The majority of the time a PC is in an idle state, when that occurs the CPU will enter a low-powered state.
When you're running multiple graphics cards and not doing anything intensive, the extra graphics cards are switched off. (AMD ZeroCore.)
Then the primary card also enters a low-powered state.

However, if you're doing a compute job, having faster and more graphics cards can actually save your power, the power increase from adding extra graphics cards isn't a linear increase due to how you can never get 100% utilisation out of every functional block in a GPU.
Then add the "hurry up and finish" ideal, where the faster something can be processed, the sooner it can power down and return to an idle/switched off state.

If for instance you had one CPU with a 95 watt TDP and a second CPU with a 95 watt TDP at load, but one processor was twice as fast, the faster processor would actually use less power as it can get to idle faster.

Power Supplies can have a fairly large impact on electricity consumption as well, the more efficient the less energy that is lost in the conversion.
I live in Australia myself, so my energy costs are roughly 20% lower than yours in Germany, so it's still expensive, but having a powerfull PC with 4 GPU's hasn't impacted my energy consumption to any great degree compared to about 4 years ago when I was only running 2 GPU's.

i assume an equivalent considering worth