Amplifier power and speaker efficiency determine how loud something sounds. When it comes to "volume," many musicians only consider the amplifier's power or wattage rating, and in general, more watts does mean "louder." But while wattage is an important consideration, the efficiency of the speakers that are connected to your amp also play a big role in how much sound you can push out through them - so don't forget about those either. The loudest sound possible from an electric guitar can be achieved with 100W of amplification through high-efficiency (i.e., 90dB) speakers like JBLs that are capable of moving lots of air without breaking up at such volumes; since both factors play into volume levels equally.