Re: "i think the person wants to know about a stereo not a amplifire" I am afraid that your understanding of what constitutes a "stereo" is at fault. An automotive stereo may contain any of the following sources: analog tuner, digital tuner (terrestrial and/or satellite), tape deck, and CD player. The stereo also contains a preamplifier, which provides volume control, input selection, and equalization. The output of the preamplifier stage feeds the power amplifier stage, which, in turn, drives the speakers. Even if all of the functions are contained within one box, they are all present. For the purposes of this question, the term "stereo" and amplifier are synonymous. The stereo's power amplifier is what drives the speakers. ------------------------------------------------------------ The first thing you need to do is decipher the specifications provided by the manufacturer. Audio products for the automotive industry are frequently wildly overstated to make the products look better than they actually are. You need to determine the amplifier's continuous RMS (Root Mean Square) power rating into an 8-ohm speaker load, for example, 100 watts RMS into an 8-ohm load. Your 180W amplifier may be measured quite differently. Use this value to select appropriate speakers. The amplifier power rating should not be substantially less than the speaker's rated capacity, but it can safely exceed it by up to 25% in most cases. Note that if you routinely operate an amplifier at high volume levels, you increase the risk of damaging the speakers when the audio signal clips. Clipping greatly increases the distortion, which can ruin a speaker. Most speakers are 'blown' by using an underpowered amplifier driven into clipping. The power rating can be inflated by a company if they specify the output power into a 4- or 2-ohm load and/or specifying the peak output power, rather than the continuous RMS value. The output power needed to drive an 8-ohm load will increase as the output load decreases. Ideally, the output power doubles when the output impedance halves (e.g., 100W into 8 ohms becomes 200W into 4 ohms and 400W into 2 ohms). This rarely happens in practice, because most amplifiers cannot deliver enough current to satisfy the increased load demands. This is especially true for automotive applications, where the available current is quite limited. You can safely assume that an amplifier rated at 180W into a 2-ohm load is not likely to provide more than 100W into an 8-ohm load. If the rating uses the amplifier's peak output power, it may be very difficult to determine the RMS power level. Depending on how the company has calculated peak power, the RMS rating is likely no greater than one-third the peak rating. An amplifier that is rated at 180W peak power into an 8-ohm load is not likely to deliver more than 60W continuously. Another factor is the THD (Total Harmonic Distortion) rating of the amplifier. Although THD is almost useless as a measure of quality, since it is very easy to design an amplifier with very low THD, it may tell you something about the way the manufacturer has rated their product. A low THD does not guarantee better sound, since the large amounts of global feedback used to reduce THD in cheap amplifiers also reduces the sound quality - but they look good on paper. Amplifiers are normally tested at their full rated power, so their design is optimized to produce the best-looking specs under these conditions. The THD in solid-state amplifiers tends to fall as the output power increases until it exceeds the maximum rating, when distortion sharply increases. An amplifier rated a 0.1% THD at 100W might produce 0.5% at 1W and 25% at 120W. The increase in THD at lower power levels is due to the design optimizations at full power. If the THD listed in the specifications exceeds about 1% (e.g., 10%), you can safely assume the measurement was taken above the 'normal' rating point and the output signal is severely clipped. This increases the risk of damaging the speakers. You should downgrade the power rating by about 10% if the rated distortion is above 2%. Amplifier power and distortion become important when you listen to music at high volumes or music that has a wide dynamic range. Doubling the volume requires ten times the amplifier power. An amplifier that is humming along at 1W, may easily need to produce 100W or 1,000W during transients. The amplifier output will clip, producing large amounts of distortion and potentially damaging the speaker. Look for reliable specifications, select the components with care, and don't overdrive the speakers.

Having a livingroom quality stereo system in your car is a waste of money. You should stick with the low end factory default units since the the background automotive noises will ruin everything. However, If your stereo is rated at 180 Watts RMS a side, then the speakers you install need to be rated at 180 watts or above.

if the stereo is 180 watts total then the speakers only need to be 40-45 watts but i would go with 50 watts

thats quite a powerful stereo. divide the power output between the number of speakers. multiply the result by the average impedance of you speakers. works every time for me.

My guess is that your stereo says 180 watts "Max Power". If this is correct, pay it no mind. The RMS per channel on any radio making a "Max Power" claim is between 15 and 22 watts. You can run any speaker that doesn't need an amp to function properly. Pretty much any entry level Coaxial speaker will do.