T5's are definitely not power compacts. T5's actually produce more lumens per watt than power compacts. Lighting is a big source of debate and talk on a lot of forums. The use of watts is not exactly like comparing apples to apples. Read the
www.ahsupply.com web site to see of the differences in light quality based on spacing and reflectors. There is also the issue of heat which Tindo hit on.
However it is a commonly used term so I will try to address it a little bit. Some people suggest about 1 watt per gallon for terrariums, since you are not having to cut through water. Others say closer to 2 or 3 watts per gallon. It also has a lot to do with what types of plants you are going to have. Bromeliads require more light than pothos, and the list goes on forever. From what I have read many plants, in say a 55 gallon tank, will due quite well with two 4 foot T8s over them producing 5600 lumens with 64 watts. I am not sure how well the broms will color up, but you may not care. (They may color up great, I just do not have any experience with it, but I would like to know, as I will be helping a friend soon with a 55, and I would love to able to use T8s to save money.)
My only personal experience is with my 40 gallon tank. I decided to use a 96 watt 6700K bulb from AHSupply. I have 7 broms, more I will be adding, some Pilea Glauca, Peperomia Japonica, Fittonia argyroneura, and three types of Dischidia and the plants seem to be doing well. The broms have colored up very nicely. My tank is 36 long, so this bulb fit over it perfectly.
That brings me to my questions for you. What are the dimensions of your 125 gallon? And what types of plants do you want to grow? Also, what type of animal do you want to dwell in this enclosure?