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.