kWh per 24hours from system that uses 950W

call it 1000 watts , its 1 kilowatt hour. (at max cpu and gpu use) . Obviously normal tasks will not use more than about 100 watts (0.1 kilowatt hrs).
so 8 hrs gaming would use 8 units.
Thats about £1 in the u.k.
 
If it constantly draws 950W, just multiply that (NOT its maximum capacity) by the number of hours and you get 22800Wh (22.8kWh)

He could also get a digital wattmeter to get the actual figure.