiency and new power generation will need to be built, which will push energy costs up.

Propane is cheap but it has a couple drawback. Flammability, and the refrigerant must be pure so no mercaptan can be added which means if there’s a propane leak, nobody will smell it. This is more of a problem for commercial and industrial units than residential.