It really can't be calculated unless you know the power comsumtion of the componets inside but the rule of thumb that I use is.
If you have a 24v, 60 watt (2.5amp) power supply with a 120 volt primary.
60/2=30 watts or 2.5/2=1.25 Amp
Hope this helps
But if you really want to know read the lable on the Power Supply!!!