I don't know how many people still power their pedals with 9V batteries, but I'm one of them. I know it's not very green, but I like the simplicity, and the batteries usually last a long time.
Well, I just got a Line 6 compressor pedal. These Tonecore pedals are essentially little computers, so they draw a lot of power. I measured this compressor pedal and it draws 56 mA. Compare that with 4 mA or so for a typical distortion pedal.
I expected the Line 6 pedal to eat through batteries, but I was very suspicious when it wouldn't even turn on with known working batteries. I did a little research and found a discharge study of different brands of 9V batteries. Here's a plot of battery voltage versus output charge:
Guess which brand of battery I was using? That's right, my pedal will only last 1 hour with this brand of battery. That explains why those batteries were so cheap.
Here's the whole study for those interested: Discharge tests and capacity measurement of 9 volt transistor radio batteries
This reminds me of the fact that my guitarist swears by the Danelectro 9Vs that release lower amounts of current than normal so as to sound "vintagey" and "broken in"
I am a big fan of plugging my pedals in most of the time. It means that the batteries still function when I need them most.