I'm using the tda2002 amp.
The TDA2002 amp is obsolete and is not made anymore. It was replaced by the TDA2003 that has improved performance.
Will volume control limit the amp output in wattage, and does limiting the gain lower the amp output in wattage?
The volume control can turn down the output power to zero. It is used to adjust the loudness of the speaker. At a certain input level the volume control or gain control can cause the output to clip and produce severe distortion for louder signals.
Can the gain of an amp be over 100?
Usually a power amplifier does not have too much gain so that it has plenty of negative feedback to reduce distortion. A preamp can add more gain.
How do you calculate amp output in wattage? Is it ampwatt x (speaker impedance / amp impedance rating)?
A speaker power amplifier has an extremely low output impedance (0.04 ohms or less) so that it damps the resonances of a speaker.
Manufacturers of amplifiers lie and exaggerate the power number by saying the power output when the output is severely distorted (10% distortion) and is clipped into a square-wave.
Frequently a power amplifier manufacturer advertises PEAK or MAXIMUM power which is simply double the RMS number.
The TDA2003 amp is rated for an output power of 6W into 4 ohms or 10W into 2 ohms when the supply is 14.4V and when the input level is too high producing horrible 10% distortion.
Frequently it is used with a 12V power supply then its output is only 3.2W at clipping with a 4 ohm speaker. With an 8 ohm speaker the output power at clipping with a 12V supply is only about 1.8W.
The output power at clipping is calculated by the peak-to-peak voltage of the output sine-wave divided by 2.828 which gives the RMS voltage of the sine-wave which is squared, then divided by the speaker's impedance. So the p-p output with a 12V power supply is 10.12V then the RMS voltage is 3.58V which is squared to 12.8 divided by 4= 3.2W.