The power consumption of the WebSDR hardware is mainly the consumption of the laptop.
In total we expect about an average of 70W.
Based on the electricity rates in the Netherlands, it is approximately 12.50 euro per month.
A WebSDR is energy efficient, because it is used by multiple users at the same time.
Suppose there are in average 10 users. Each user then uses only 70W/10 = 7Watt.
Of course the internet needs also energy. The 100kbit/sec per user data rate however is very low.