A good blender isn’t necessarily defined by how many watts the motor has, but generally the higher the number the more powerful it will be.
If you’re looking at a single serve blender, then you’ll find the sweet spot is somewhere between 250 and 500 watts, although you’ll find some that go as high as 1000 watts.
It all depends on what you want to make at the end of the day, since if you aren’t concerned with grinding ice or tough ingredients then you’ll be fine with just about any wattage.