How Many Watts Guitar Amp Do You REALLY Need?

Before we start to discuss how many watts guitar amp do you need, we must understand the distinctions between electric power and loudness of one amp.When we talked about power of amp, we pointed at electric power of amp.The back of this question, we really are focused in the loudness of amp. How many loudness amp do you need use? or simple said: How loud does an amp need to be?
Before we answer this question , we must understand below concepts: Power and loudness of amp; Tube and solid state amp; Speaker(cabinet).

The relationship between power and loudness of amp
The unit of measure loudness most common is the decibel(dB), the dB is a unit of measure for sound intensity or level. It is a logarithmic scale developed to express wide ranging quantities on a simple scale. Because it is logarithmic, it can be a little bit confusing. As an example, a doubling of volume is not equal to twice the number of dB. The chart below will help you to better understand the dB and how it relates to what you hear, and what is happening throughout your system as volume levels change. But first, we need to look at the other end of the equation.
 Power,in this discussion, refers to watts. Power defined is the rate at which energy is converted or dissipated, as in the case of an amplifier driving a loudspeaker. It is important in our discussion in defining the general relationship.

dB Change

Voltage

Power

Loudness

3

1.4X

2X

1.23X

6

2.0X

4X

1.52X

10

3.16X

10X

2X

20

10X

100X

4X

40

100X

10,000X

16X
Looking at the chart, and starting at the left, we can see than an increase of 3dB results in a voltage increase 1.4 times the original, a doubling of power, and yields only a subjective increase in loudness only 1.23 times the original. To get a doubling of loudness, it is important to note that an increase of 10dB is necessary. And to reproduce that volume through our loudspeakers, note that we require ten times more power from the amplifier!
The chart below is relationship between power with loud
100 watts is 1.23 as loud as 50 watts.
80 watts is 1.1562 as loud as 50 watts.
60 watts is 1.0578 as loud as 50 watts.
40 watts is 94% as loud as 50 watts.
30 watts is 86% as loud as 50 watts.
25 watts is 81% as loud as 50 watts.
22 watts is 78% as loud as 50 watts.
20 watts is 76% as loud as 50 watts.
18 watts is 74% as loud as 50 watts.
15 watts is 70% as loud as 50 watts.
12 watts is 65% as loud as 50 watts.
10 watts is 62% as loud as 50 watts.
9 watts is 60% as loud as 50 watts.
8 watts is 56% as loud as 50 watts.
7 watts is 55% as loud as 50 watts.
6 watts is 53% as loud as 50 watts.
5 watts is 50% as loud as 50 watts.
4 watts is 47% as loud as 50 watts.
3 watts is 43% as loud as 50 watts.
2 watts is 38% as loud as 50 watts.
1 watt is 31% as loud as 50 watts.
3/4 watt is 28% as loud as 50 watts.
1/2 watt is 25% as loud as 50 watts.
1/4 watt is 20% as loud as 50 watts.
1/10 watt is 15% as loud as 50 watts.
50mW is 13% as loud as 50 watts
20mW is 10% as loud as 50 watts.
10mW is 8% as loud as 50 watts.
5mW is 6% as loud as 50 watts.
All other things being equal, a 100-watt amp will be just perceptibly louder than a 50-watt amp. It takes about a ten-fold increase in power to double perceived volume. That's means, you'd need a 500-watt amp to be "twice as loud" as your buddy's 50-watt amp. Even more thought-provoking is the fact that a 50-watt amp will only be perceived as a little bit louder than a 15-watt amp driving the same cabinet!
  • There are so many variables controlling sound pressure level (SPL) and perceived volume that it is quite common for a small amp to sound louder than a much more powerful amp.
  • The initial purchase price of a 100-watt amp will be significantly higher than a 30-watt amp with similar features.
  • It will cost you signficantly more to re-tube a 100-watt amp.
    Tube (Valve) vs Solid State loudness
    Other one factor affecting loudness of amp is device of amp : tube or transistor. It’s important to be clear about the difference between tube and solid state amps, when it comes to volume. 
    Tube amps are often perceived as being louder than solid state amps and this is because they actually are. A low wattage tube amp, of say 10 or 15 watts, will actually sound as loud or louder than a solid state amp of 50 watts or more. Speaker impedence, which is the amount of resistance that a speaker offers to the current that flows through it, actually depends largely on the nature of the frequencies that the speakers are reproducing, and the hard clipped signals produced by solid state amps strongly reduce the available power, giving them higher speaker impedance and less overall power.
    Tube amps, on the other hand, faithfully produce between 80% and 90% of their power regardless of the frequencies being produced by your guitar. Also, the full, smooth and fat over-driven tone of a cranked tube amp are less harsh and more agreeable to the ear, making them more tolerable to listen to at high volume than solid state amps. In this way tube amps are both actually and subjectively louder than solid state amps.
    The Speaker (Cabinet)
    There are some other factors that you need to consider, besides the power. The cabinet (speaker) dimensions are also important. For example, One 2x12 cabinet is obviously lound than 1x12 cabinet , when they are drived by one same 100W amp. Because the 2×12 cabinet will generate twice as much air movement, hence causing it to sound louder. 
    Other one factor is back cabinet: open or closed. The back open cabinet allows sound of speaker emission from both the front and back of the cabinet, This tends to let sound fill the room a bit more, which is good to a larg venue. Back closed cabinets tend to sound more focused, since the sound is only coming out of the front of the speaker.
    Now we know what affects loudness, and where power (watts) comes into play, let’s talk about the different uses of amplifiers, and how loud they need to be for each purpose
    How much clean headroom is required and select the amp on that basis.
    Headroom is defined as being the volume at which the amp starts to overdrive or distort the incoming signal from your guitar. Fender Twins are known for producing loud clean tones – it’s extremely difficult to get that amp to overdrive. Therefore, it has lots of clean headroom. A 1 watt amp designed to produce overdriven and distorted tones (basically more of a distortion pedal than an amp!) will overdrive at very low volume. This type of amp has very low clean headroom. So,how do we figure out how much clean headroom and output power is required?
    Does practicing amplifier need 100w amp?
    Below are relationship between hearing and volume:
    50dB-60dB the volume of electric guitar without power.
    70dB-80dB the volume of watching TV and wood guitar;
    80-90dB Volume of solid guitar amp, this volume don't noise your neighbor on day.
    90dB-100dB the volume of ROCK ,you can start feeling cool ,although it still need more headroom. It is only one fifth of full volume of 30w-50w amp damaging your hearing. Not face to speaker.
    100dB-110dB the volume is very cool. half of volume of 50w amp, good headroom, damaged hearing serious .
    110dB-120dB It is limit of amp. Within a few years hearing of player will be permanently damaged.
  • There is absolutely no venue that requires a larger amp and cabinet than one suitable for use as a stage monitor – provided that the members of the band don't get into a juvenile competition to outdo each other. Need more feedback? Fine, move closer to the amp, for crying out loud! Typically, a really good 30-watt amp with a suitable cabinet is ample! Depending on the composition of your band and the variety of venues you play, an even smaller rig mic'ed to the PA and then pumped to stage monitors may be even better. Note that while a good 30-watt rig has ample volume you may end up having to buy a more powerful rig to get other desired features.
  • At a small venue, a 15 to 30 watt amp can be placed behind the band in a traditional position and used without a mic. In this position the rig serves as a monitor for the guitarist and as the primary amplification for the guitar.
  • At a medium venue, the small rig can be placed in front of and facing the guitarist as a monitor, and be mic'ed and run through the band's PA. The band's PA speakers are placed along the front of the stage and facing out. This arrangement allows the sound to be balanced at the mixer and allows the band to flood a medium to large venue without damaging their hearing.
  • At a large to giant venue, that same small rig can be arranged much as described above but now the band's mixer feeds the house PA. This is the only way to reach the back of the room without dangerous SPLs near the stage at a large venue and would be required no matter how powerful a rig the guitarist has.
  • Smaller equipment takes up less space on cramped stages.
  • The money you save by purchasing a smaller amp can fund a nice power conditioner to protect your equipment and clean up power line noise the conditioner will cost less too since now you don't need one that will handle a billion watts.
  • If the band members will agree not to compete, all of them can recognize significant savings by not having to purchase unnecessary equipment.
  • Your band is far more likely to be called back for a repeat engagement if you provide a well-balanced, easily managed performance than if you are just obnoxiously and uncontrollably loud.
  • Many guitarists already have the monster amp it isn't necessary to get rid of it. You can use an attenuator (see the attenuator myth) and perhaps a smaller cab.

    • What is the best combination of amp for practicing ?
    We have discussed 100w and 5w guitar amp , know disadvantages of two kind of amp. What is good choice for practicing amp? One amp, which power is within 50w, is good selection. At same time combination 50w amp with one attenuator will help you more.
    How about amp which power is within 10w?
    It depend on whatsound you want. If you like clean sound, No problem, one 5w combo which has smaller speaker is good for you.But if you want distortion sound(many people want it). It is different.
    There are fours components in total guitar amp syetem.
    No.1 preamp tube distortion( sound hard ,more metal).
    No.2 power tube distortion( sound warm ).
    No.3 speaker distortion ( No-Line of speaker)
    No.4 the physical movement of air that your speakers produce at high volumes.
    When the power of amp is smaller such as 5w. It is difficult to obtain more distortion from power tube. At same time, because the capacity of power tube is smaller, the distortion of preamp is limited. The power push coil of speaker is too smaller . The response of speaker will became bad.
    Amp used in clubs and bars
    The number of watts you need to gigging in a club or bar that holds around 50-100 people is usually around 20 watts for a tube amp, and 40 watts for a solid state amp. If you're playing without a ban, that is.
    If you are playing in a band with a drummer, you’ll need a boost in volume so your sound doesn't get completely drowned out. You’ll need a solid state amp that has around 100 watts, or a valve amp that has around 50 watts. 
    This will usually give you enough volume that you can be heard over the drummer, without having to push your amp’s volume too hard so that the distortion becomes overbearing. 
    Amp used in larger venues
    If you're playing in larger venues, then you'll normally have your amp’s volume boosted by a microphone. This is the easiest way to get that boost in volume without having a ginormous speaker and amp.
    For large venues, holding few hundred to a few thousand people you’ll normally need either a 100W tube amp, or a 200W solid state amp. Of course, you can then adjust the microphone volume depending on the specific venue size. 
    Amp used in recording
    Typically, if you're using your amp so recording purposes, you'll normally want a solid state amp that has around 20-40W or a valve/tube amp with around 10-20W. 
    Volume isn’t as important in the recording studio. But you do need to get the balance between having an amp that’s actually loud enough without you having to push it too much or little, so that the distortion effect is either too obvious or too weak.