Really 2 years I was wondering cause I have new lamp and a used 1 same brand and 1 side growth is lower.
Depends on the bulb. Different bulbs are completely different animals. The specs I listed were for a
Hortilux Super HPS, 400W bulb that I use, obtained from their product specs...other bulbs can and will be completely different. In addition, lot's of other things affect will affect growth and harvest....pot size, feeding, individual plant vigor, phenotype/genetics, etc. In addition, YES, you could see decreased growth under a year old bulb versus a new one, depending on your setup. Here's the question you've got to ask - is it worth it, for your personal situation, to replace a bulb because you would yield "X" amount less? ("X" being some unknown amount of harvest you might not get because your bulb's output is 90% of maximum) Consider also that just because your light is at 90% of it's maximum output doesn't necessarily mean your yield will be 10% less than when your bulb was new. Light is not always the limiting factor in every grow. I guess the point I was rather vaguely alluding to is that in the end, you have to weigh the cost versus benefit and go with what you feel is best for your situation.