welcome to the forum. We from lab10 collective did a calculation on that and were surprised about the outstanding efficiency of networks like POA, xDai or ARTIS.
Calculated with 100W/Server
POA - about 17,500 kWh/yr or 1.7 US households
xDai & ARTIS - about 8,760 kWh/yr or 0.8 US households
Bitcoin consumes 73,100,000,000 kWh/yr (7M US households), Ethereum consumes 7,600,000,000 kWh/yr (730,000 US households) and Visa (without the needed banking infrastructure!) consumes 444,063,000 kWh/yr (42,700 US households).
All three mentioned networks have 4-5 sec. Blocktime and consequently these networks are in the range of 50 TPS. This can be increased but to avoid compatibility issues with Ethereum all the networks use the same block gas limit as Ethereum.
We calculated the power consumption / tx based on the capacity of the blockchain:
POA - 0.011 Wh/Tx - about 200x better than Visa
xDai and ARTIS - 0.006 Wh/Tx - about 400x better than Visa
It is not perfectly fair to compare a smart contract blockchain system with Visa, but if we only would do transactions then it is fair to say, that well designed blockchain networks are superior to legacy systems in terms of energy efficiency.
With regards to decentralization, systems like POA, xDai and ARTIS are already far better than Visa but of course can’t match Ethereum. Therefore the next step is to upgrade these networks to POSDAO / DPOS governance and increase the number of validators. Even further, xDai and ARTIS are planned to move to Honey Badger BFT consensus and our expectation is that these networks should be able to handle 1000 TPS. Taking this into account and considering a network of 100 nodes + 100 backup nodes will lead to an energy efficiency of 0.004 Wh/Tx - about 500x better than Visa.