No offence, but I believe that was a too technical answer for a simple question. This portion of answer could have arrived at later stages after you know how much OP is into these things. Nevermind.
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@OP- Some quick points from my side:
1) Don't look at PMPO (stated in watts) which people normally do.
2) So what should you look for ? First thing you should look for is RMS which actually tells you the raw power of that speaker.
3) See the build quality of the speaker. Cheap plastics should be avoided at any cost. The implication, a good sub is always on the heavier side. Not to generalise, but yeah, it is mostly true.
4) Look for the quality of the wires and connectors. Cheap wires & connectors result in loss of quality during audio signals transmission. For connectors (cords), look for gold plating if available or as best as possible.
There are other points to be considered (obviously much more technical), which others can help you to deal with.
Good luck.