How many watts of light do I need for plants?
Typically they rate 2-3 watts per gallon as the rule of thumb. This does not take into consideration the efficiency of the lights, and other factors in the tank. For example a 175 watt Metal Halide lamp is two times as bright as the same amount of Fluorescent bulbs. A better measurement is that of PAR (Photosyntehetic Active Radiation) but unfortunately PAR meters are expensive and generally unavailable. Also almost no bulbs are shipped with PAR ratings.