I want to run a 12v inverter from my truck to power two speakers that have a total continuous power of 8 watts RMS.

How long will it take for the battery to drain?

How long would I have to leave the truck running to then recharge the battery?

:-?

You have speakers that require 120 volts AC?

An inverter changes 12 volts DC (battery power) to 120 volts AC (household power)

To measure inverter drain, you need amperage draw of whatever you are powering.

Inverters are inherently very poor choices to power anything for very long, unless you have a huge battery bank.

I can help with more information.

Cheers,

Mark

Lots of factors to determine.  

Whats the reserve capacity of your battery,  this means, how long can you operate your battery with out charging.  Normally, thats 25 amps of power for about 80 minutes.

Or, if you used 120 watts, thats 10 aph, which would give you about 3 hours.

Most inverters have alarms to indicate its time to charge the battery before the battery is too drained to start the vehicle.

3 types of inverters are made.      

Squarewave, the cheapest, less efficient, not for electronics, power tools and motors

Modified sine wave, middle of the road in price, will work for Tv, Computers, may have fuzz in volume, and line distortions, but will work, will not run laser printers.

True sine wave, same as outlets in house,  will run everything, higher priced.

Standard Safety, always figures convertors at 90% effiency of actual rating.   Better to err on the safe side.

given current is amps,   volt is voltage, and Power is measured in watts.

How many DC watts will the Invertor draw from the battery to covert AC?  

using standard formulas, P=IV and I=P/V,    

simple math,   If you have 2 of the 3 givens, you can figure Power mesured in watts, current measured in amps, voltage n volts, or resistence measured in Ohms,  knowing the formulas.

     We know DC is 12 volts, and AC is 110 volts,   We know the Watt or power, is 8 watts.

First you have to figure how much the convertor is pulling from the battery.

  I know the AC is 110v and rating is 8 watts, so     current, I is equal to W/V, so   8 watts divided by 110v = 0.072727 amps

Now I know current, I can find P, P=VI, orP= VxI,   12Vdc times 0.0727=0.87 watts.

simple,  AC watts/12 v X 0.87 amps, so 8 watts divided by 12v =0.666 X o.87 amps =0.58 DC amps from your battery.

You would be using about .6 of an aph,  or amps per hour from your battery.    

just multiply .6 amps x hours of useage, and the rating of your battery.    you should have no problem getting 6 hours easily out of your battery.

A deep cycle battery would double that, it has larger reserve capacity, again, it depends on the reserve and wattage pulled.

Hope that helps.  

DOH!  what Matt Fly said.

That does help. Thank you.

What we're doing is running a small set of computer speakers so that we can listen to an ipod while camping. We're already going to have the inverter for airbed pumps and what-not. I just figured we'd use the inverter to run a set of speakers also.

Thanks again.

Very nice Matt M you paid attention in class!

I'm awfully rusty but the only thing I could think of is with an AC load you'd also have the cos of theta ( for non resistive loads). In theory anyway but in practicality it is more often than not ignored. Again I'm rusty so correct me if I have it wrong.