But from purely mathematical point of view - if the output > input, it's more than 100%

Indeed, but a heat pump has two energy inputs, its electrical power supply and the heat absorbed from the cold side.

The two summed is the energy output, which cannot be greater than the sum of the inputs.

If those Hydlon heaters can handle the flows needed then they will do the job fine even without the claimed over-unity efficiency.

Do watch for the power requirements, 150A single-phase is not available as a domestic supply in Thailand.

Decided to go for the proven quality of Stiebel. Cannot justify the extra $500 investment in a technology that is not proven. If the company is still around in 8-10 years, when time comes to replace the tank heater, I will definitely consider purchasing their unit.

Meanwhile, I've settled for the German pipe and fitttings ("pimaflex"). It is by far the best option available on the Thai market now. They also sell pipe insullation, which I plan to use for the hot water pipes.

My only problem is that I cannot find manifold to organize my piping. I really like the plastic Viega PEX manifolds (manabloc) or the Uponor ones, but they are only compatible with their own PEX pipes. So, the only option I've found so far is to use brass manifolds lile this one:

Does anyone know where I could get these in Thailand? They are usually made in China.

Looks as though InvadeIT.com no longer has the tank style electric. Anyone have another source, Khon Kaen preferred.

Not sure what you want a manifold for, but can't you just use a few tees and close nipples?

No need to insulate hot water pipe.