How to calculater total watt


New member
Username: Zen936

Post Number: 11
Registered: 01-2004
Just wondering. Exactly how do we calculate the amount of watts our audio system bring out. This is to determind the type of capacitor or alternator to get for your car.

Is it the total of rms on all your units (HU, Front Spkr, Rear Spkr, Amp, Mono Amp and Sub)?

Also suppose your amp is rated 50watt x 4 rms, does that mean we should calculate it as 200watt rms? Kinda confuse on this one. Hope you can help. Thanks.

New member
Username: Its_bacon12

Post Number: 41
Registered: 12-2003
watts = voltage x amperage
caps usually go by 1 farad to 1000 watts rms
if you have a 12.6v regulator, which comes stock in almost every car, then divide your total watts by 12.6 and that will be you total draw of amps and like glasswolf said, cars usually use about 60% of their stock alternator to power all the electronics around the car, so say you have a 75a alternator idle, 110a max output. your car would use about 40-45a the alternator produces so theres an extra 30-35a left over for you to use as you please whether it be sound system, fog lights, or anything else.....

i hope this helped a little and for your last statement, an amp thats 50w rms x4 is a total of 200w rms. and yes you should calculate it as 200 watts RMS and not 50.

New member
Username: Zen936

Post Number: 12
Registered: 01-2004
When you say Total Watts, do you mean RMS or Peak? So if alternators usually give out only 60%, then one method of getting more power is by changing the wiring (as was mentioned in another post somewhere in here) to 4gauge?

Now I see where it's all going. I've got the standard 12v 80a alternator. So the standard alternators use 8gauge wires right? Which is why my lights dimmed because the standard wires have a limit to the amount of current it allows through. Is this a safety precaution? Are there any dangers to using 4gauge wires?

I was just looking at my alternator just now and it's difficult to reach as it's hidden underneath the engine. On a last note, you mentioned that we need to count it as a total of 50w x 4, but if we're using only 2 channels, can we take out 100w from the total? Not that I need to coz im running two amps.

My 4-channel amp:
Power Output at 2 Ohms, at 14.4V (1kHz, 0.8% THD): 90 Watts x 4
Power Output at 4 Ohms, at 14.4V (20Hz-20kHz, 0.08% THD): 60 Watts x 4

My sub-amp
Power Output at 2 Ohms, at 14.4V (100Hz, 0.5% THD): 400 Watts x 1
Power Output at 4 Ohms, at 14.4V (20Hz - 200Hz, 0.5% THD): 200 Watts x 1

In this case, I was wondering if you could help me figure this out. My total watts for front and rear speakers are 200w (50w x 4RMS)

Thanks. Man...sooner or later I'm gonna start becoming an audio freak! (P.s. Sorry if this is long)

New member
Username: Its_bacon12

Post Number: 42
Registered: 12-2003 misread something, your car uses about 60% of your stock alternator not it gives out 60% of its rated power. and if i were you, find out what your amps put out at 12.6V instead of 14.4v because thats a huge differencein power usually about 50%. by total watts i mean RMS. never go by peak power ratings- if the RMS isnt listed, ask the manufacturer or just look around some more.

to your alternator question about 8 guage. If it uses 8 guage to connect to yuor battery, there is not problem, as long as the wire can withstand 80a of current flowing through it. You lights dim because there is alot of current being drawn to your amplifiers and not because of the 8 guage cable connecting your alternator to your battery. To correct one of your first statements, the only way to get more power from your car is to get a higher output alternator, say 125a idle. since your alternator is 80a im guessing idle, then your car uses about 60% of that, not the alternator only produces 60% of that. 4 guage wire you only use if your amplifier requires roughly 90a-150a of curent draw. Before you do that though, you have to have an alternator that will put out that much, and then more. If you're only going to use two channels, i would suggest bridging the 4 channels down to 2 channels, which would give you about 100w x2 RMS power...which will also equal the total 50w x4 RMS power total to 200w RMS. You can use only 2 channels, non bridged but you'd be losing the amplifier potential by 50%, and only using 100w RMS total, and thats if you DID NOT bridge the 4 channels into 2.

I'm not quite sure by what you mean in "In this case, I was wondering if you could help me figure this out. My total watts for front and rear speakers are 200w (50w x 4RMS)"

ok...i know im kinda scattered but i think ive covered it all, and on a last note, add the total amount of RMS power your amplifiers will use (i.e.: 50 + 50 + 200 = 300) and to say this again, running a 4 guage cable from alternator to battery will NOT increase the power, it will do nothing if your current flow is 80a. i hope this helped you out a bit too :-)

New member
Username: Glasswolf

Post Number: 744
Registered: 12-2003

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