Turtles need a day/night cycle, just like humans. Most people around here run their lights between 10-12 hours. Many around here get light timers (you can find them in lighting part of the hardware area in Wal Mart--$5) so that the lights turn on and off for a set time, and you don't have to remember to do it.
As far as power consumption for your fluval filter, I'm not sure. I have the 404 model and I didn't see any mention of power usage. If you contact the Hagen company that makes the Fluval filters, they might be able to tell you. Check their website for contact info: www.hagen.com
Hope this all helps!