It depend on the type of gig, size of the venue, shape of the venue, expectations of the client, whether it’s inside or outside, and the style of music that you will be playing. If it is an indoor gig, you should aim to have as a minimum around five watts per person. If you are playing outside, then you will probably want to double that and have 10 watts per person. Naturally, this is a loose rule of thumb and if you are going to be playing bass-heavy music, you might want to have extra sub-woofers. In this case, a 2:1 ratio between subs and tops is common.
Most importantly, and something that is often overlooked, is to make sure that your speakers are being amplified efficiently. Bigger doesn’t necessarily mean better (or louder for that matter). Make sure the amps are rated at at least a third more power output than the speakers are designed for – half as much again or even more is also fine. Contrary to popular belief, it’s speakers being driven by under-powered amps that tends to blow them, not overpowered. Avoid this entirely by using modern active speakers, where all of that has been handled for you.
Indoor gig = Number of guests X 5 watts = speaker watts (+ sub-woofer)
Outdoor gig = Number of guests X 10 watts = speaker watt (+ sub-woofer)
On the Crown website is a calculator that determines the amplifier power required to achieve the desired SPL at a certain distance. It also accounts for the number of dB of amplifier headroom needed for audio peaks. Text accompanying the calculator gives the equations used. Click on the following link to go to Crown’s power calculator: Calculator