Someone else told me .
Its probably around 500W in total (a good enough guess anyway) with everything running, I use that as a guess to work out the cost in kWh (i.e. 5 hours at 10p/kWh = 25p).
Your PC power supply will have a rating on it somewhere (as will the monitor), but keep in mind that these are max ratings and it might not ever go near that. 500W could be a very generous guess for many systems