The output of an amplifier is measured in Watts, but not all watts are the same! We suggest you look at watts measured in RMS - basically this is an ''average'' level of power that the amp can deliver or the speaker can take. Ignore anything that says ''peak power'' or ''max power''. Do not be misled; you want to know what it's capable of all night long; not during some nano-second performance measurement.
Another important piece of the puzzle is Impedance or Resistance measured in Ohms. Again, we advise looking at the amplifier output (measured in Watts RMS) at 8 ohms. Most manufacturers and many suppliers want to quote output at 4 ohms as it is higher (and so looks like better value!), but most speakers are 8 ohms; so compare like with like. You can always put more speakers together in parallel to bring the resistance down to 4 ohms and then you will get more power from the amp.
Some amps will go as low as 2 ohms - particularly in what is called ''bridged'' mode where both channels are linked together. If your amp does not support bridge mode then don't link them together as it will all go bang in an expensive way!
The lower the impedance of the speakers, the more power you get from the amps. Sounds like an easy and cheap way to get more power, but tread carefully!
So look at the Amplifier and the Speakers in terms of Watts RMS at 8 ohms, and you'll be off to a good start.