To clear up some misconceptions before they become mythology;
Run the right bulb for the ballast, for safety reasons.
You can 'get away with' running a 400w bulb in a 600w fixture dimmed down... sometimes. If your digital dimmable ballast likes to strike the bulb at full wattage and then dim it down, watch out!
The dimmer switches do reduce the wattage, and do reduce power. Anyone who says different believes in magic- the magical ability for watts to vanish, leaving no trace behind!
Running bulbs at less than their full rated output shortens their lifespan. The more they are dimmed down, the worse this effect: an hps is not as susceptible to this, but you can shorten the lifespan on an mh bulb by 90% by running it at half power! The short rule of thumb? Never dim mh bulbs.
Someone said above that running a bulb on a dimmed setting is wasteful- he is right, and here is why; the engineers who design these bulbs are working with design compromises. One of these is how efficient the bulb will be, or how many lumens per watt it will produce. That number changes with wattage, so if a bulb is designed to be most efficient at its rated output of 1000w, it will be much less efficient if it's only seeing 60 or 75% of that. In other words, 75% power gets you 60% of the lumens, and 60% power nets only 45% of the rated lumen output. As if that wasn't bad enough, the light spectrum output also shifts, and never for the better. This hurts PAR even more than it hurts lumens!
To sum up; dimmed bulbs are inefficient, put out the wrong spectrum, and wear out more quickly. Never dim an mh, you'll just destroy it.
Finally, if you must run your hps bulb at a reduced power seeing, don't do it with a fresh new bulb without a 'burn in' period at full 100% of its rated power for at least 10 hours first. Otherwise, it will Jack up your bulb and it will never work properly.
