This is why when I was talking to a solar hot water installer about heat pump water heaters he got worried, 550 watts to run the heat pump, they claim with the other elements it's around 5kwh per day... seems kind of high so with those specs you would need over a kw of pv, but compared to the cost of solar hot water panels, pumps, plumbing, fluid... and they still recommend having a back up for those, this the back up would be a switch.
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