It would be great to measure the efficiency of a heat pump (or Dino fuel heater for those stuck in the 1900's

Water flow could be measured with a cheap YF-G1 flow meter on a pulse input, and two 18ds20 would measure delta-T.
Assuming
V= Volume [l]
dT= Delta-T [degree]
E= Energy [kWh]
Then
E=(1,16 E-3)*dT*V
needs to be calculated and sent to Domoticz