I think that the resources required deserve to be considered. If these computers are running 24 hours per day, and they require an extra 100 watts to look for p

I think that the resources required deserve to be considered. If these computers are running 24 hours per day, and they require an extra 100 watts to look for primes (over their idle power consumption), then each computer is using the equivalent of 10 kg of coal per month. In some cases, they're using renewable energy, and in some cases, they're offsetting heating costs. But the important thing is that the people installing the distributed computing software are not the people looking at the electricity bill. And that kind of breaks the whole economics of it.