In order for a small class D or Tripath amplifier to deliver more than about 15 watts, it needs a high voltage battery power source, greater than 12V DC. The amount of wattage depends on how much voltage your battery system can deliver, the impedance of the speaker system, and the battery’s available current. .
Lots of battery-powered suitcase speakers claim high wattage. It’s nearly impossible to accomplish without big and heavy, expensive or multiple batteries. There are so many factors that go into actual performance numbers. Besides power compression, poorly designed crossovers with too much insertion loss, amplifiers that simply lie about what they can do, impedance of the total system, mechanical resistance of the enclosure, flexing baffles that rob output, poorly tuned ports, improper enclosure volume, efficiency loss due to improper enclosure design and lots of others considerations; DC voltage considerations should be the starting point of any portable system design.
In a perfect world where amplifiers were 100% efficient (they’re not), an amplifier that makes 72 watts into 8 ohms at 24 volts will make 18 watts at 12 volts. I’ve seen too many suitcase speakers using 12V batteries, that give wattage specifications for 24 or 36 volt inputs. 18 watts is not a lot of power but a system that uses the correct speakers in the correct enclosure that’s built for maximum sound quality and efficiency is surprisingly loud.
To forecast an amplifier’s performance you must also consider the impedance and mechanical resistance of the speaker system. Nominal impedance is simply a reference point because impedance is frequency dependent. It can vary wildly over the bandwidth of a speaker. Your 12V DC amp that makes 18 watts into 8 ohms makes 2.88 watts into 50 ohms. A quality speaker system has a workable frequency vs impedance curve. This is of the utmost importance when dealing with limited power. Properly sized and damped enclosures are crucial for efficiency. Tuning and venting an enclosure is a valuable way to affect the output and sound of a speaker system; selecting the proper speakers and crossover design are also significant contributors to a speaker system’s sound.
I’ve made my reputation by building great sounding suitcase speakers and by telling the truth about how they work. I’m not saying that other builders lie about what their speakers can do, i’m saying they don’t know enough about amplifiers and speakers to know better.
Without serious test equipment, no builder knows how much power his amplifier is making into his system; therefore you should be very skeptical of any specifications other than dB output measurements. An amplifier’s wattage is limited by the available voltage, current, speaker impedance and efficiency of the system. A builder that promises lots of power but doesn’t discuss battery voltage, impedance curves or the efficiency of the system is just another guy that screws speakers into a suitcase.
What’s the most important watt? The first one.