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I'm building an off-grid summer cabin with a solar PV system. I'm planning on using this equipment:

  • 3 kWp PV panels
  • BlueSolar 150W/70A MPPT charge controller
  • Victron Multiplus 3000W inverter
  • 4 x 303 Ah AGM batteries (probably as 48 V)
  • A 2 kW water heater (100 liters / 25 gallons) using normal 230 V AC energy (*see below)

When the sun is shining and the batteries are full, I'd like to divert excess energy to bathroom floor heating. It would keep the bathroom a bit more dry, and feel nice. So, it's mostly a nice-to-have feature.

Is there a relatively inexpensive and simple way to build this behavior? I'd prefer a solution that would not stress the batteries. Can I use low-voltage DC (eg. 48..54 V) floor heating? How should I connect the system?

*) Why use electricity to heat water? Yes, the efficiency is poor, but PV panels are cheap and I want to have 3 kW for other purposes anyway. Compared to solar thermal panels, this system is a lot cheaper and simpler - and fulfills my needs. (And why use 230 V and not low-voltage DC for it? I'd like to use off-the-shelf hardware, for eg. insurance reasons.) I also plan on having the system switch off the water heater circuit when there is other load in the system (eg. a washing machine).

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    \$\begingroup\$ I am not sure what you mean by "easy" but a lot of generator managemant systems have the ability to connect a dump load \$\endgroup\$
    – PlasmaHH
    May 29, 2016 at 19:43
  • \$\begingroup\$ Thanks - I changed "easy" to "relatively inexpensive and simple". If the solution would cost hundreds of euros/dollars/etc, it would not be worth it. If it would need significant amounts of maintenance, it would not worth it. \$\endgroup\$
    – korona
    May 29, 2016 at 20:16
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    \$\begingroup\$ In a summer cabin, when ambient air temperature is not too cold, solar hot water is super cheap and super easy, and probably around 80% efficient. You can just coil black plastic irrigation tubing and cover it with plexiglass. \$\endgroup\$
    – user57037
    May 30, 2016 at 3:44

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Dump is a commonly available feature of better solar charge controllers. You'll have no trouble finding such a setup.

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