I want to buy a simple inexpensive add-on speaker but have no idea how many watts to get - the tablet's speaker is a bit too low volume, I don't need the add-on speaker to be ultra powerful.
Teejai80 is right, watts doesn't really measure quality of the sound output or how loud it will be in your desired area, it's just a power consumption measure which is really misleading. Think about your budget and then test some out. I have a $30 speaker we use in the house while getting ready for work and a $400 we use at the beach/camping. Both work great for the conditions I purchased them for.