That's a really good question. It's important though, before you start comparing amp power ratings, that you don't compare apples to oranges. There's two different kinds of power ratings out there: peak power and RMS power. Peak power is what the amp can produce for a very limited burst of power. It's often the number you'll see on the box but it's not very useful. It's better to look at RMS ratings. RMS power is the measure of the amp's continuous power output. It's much better to look at the RMS numbers when you're shopping. And if you really want to be sure about an amp's power rating, compare amps that have a CEA-2006 power rating. Now, this standard establishes specific guidelines for how the amp's power is measured. So amps with this kind of power rating have all been measured on a level playing field. But once you're aware of all this you still need to figure out just how much power you need to drive your speakers. The first thing to remember is that it's better to have more power!!
Right. You want your amp to be able to play all your music, loud and soft, without distortion, and be able to handle sudden changes in volume easily. More power gives you just that. It's something that we call "headroom." As a starting point consider the RMS power ratings of your speakers or subs. Match or better yet, exceed the speaker power ratings with your amplifier. For example, to get that headroom, if you have a subwoofer that handles 200-watts RMS, power it with an amp that puts out 250-watts RMS. It'll drive it cleanly and without distortion, especially when the volume is cranked and it'll do it better than an amp with less power. More power is always better. But we haven't talked about one of the most vital factors in amp performance: amp wiring. Your amp depends on a steady supply of power to operate at its peak and that's why you should invest in a high quality amp wiring kit when you buy the amp. If you use too thin or cheap wiring to hook up your amp, you're starving it of the power it needs to do its work. Yeah, the bottom line is you're not going to get the performance that you paid for.