there is a little more to it than that...measurements are made 18" from the edge of the lamp and the value quoted are the initial starting output, but they drop to 80% within 10 operating hours and continue to drop off. Commercial growers often change bulbs with every grow because of how much the lamp changes over 2 or 3 months.
NOT MY BULBS, mine last a long time. I have bulbs that are 5yo and they still work good. they make my plants grow like crazy. I have 3 plants under these bulbs that are 3wks old and already they are 36" tall. they only have 4 nodes though for some reason :doh: