They can sink 200W at the evap head, but it's 75% efficient and would use 300W of power under full load. This is approx 3500 kWh per year when on 24/7 which would prob cost £180 to £215 or £3 to £4 a week. However, the heat the unit pulls out of your CPU and its own excess heat are thrown into your house 'for free' so your heating bills would go down a bit - in winter at least. I gather a hot summer with a prommie is rather fraught!
sw