Watch the Power online guitar lesson by Jeff McErlain from Guitar Effects Survival Guide
Many classic stomp boxes use 9v batteries. The milliamp draw on each effect varies wildly, classic fuzz pedals can draw under 1 milliamp (1mA) and your battery can last for a very long time, months in fact if you remember to unplug the input after using the pedal. Conversely a boss digital delay pedal can draw 55mA and last maybe 6 hours (so they say) on a 9 volt alkaline battery. So to add more confusion and annoyance, classic fuzz pedals generally sound better with batteries. So in a word, fuzzes, one of my favorite pedals, is a finicky customer.
Alkaline vs Carbon batteries
Okay, we have all heard about Eric Johnson hearing the difference in batteries, well I am here to tell you there is a lot of truth to them sounding different IN CERTAIN effects. I tested it myself, in classic fuzz circuits it is very noticeable, the cheap carbon 9v battery that you can buy at the dollar store sounds warmer to me. I can also tell a difference or perceived difference with some overdrives. Don’t believe me? Try it! In digital pedals I did not hear a difference but a carbon battery will last you about an hour, so we wouldn't want to do that anyway.
Many newer pedals don’t even take batteries, AC only. This is because digital effects use much more current and milliamp draw, also there may not be enough room for a battery!
Power supplies are basically a must for the guitarist. Of course many pedals use batteries, but many do not or they will drain them fully by the end of a gig. Changing batteries often is expensive and annoying, also if you have a pedalboard, you will need to unplug the cable from the input jack of each pedal, a total pain in the butt.
A poor power supply can be noisy and problematic. Always use a high quality adaptor such as the Boss PSA, One Spot, or similar. If you want to use multiple pedals you will want a good power supply like the Voodoo labs pedal power series. These “brick” style power sources are excellent because they supply isolated outputs for each unit, so no noise issues.
The next thing to consider, especially if you are using digital pedals like Line 6 or Strymon units is Amperage. Amperage is another name for current, all pedals need a certain current or amperage to run properly. Overdrives and distortions have a low milliamp draw, digital effects use much more. You need to be certain that your power supply can supply enough milliamps to properly run your effect. If the milliamp draw is too high from your effect many issues can happen. The pedal may not work, it may be noisy ( pops, high pitched hums, hiss, etc.), the pedal can also be damaged. It is important to check the manufacturer's specs before plugging everything in. The main danger, and what can cause the most damage is sending too much voltage to a pedal. For example some power supplies have outputs for 18 volts, be sure not to plug these into a 9v pedal unless the manufacturer says it’s okay.
Some pedals actually can run at 9, 12, or 18 volts like many of the Fulltone overdrives (NOT ALL OF THEM, check the manufacturer's specs) in many cases this extra voltage gives the pedal higher headroom, in other words, it doesn’t distort as quickly. I really love the sound of certain pedals at higher voltages. ALWAYS CHECK WITH THE MANUFACTURER BEFORE POWERING ANY PEDAL!