I think its a 400 watt unit
Even a 300 watt unit will use a KW in 3.3 hours (1 unit) 24/3.3 = 7.3 units per day
Say a unit costs 15p its 7.3 x 15 = £1.08 per day x 465 = £399 per year....Thats if your PC is on FULL load all the time which its not you could say it would use a 1/4 to a 1/3 of that.