> Wouldnt they have consumed some portion of the said electricity no matter what?

Some yes, but there's a lot of difference between a machine idling at 0% CPU and one fully loaded with SETI or Folding and running 100% all the time.

Would be even more worse now with deep sleep states and GPGPU, but even back then the difference between an idle and a fully loaded machine was absolutely significant.

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

Is heat turned on around the clock?

The software runs on lowest priority. So when people are using the computer the software just sits there, not doing much. After a few minutes of inactivity the seti software (which sometimes was a screensaver) would kick in. Rather than sleeping or idling the computer would be doing computationally intensive work. Some distributed stuff (especially Prime95) is used specifically because it makes the computer work hard - it's a reliable stress test. This causes extra power to be used for the processor and the fans.

http://www.mersenne.org/download/

Running Folding@Home, SETI@Home, Prime95, and other CPU intensive applications requires much more power than simply running idle. For example, I have a processor that when running Prime95 will use more than double the idle load wattage. [0]

[0] http://www.bit-tech.net/hardware/cpus/2010/04/27/amd-phenom-...

Even if they were always on running *@home increases their power load a lot during non school hours when they'd normally be at dead idle. The way @home software usually works is once the CPU has been at low utilization for a certain period it cranks up it's computation. If he was top of SETI@home then the computers were definitely up 24/7.