For the record ive allready done side by side testing for veg cycle,this is how i came to use 24 watt bulbs in fixtures rated for 75 watts,the higher wattage bulbs offered no benifit,infact i have better results from 24 watt bulbs for one reason,heat.
A 75 watt bulb will burn the shit out of plants,a space between the canopy & the bulb surface needs to be kept at about 4 inches,with the 24 watt bulbs you want the bulb surface to he touching as many leaves as possible,the low watt lights will not burn,with light fading at distance lower wattage bulbs offer much better performance.
With the t-5 being 54 watts i was running an 8 bulb fixture,this comes out to be 432 watts to light a 2x4 veg table,the t-5 are a very hot bulb compared to a like wattage of t-12 being that the bulb surface area is so small there isnt as much surface area to disperse heat,this creates a much hotter bulb surface vs a much larger t-12 or t-8.
With the t-12's im running 8 24 watt bulbs to light the same 2x4 veg table for a total of 192 watts,the performance of the 12's is every bit tue equal to the 5's at a power savings of 240 watts,thats more than 50% less power to accomplish the same task.
Keep in mind that the low wattage 12's need to ride the canopy at all times,what i did was to secure the fixtures to 2 peices of 2x4 with eye hooks on each end & use it exactly as a t-5 unit.
Don get caught up in the t-5 hype,lots of very creative marketing at work convincing an entire consumer group to buy something that allready exists,under the guise of excellerated performance.
The pay offs are not there,use your 12's as i outlined & you will save alot of cash without loosing performance,i wish somebody had shared this info with me before i spent allmost $600 on t-5 units that i struggle to find uses for.