the speaker I bought said something like 800 ohm 5W, 8 ohm 25W or 8 ohm 25W.is there a formula or something to calculate.

 

the speaker I bought said something like 800 ohm 5W, 8 ohm 25W or 8 ohm 25W.is there a formula or something to calculate.

do you have the reference number of the speaker?

 

It sounds like your max current rating would be about 1.76 A if you have a typical 8 ohm speaker. P=(I^2)R. In the same way, P=(V^2)/R. So the voltage would be around 14V. That's what your alternator normally puts out.

Are you using an amp?

 

do you have the reference number of the speaker?

it just some china-made speaker, no reference number, but i found something similar like this:

http://www.farnell.com/datasheets/107511.pdf

 

Those are very small 8 ohm speakers. The 50mm one has a max power rating of only 0.25W.
The max power might be its peak power so then its max RMS power from a sine-wave is only 177mW.

Then the RMS current at max power is 149mA RMS.

 

Those are very small 8 ohm speakers. The 50mm one has a max power rating of only 0.25W.
The max power might be its peak power so then its max RMS power from a sine-wave is only 177mW.

Then the RMS current at max power is 149mA RMS.

how do you calculate all those number?

 

See the power formulas in our Ebook - http://www.allaboutcircuits.com/vol_5/chpt_1/8.html.