I’ve become an unpaid consultant to the village of Mazomanie, Wisconsin, as they want to replace a crappy PA system at their community pool. It’s somewhat outside of my experience and expertise, but I’m the best they got and you get what you pay for. 
We’re talking about 4 speakers mounted in a rectangle roughly 150’ x 100’. The current speakers are 70v system (4x 70v x 30watts) mounted pretty high (maybe 15-20 feet) on light posts. We’re talking about a lot of ambient noise, pool splashing , kids calling out to their friends, etc.
The system had two problems, a lot of noise and not enough volume. The noise problem is fixed. Their current amp is 100 watts, not sufficient to drive the 4 speakers at full capacity.
My personal feeling is that the whole system is underpowered, and that we will need to replace the amp AND the speakers (or buy additional 30w speakers).
I could go buy a 120 or 150 watt amp, but I’d like to avoid trading up later and maybe incurring restocking charges, if I find that I really need 250 or 400 watts.
Anyone help? Know some other good site where I could get this answered.
Asking “how many watts do I need?” is pointless because that’s just an indication of how much power an amplifier will produce, it has nothing to do with loudness. It depends more on matching speakers to amps and what decibel level a speaker will produce with a given power input at a given distance (usually 1 W @ 1 meter). Since this is a constant-voltage system, finding an amp that can handle the varying impedances should but a priority.
What kind of speakers are they? What is their maximum peak and continuous power rating? dB sensitivity?
The speakerd are rated at 30 watts continuous. They are Atlas Sound AP-30T horn loudspeakers. Searching for specs at the manufacturer brings up this, which I am suspicious of because the model number does not appear on the sheet.
OTOH. the only other reference of I found to the AP-30T was this, which I found under a different model number.
The current amp is a Bogen C-100. On the linked page there are further links to that model’s spec sheet and manual.
You will want a minimum of 75 Watts RMS and preferably on the order of 150 Watts RMS mono power. If you need PA (Public Address) capability alone (no music or complex signal), you should make sure that your speakers are for voice-only applications. The narrower frequency spectrum of voice reproduction will more efficiently use the amplifier’s output signal. This punch-through capability will be important in an environment full of noisy screaming children and competitive adult voices.
You need to test all of your pole mount speakers before preceding. Some of them my have aged voice coils that are dragging down the system’s overall performance. Do a continuity check on the extension wiring as well. Test for resistance in the megohm range and check for large variances. The humid and corrosive (chlorine) environment of your natatorial facility will accelerate interconnect degradation.
Do not use speakers that are at the maximum dived power output of the amplifier. If the amplifier produces 300 Watts, you do not want four 75 Watt speakers. You’d rather use four 50 Watt speakers to better drive them in their best efficiency range. You’ll get a higher amplitude output with better clarity.