What is the most energy-efficient inline fan?

I've been scaling down my grow these past few months because of high energy bills. I decided to decrease the power consumption of the grow by getting a dimmable digital ballast and a 400 watt hps, instead of a 600 watt hps hooked up to a magnetic core ballast. To make up for the lost lumens, I am going to stick the lamp in a cooltube and place it vertically in my tent. For this, I would need a fan to keep the lamp cool. I've looked around a bit, and there are a lot of fans to choose from. The most important factors to me are low wattage and low sound output. The cooltube is 15 cm in diameter, and the tent is 1.2 x 1.2 x 1.8 meters. Can anyone recommend a fan for my situation? One that I can buy in Europe?

you can look on youtube.com and monstergardens test the watts of some fans. I have a 8" sunleaves windtunnel and its around 140 watts for an eight inch. Its not the strongest eight inch out there but its great for me. Its quiet too. Check out the 4inch, I think its 70 watts. I was reading on either lumatek or hortilux website and they said it is bad to dim bulbs because they are made to run at a certain wattage and there is a chemical reaction taking place when they are at that wattage. Im late for work or I would post it.