WATTS.
I finally bought a new power supply for my server--everyone has been telling me to since building it. With 19 hard drives and a moderately recent motherboard/processor/server chassis and fans, I was using an Antec 400 watt power supply. It ran fine for four months.
Addition: BTW: nothing bad happened, I'm changing by choice.
But I wanted to order another 500GB hard drive as an on-hand spare, for obvious reasons with RAID. So when I found the drive to buy on Newegg, I noticed they had a nice combo with a $30 discount on an already decent priced power supply. It's a Rosewill 700 watt with single, large fan.
After about 30 minutes of tearing everything apart and replacing the power supply / hooking everything up, it booted fine. I then checked my UPS to see the power usage. It is about 50 watts lower! Before the change, it was drawing around 280 watts but now it draws only 230 watts or so! That's quite a nice drop and will save us more than $2 per month, not to mention the extra 3 minutes or so of runtime on my UPS!
The previous power supply is probably 4 years old so maybe it was not as efficient. Either way, I now have an extra drive and a more capable power supply. Unfortunately, I didn't jump for a redundant PSU but that shouldn't be a bid deal. The price (including instant savings & combo savings) was $105 on the PSU and the hard drive (alotting combo savings to PSU not hard drive) cost $115.