- The baseline electricity the computer is using. This will get used and produce no S@H work either way (in an era where our computers didn't go to sleep)
- The added electricity to do S@H work. This certainly was not idle, but also was a relatively small amount relative to an entire household's use.
- The hardware itself.
I would say that S@H were making use of hardware that was "going to waste" (this is CAPEX the project didn't need to spend), was having volunteers donate the added electricity, and could be said to be making use of the baseline electricity inasmuch as it would be used either way, and if S@H were to set up their own compute cluster, they would also incur similar overhead.
I don't know what system efficiencies were like in the late 90's, but I would totally believe someone if they said "idle" was 40% the power consumption of "max usage". Using that number as an example, "donated electricity" gets (1/0.6=) 1.6x as much compute-per-watt as if it was donated to run an equivalent, dedicated, machine. Maybe you can convince yourself that you were using "idle cycles" and donating on top of that?