[Electrical] Current consumption

[shepd]

100 watts is also a LOT of power for a speaker. I know amplifiers can be rather inefficient (Class A can be around 60%, though nowadays you'd find subwoofer amps are Class D which would typically be 90%+ efficient).

I'll use my subwoofer speaker as an example, though in the "small sealed car" design to show how much sound 100 watts *should* generate.

http://www.eminence.com/pdf/LAB_15_cab.pdf

If we take a 35 Hz sine wave (typically, for cars, where you get the best sound energy for your input) you'll notice the maximum safe input power is 100 watts. At that you'll experience 110 dB at 1 meter. 110 dB is equivalent to standing beside someone operating a power saw, and only slightly less loud than a rock concert. It's only *somewhat* below the threshold of pain.

Of course, 100 watts is going to draw 110 - 160 watts.

So, I think you can see if you're going for reasonable, you have plenty of power already. If you're going for "blow out the windshield" you'll definitely need to figure out a way to install a secondary alternator.

FWIW, many companies lie about watts and write on the product the output power at which the amplifier fails (Often spectacularly, we always joked that PMPO = CSA fire rating) rather than what it outputs before clipping. I have a 2100 watt "real" output amplifier (plugs into the wall). It weighs about 100 lbs, has a 12 AWG cord (and would need a 20 A outlet for full power output), and has shock hazard warnings on the output (2100 watts @ 8 Ohms = 130 volts).

I have that exact speaker in the "Large Deep Bass" cabinet. Before reaching 1/4 of the amplifier's output the floor above in my house starts to shake in ways that break dishes and worry me that I might damage something. At 20 Hz (typical house resonance) the speaker only outputs 112 dB to get things to that point, consuming just 200 Watts of power. Yes, I have to sit on the speaker or it tries to walk away.

That's not to say that having a large amplifier isn't useful (the extra wattage means small bursts of musical energy are handled cleanly and easily), but it will normally not draw anywhere near that much power constantly, just in short bursts. And your battery can handle those bursts for one reasonably sized sub (or 4 regular speakers) while the alternator charges the rest of the time.