I am currently using 2 x Shelly 1PM devices, 1 on a 3Kw 150L geyser and 1 on a 2Kw 150L geyser(I changed the element from 3 to 2). Both have been working well so far the 3Kw geyser actually pulls 2780watts and the 2Kw pulls around 1700w, I only run them during the day as I have a grid tied solar system with no batteries, so make use of the solar power to heat the geysers. Might look at a Shelly EM to measure when I have excess solar to heat the geysers.
