Many types of wireless speakers have fairly low wattage. In particular, Bluetooth speakers typically only have a few Watts of power. That is not surprising given that they are portable and often run from batteries. However, for outdoor applications, this wattage might not be enough.
Portable wireless speakers typically have less wattage than high-power wireless speakers as shown here. That is mostly because they are designed around batteries. Batteries only have a fairly low voltage depending on the number of batteries. One battery will typically provide either 1.2 V or 1.5 V. However, the voltage of the battery will actually decrease as the battery is being discharged. Also, the current that the battery can supply is fairly limited. In particular, batteries have an internal resistance which means they cannot easily provide large currents without the voltage dropping significantly.
All of this poses limits on the maximum wattage that you can get when running a portable wireless speaker. Of course, the best way to improve wattage is to instead run the speakers from a wallwart power supply. Many suppliers of the speakers will bundle a suitable power supply. This power supply is designed not only to recharge the internal batteries but also to run the speaker. When running from a power supply, the wattage of the speakers typically much improved because the voltage is higher and also the current that the power supply can deliver is usually greater than that of the batteries.
If you need portability and don’t have the luxury of plugging in the power supply then I would recommend using external batteries. You can then purchase a DC voltage converter which will convert the battery voltage to a fixed voltage. As a fixed voltage you should select the voltage that is identical to the voltage of the power supply that came bundled with the speakers. You shall not exceed that voltage because the speakers can take damage. The advantage of using a DC converter is that even though the voltage of the external batteries decreases the output voltage of the converter will remain steady. Also, the setup can deliver larger currents because even though the external battery voltage will fluctuates during high current peaks, the voltage provided by the converter will pretty much remain stable. Of course, pay attention to the current rating of the converter.
You can also use batteries with a voltage less than the voltage that is required by the speakers. In particular, car batteries are often used. Car batteries will typically provide 12 V voltage but in fact can deliver as much as 16 V when fully charged. For that very same reason I would not recommend connecting speakers directly to a car batteries because if the speakers are rated for 12 V than the higher voltage from the car battery can certainly do some damage.