During the winter months, the electric heaters that were also performing computation would supplement the normal dumb heaters. The cost of electricity or fuel saved on heating should therefore be subtracted from the cost of running @Home. Likewise, additional cooling costs would have to be added for the summer months.

Once those kWh are used in the CPU, they don't just disappear. They become heat.

http://www.technologyreview.com/news/414050/computer-cluster...

http://www.bbc.com/news/magazine-32816775