The power consumption of the WebSDR hardware is mainly the consumption of the two laptops and the PI4, including all receivers.
In total we expect about an average of 80W.
Based on the electricity rates in the Netherlands, it is approximately 15 euro per month (April 2022 level).
A WebSDR is energy efficient, because it is used by multiple users at the same time.
Suppose there are in average 20 users. Each user then uses only 80W/20 = 4Watt.
Of course the internet needs also energy. The 150kbit/sec per user data rate however is very low.