Update on Feb 22, 2024.
The solution discussed in this thread evolved to a NPM package openhab-spot-price-optimizer.
The blue area in the figure above represents the spot prices. The yellow bars indicate when the heating of the house will be ON and the red bars indicate when the water boiler will be ON to heat domestic hot water. The solution can be used with any devices you can control with openHAB.
Installation instructions and full documentation with examples can be found at Home · masipila/openhab-spot-price-optimizer Wiki · GitHub
The comments between this and comment #302 are related to the older version of the solution. Comments from #302 onwards are related to openhab-spot-price-optimizer linked above.