Hmmm lots of confusion going on around here.
First, with DE bulbs, you do not have to have a glass top on your tank, only a glass UV protectant shield at the bottom of your reflector.
Second, keith mentioned that his ballast said 290w even though it is a 250w MH. The thing is that you need either an electronic or HQI magnetic ballast to fire DE bulbs, and because of the nature of the HQI bulb, you need more wattage running through it than a standard magnetic ballast gives. Hence, HQI ballasts tend to over-drive the HQI bulbs, but they're usually built to take that into account. So HQI ballasts output more than 250w more often than not, and while that creates more light (it's overdriving the bulb), it also shortens the life of the bulb, and probably puts out more heat.
You can run a SE bulb on HQI or electronic with no problems, but it's the same deal with overdriving the bulb, shortening the life of it, and putting out more heat.
Electronic ballasts tend to put out less wattage than the HQI ballasts, because they've got circuitry that allows them to fire large wattage to get the bulb burning, and then they can lower the wattage as the bulb warms up, therefore not using as much power over the rest of the burn, extending the life of the bulb, not overdriving it by as much, and putting out less heat.
Now, one thing that I've realized after doing much research into it and talking to people online, is that if you're using quality reflectors, it's better to get a DE setup, as the glass protects the reflector and bulb in one shot. With a SE setup, if you want to put glass over the reflector to keep it clean, you're hit with the light subtraction from the glass envelope on the SE bulb, and then hit again with the light subtraction from the reflector's protector shield. So if you go DE, you only need one protection layer reducing light, not two.
I run a Coralvue 250w electronic ballast and an IceCap 250w electronic ballast, and I know the coralvues drive the bulb harder than the Icecap, as the light is brighter on that side, using identical bulbs, identical reflectors.
Someone mentioned that the bulbs will burn out faster (and I did as well). MH bulbs do eventually burn out, but they'll burn out LOOOONG past their useful life. That means that they will not be useful to you when they near the end of their life. So you should not wait to replace your bulbs by when they burn out.
The thing is that the color spectrum changes over the life of the bulb. In a 20k bulb for example, the color goes from bluer (20k) look to a 18k to 16k, and probably ends around a 14k look if you drive it long enough. The light intensity itself does not diminish much after the first 3 months though. The first 3 months they drop off about 20% or something to that effect, and then after that it's only like 3-6% per year drop-off of intensity. But during that time the color also shifts to the less-blue color (more to the red/yellow spectrum).
Anyway hope that makes sense, hope it helps, and if there are any more questions that I can help with i'm happy to help out
