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I am running a PV-Solar system together with a heat-pump in Germany. Excess electric power is fed into the grid, however I am only allowed to feed in 70% of the peak capacity. During solar noon, I find myself with around 2 kW of excess power capacity which I would like to put into my warm-water tank via a resistive 2 kW heating element (which has been installed when the entire system was put in).

I assume that I know how much excess electricity I have via the Bus-interface of the PV-inverter which I read out with a suitable microcontroller or single board computer: What element or what system can I use to put 0-2 kW of power into the resistive heating element which I can control over said microcontroller or single board computer? Perhaps some kind of dimmer or controllable AC/DC element?

To clarify: I don't know which electronic "building block" is right for this type of application.

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    \$\begingroup\$ 2kW of excess energy - that is power not energy; you can't have excess power. \$\endgroup\$
    – Andy aka
    Commented Mar 1, 2022 at 10:44
  • \$\begingroup\$ I can imaging the "warm" water getting to over 95°C if there is no thermostat to prevent that. Does the hot water come out of temperature-limited mixer taps? \$\endgroup\$ Commented Mar 1, 2022 at 11:03
  • \$\begingroup\$ @Andyaka: Why can't you have excess power? The PV system could generate more power then I can use, therefore it throttles down via its mppts. I do not understand why that does not qualify as excess power? \$\endgroup\$
    – U_flow
    Commented Mar 1, 2022 at 12:11
  • \$\begingroup\$ @AndrewMorton: I did not clarify. The same Microcontroller/SBC which monitors the excess power of the PV system, also monitors the water temperature, so as to not overheat. I did not mention as I deemed this bit of information as irrelevant to the question thank you for mentioning this. \$\endgroup\$
    – U_flow
    Commented Mar 1, 2022 at 12:11
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    \$\begingroup\$ @U_flow engineering is about detail and precision. Power cannot exist unless it is being burned, transmitted or converted by some object therefore excess power is meaningless. If you meant "extra power capacity", that's different. Call me a pedant if you will but, that's what engineering is about if you are to avoid mistakes and ambiguity. \$\endgroup\$
    – Andy aka
    Commented Mar 1, 2022 at 12:40

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An easy way to switch high-current AC loads with a microcontroller (beside normal relays) are solid-state relays (SSRs). Unlike mechanical relays, which need a reasonably control current and therefore need to be controlled with an extra transistor, SSRs can typically be controlled with 3.3V logic voltage directly. Though you may find SSRs with very high current rating, they will require sufficient heatinks as they dissipate more power than mechanical relays.

There exist SSRs designed for DC and AC. Make sure to get the right one, AC-SSRs are based on triacs which won't switch off when used for DC (they need a zero-crossing of the voltage that is to be switched).

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  • \$\begingroup\$ would I need buffer the energy over a capacitor to smooth out the voltage? Could you give some mor detail of how such a circuit might look like or if there are ready-made systems of this sort? \$\endgroup\$
    – U_flow
    Commented Mar 1, 2022 at 20:34

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