You are not in danger of wearing out the ballast at all. The only problem that you could see is underfiring the bulbs. VHOs are not like Metal Halide, it is the time burned not the number of start ups. I would love to see more info. on the amount of lumens produced. For example an Ice Cap 660 use 30% less electricity than the bulbs need. Rumor has it that it underfires bulbs and that is why they can advertise that your bulbs will last longer. I called World Lighting (Local Company that carries WH7 for $35) they told me that using more than one bulb might under fire the bulbs on the WH7 ballast. The big negative I have read on the WH7 is that a few people have claimed that their end caps caught on fire. I believe that it was the ballasts either must have been wired incorrectly or were deffective. Many people use WH7 and grainger magnetic ballasts over their suggested capacity with out problems. If you are getting good growth and your bulbs last a little bit longer I don't think it is something to be worried about. If I had not gotten a IC 660 dirt cheap I would be running 2 WH7s now.
