If you can accurately measure the available surplus power, you can control an AC to AC converter to precisely deliver that power to your heating element. You would need an element with a mechanical (not electronic) thermostat as the voltage will vary. If you intend to feed the heated water in to a gas boiler, you will need to know the maximum supported inlet temperature.

Hi,

Thank you a lot for your interesting contribution.

Yes I can accurately and continuously compute the available power. The 2000 Watts heating element will be simply controlled by a Triac. The firing angle of this triac is managed by the microcontroller. Depending on this firing angle, more more or less power will be fed to the heating element. A LEM current sensor and an RMS (Root Mean Square) algorithm compute the actual absorbed power. A numerical PI (Proportional + Integral) controller manages the regulation loop so that the absorbed power is always very close to the set point.

This already works very fine on my prototype and I will publish the circuit soon.

Yes, The max. temperature of the water is controlled by the built-in safety thermostat of the tank. For obvious safety reasons, I will not make any changes to this part.

I guess that the input temperature of my gas boiler tank can be rather high because it is its job to heat water... I will check nevertheless.

Regards,
Michel

heat pump

Hi Bruce,

I have equipped my house with an Off-grid PV system and I have currently a lot of unused power. My plan is to heat a 200L water tank with the surplus of electrical energy.

See the log called "What comes next ?" of my project :
hackaday.io/project/2333-My-Off-grid-Solar-System-Monitoring

Regards
Michel