How much power will a car stereo and speakers draw from a battery?

I’m setting up a 12v audio system which will be run from a battery, and i’m just trying to figure out what power the system is likely to draw from the battery? So for example:

I have a stereo in my van at the moment which is rated 19W RMS and 50W Max per channel.

I have 2 speakers attached to it, rated at 140W Max, i cant find any info on the RMS for these speakers: (http://www.amazon.co.uk/Juice-JS43-10cm-140W-speakers/dp/B001IZZ2CC/ref=sr_1_1?ie=UTF8&qid=1365944635&sr=8-1&keywords=juice+JS-43)

So what i’m asking is; if i had this system running on 12V battery (not in my van), what would be the total wattage that the Stero and 2xSpeakers would draw? And how could i work out the power draw for other stereos and speakers?

I apologise if this is asked often but i’m not electrically minded and cant find the answer to my question anywhere.

Many thanks in advance. 🙂

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3 Responses to “How much power will a car stereo and speakers draw from a battery?”

  1. N2Audio says:

    If you’re just running a pair of speakers you’d be drawing ~38w at full volume.
    It’s important to understand there aren’t any "exact" calculations for this sort of project because the electrical requirements of a music signal vary so drastically. Also, listening at high volume obviously requires more energy than low volume.

    It has been approximated that music on average requires about 1/3 as much energy as a sine wave.
    There is also amplifier efficiency to account for. Your HU may produce 19w per channel, but it will require 20-40% more power from the electrical system to do it.

    So if you’re only using 2 channels you’d have 38w/.75 (efficiency) = 50w

    P=IE or I = P/E
    I = 50w/12v = 4.2A

    From there you need to know the amp-hour rating of the battery.

    A car battery might be 70ah or so.and you could expect 10+ hours of run time.

    If you were to add 2 other spekaers you’d double the current demand so your average current draw would be around 8A.

  2. biggjayp says:

    Look at the fuse ratings of the equipment that you are using, total up the amperage from all fuses, this will give you your max current draw.

  3. tom7railway says:

    biggjayp is right about the fuse rating & current draw. But the power is supplied by the alternator if the truck is running, that’s why if people are running lots of spotlights and a huge sound system, they sometimes have to fit a bigger alternator because the standard model can’t cope. Your amp is rated 19w RMS per channel, about 40W total, or a little over 3 amps. You’ll probably find the amplifier has a 5 amp fuse.

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