In 2021 a study by the University of London found that in general the energy consumption of the proof-of-work based Bitcoin was about a thousand times higher than that of the highest consuming proof-of-stake system that was studied even under the most favorable conditions and that most proof of stake systems cause less energy consumption in most configurations. The researchers also noted that the energy consumption of different proof-of-stake systems was divergent with permissioned systems that used less validators being more energy efficient then permission-less systems that don't. They also couldn't find the energy consumption of a proof-of-stake system on a large scale as such a system does not exist at the time of the report.[16][17]