As I say, Bill ... I already have a number of the cheaper (manual) versions. They're commonly known as "off switches".

But I'm just wondering what's actually inside those boxes. Not just a transformer, surely? Some sort of fancy SMPS, I reckon. All very well until it fails, and even something like that would itself dissipate (that is, waste)
some energy.
Well I'm damned ... just look at
this .pdf! A transformer switched in and out (bypassed) by a thermal cut-out? H'mmm. I had been imagining a voltage sensing circuit controlling trigger pulses to a triac, or a couple of SCR's, or something like that. Perhaps that's what it is. But maybe I
will make one for myself after all, Bill.