It depends on the bulb. There are two different kinds of T5s.
One, T5NO (or just plain T5) are much MUCH weaker in output than T5HO (T5-High Output).
For 10 gallons, and high-light plants, I'd aim for 3 watts per gallon of T5NO (T5). With T5HO though, you'd be fine with probably around 1.5-2 watts per gallon.
Your FIRST priority when choosing lights, however, is the Kelvin temperature.
Make sure they are between 5500 - 7600k. This range is best for plants.