For understanding power output i'll try to provide you with more insight
amps (ampere) - consider amps to be the flow of electricity like the flow of water in a pipe.
volts - consider voltage to be the water pressure of the pipe, how much force it has.
watts - consider watts to be the rate of transfer of the pipe to the sink it feeds.
Ohms (impedance/resistance) - consider impedance to be like blockage in the pipe.
So, in essence your amplifier works off these principles. The amplifier needs the rest of the electrical system to be able to supply it with plenty of power. So like in our water pipe example the amplifier is like the faucet and your subs are like the sink. If you have a good water supply, the faucet with shoot out plenty of water no problem and the sink can fill easily. Now interupt one of these elements....say for instance your wire carries a lot of resistance, or is too small, thats the same as someone crimping the water pipe, now the faucet is strugling to get water, and the sink doesn't fill very well, it may be functional, but everything is starved. Or take for example your amps, say the alternator on your car is rated at 65amps, but your subwoofer amplifier uses around 300amps. Now you have water flowing through your faucet, but it is about 1/2 the volume. You still have trouble filling your sink. Or lets say for instance your voltage starts to drop when you play music. Now the water if flowing through your faucet, but it's not spraying as fast as your sink needs it to.
See all of these elements are essential to the function of an amplifier. This must be understood to understand how to properly install and provide correct electrical for an amplifier. This is important because if your amplifier is suffering from poor electrical from your vehicle it will have problems, and cause problems, potentially even to cause damage to the amplifier or subwoofers. To avoid this damage we take time to understand what an amplifier needs as far as electrical, does it need 4gauge wire, 8gauge, or 0gauge. Does the amplifier require more amperage, is there a need to upgrade the alternator. Is the voltage dropping when playing music, does the system need a big secondary battery?
Look at these things, ask advice and shop around. Look at how much power you want/can afford/can do with current electrical of vehicle. Once you decide how much your vehicle can handle or how much u can spend, then look at finding products that fit your needs.