What Watt is the bottom line. Bulbs put out light by the coating on the bulb, being excited by uv from the gas with the flow of electrons from the heaters.
The basic way is by the watts used and the company real output numbers. Now that is a base line when the bulb is new. The output goes down over about 3 months with normal ballast (magnetic). With the electronic ballast it goes even faster.
The bottom of the stack is CPF and goes to the top at UhO The best bulbs for a magnetic ballast are 38 watts for a 40 watt ballast. The cheap shop lights are electronic ballast and kill bulbs very fast. Not worth it if they are free, unless your rich. Regular mag. ballast last 30-40 years.
Remember for each reflection you loose 6-10 % mim. Thats why a coiled CPF bulb must be the straight type for max output.
I have used standard mag ballast for over 30 years and still get a good responce from the plants. Noting you should replace them every 3 months max. You can buy them in case or 2 bulb lots from around 2 bucks a stick. To add them to your grow its best to use the standard sizes 18/24/48/92 full size bulbs, usualy 1 1/2 in dia.
Bastard sizes 36/60 can be used but the cost is out of this world.
Oldsanclem aka sanclem