I don't really use my computer that much. Maybe a couple of hours per day to check emails, so it is safe to assume that most of my consumption comes from running BOINC 24h/7.
You do make a valid point though and the numbers I mentioned should only be used as a ballpark to give you an idea. Everyone has a different system and the electricity costs also vary a lot from country to country. This is one of the reasons I have given links to the websites I used above, so that each user can do this calculation for themselves and decide how much GRC they should be buying every month instead of crunching.
In that case the power consumption calculator should give you a good estimate.
Even with other usage It is a quick way to find the maximum monthly cost of running BOINC on your hardware.