The water heater will generally draw the same amount of current on both legs of the 240V circuit when it is operating. This means it is possible to determine how much energy it uses by monitoring only one leg, and then doubling the energy reading.
If your Shelly is reporting the proper voltage for that one leg, and then reporting a kW reading which matches the kW reading from the water heater’s app, that’s very odd.
The shelly is reporting the correct voltage and 1/2 of the wattage when the element is ruining so it should be correct to double the wattage in HA.
But if you look at the consumption for Wednesday the water heater app thinks it used 8Kwh while the shelly reported 6.41Kwh (water heater) 2 x is 12.82Kwh (Water heater energy)
That’s odd… can you be certain they are using the same definition of ‘day’? It’s possible one of them starts the day at midnight and the other at 6AM (or something), for example.
I have this same issue – monitoring power on a 240V HVAC system, but powering the Shelly with 120V. I could make a custom card to display in HA. Kevin mentions that the Shelly has to be powered by 120V, but Shelly specs say 110 ~ 230 +/- 10%. So I am thinking that a dumb but simple workaround is to use a travel transformer that will step up 120V to 240V and power the Shelly with that. That it will always be correct via HA or the Shelly App. Here is an example ($15):
Any reason not to do this?
