Loudspeaker power is a recurring question. I'm not gonna answer your question exactly, rather try to put you on the right track.
We perceive sound power on a logarithmic scale. It's the way our ears are built. This means 2x the power is not "twice as loud"... rather it is "a bit louder". However, it will still need 2x more batteries (heavier, more expensive...) and a 2x more powerful amp!
Acoustic power is expressed in dB. This is also logarithmic. Adding 10dB means 10x more power. 3dB is 2x more power (approximately).
Also, loudspeakers have efficiency/sensitivity ratings. For example, one loudspeaker might output 85dB at 1W 1m, which means if you feed it 1W then the sound pressure at 1m will be 85dB. Another speaker might do 91dB at 1W 1m, that's 6dB more, so it makes just as much noise with 1/4 the power.
So the "watts" number on speakers is half marketing. Regarding power output, it is meaningless unless you know the efficiency/sensitivity rating too.
Also, most people think they need humongous amounts of watts. This is again marketing. Put an oscilloscope on the output of your amp and check it out. In my living room, I have 92dB et 1W 1m speakers. It's a large room. When playing loud, the amp delivers sometimes 20-30W peaks, but average power is tiny, less than 1W... because 92dB is REALLY LOUD !!
For example, I sit 3m away from the speakers. At 1W power, the speaker puts out 92dB at 1m. Since 3m is 3x further, power is reduced by 3^2=9 or 9.5 dB. However there are two speakers, each using 1W, and they are correlated, so +6dB. So there's 92-9.5+6=88.5dB average on my couch.
90dB is a gas-powered lawnmower.
Most of the times, I'm listening way, way below 1W.
For portable speakers, you'll put them on a table, very close to yourself. Proximity makes it louder. With small inefficient speakers, you won't need more than 10-20W peak, 1W average. Probably much less.
So:
- Use a class D amp, which is efficient at low power. This will extend your battery life greatly.
- Try not to use inefficient speakers. This is a compromise, as efficiency in the bass needs size and weight. So, consider the acceptable dimensions and weight for your speaker, and pick one with reasonable sensitivity.
- Do not expect huge bass, or extra loud sound, from tiny speakers. After all, the amount of air they can move is: (membrane surface area * displacement). A 38cm woofer with 1cm displacement moves 1134 cm3 of air, a bit more than 1 liter. A 5cm driver with 5mm displacement on a bluetooth speaker moves 6 cm3 of air. About 200x less. There is a difference...
