A few comments (some of which have come up before regarding HPWH & 208/240V monitoring):
e.g. Remote sensors for 240V - #4 by ixu
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I took apart a Wemo thinking “How hard could it be to hack this thing?”. <<Don’t do it>>. The key thing about UL listing is non-modification and no matter what you do you’re going to end up with something at least potentially more dangerous. Even unmodified a Smart Plug certainly adds more potential danger to an electrical system as does pushing current through anything “small”.
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IF (and only if) you were running a HPWH in HP mode only (i.e. not using the 4kW-ish elements) you could theoretically connect it to 120V only and use a regular Smart Plug.
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Have you chosen/got a tank yet? @andy has recommended Vaughn and it seems they have a hybrid with nice specs that can actually be built single-phase!
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The beauty of such things as heat pumps is, as we all know, that they use less energy. They also, by their nature, can use less wire and so less copper. This is of course a pretty irrelevant observation in the scheme of things but I think philosophically it helps. Meaning: If you can go with a lower energy feed, do it! I migrated some of my lighting to using a Power over Ethernet switch (ports limited to Class 4, 25W) and strung things using Category 6 cable. Having run quite a lot of conduit and BX in my life there’s nothing like throwing a few skinny cables around knowing they are safer and encouraging lower energy use … and actually getting more convenience. I also, btw, can per-port monitor the consumption at the PoE switch at 10mW resolution.
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This thread has the potential to go over well-beaten territory on a 240V “Sense Plug”. @kevin1’s advice is currently the best option for low energy devices (<<1800W) but won’t help you on a “standard” HPWH (with resistance element mode). And btw neither will a hack (due to the high energy load). Here’s my very recent reiteration of a reiteration on the topic with my solution: