Determine your needs. Do you want to play some PA speakers as loud as possible without blowing out or damaging them? Or are you trying to achieve a certain loudness in a specific size of music venue?
Determining how much power the speakers can handle. Look at the specifications sheet that came with your speakers. Look for the impedance. It will be 2, 4, 8 or 16 ohms. Now find the Continuous Power Handling or Continuous Power Rating. This will be rated in watts.
If you have a limiter, acquire a power amp that supplies 1.5 to 3 times the speaker's continuous power rating per channel. This is to ensure a couple dB of headroom for extra-loud short-term peaks. If you're not planning on using a limiter, the amplifier power should equal the speaker's continuous power rating per channel. Without extra headroom with this setup, you'll be forced to drive the speaker at less than full power to avoid distortion. Remember to match up the impedance of the amplifier power to the impedance of the speakers.
Determe how much amp power you need for a live sound venue.
On the crownaudio.com website there's a calculator that determines the amplifier power needed to achieve a desired SPL loudness at a certain distance. It also accounts for headroom for peaks. To use the calculator utility, you'll need to know the speaker sensitivity rating, headroom, distance to farthest audience members and desired loudness or SPL. Find the speaker-related information in the speaker specification sheet, and acquire the distances at the venue from the venue manager or make your own measurements with a measuring tape. Find the desired typical SPL from this list:
New Age: 60-70 dB
Folk: 75-90 dB
Jazz: 80-95 dB
Classical: 100 dB
Pop: 90-95 dB
Rock: 95-110 dB
Heavy Metal: 110 dB
Go to the crownaudio.com calculator, which is linked in the Resources section of this article. Enter the required information from your speaker spec sheet into the calculator and hit "Calculate" to get the amount of amp power required for your application. If you have a three-way system, determine the amp power for subs, midrange and high-frequency drivers separately. If you are in a venue with multiple speaker clusters that extend into the crowd, apply the calculator to each cluster or speaker. Add up the calculations for all the speakers on both channels to get the total power requirement for an amplifier you need to get for the application.