After further research:
Watts is merely a value derived by multiplying volts x amps, and 10 watts means that the other two values are about right for charging a mobile phone (5v/2a). There is no output wattage specified on any of the chargers in my house since there's no need of it, you get it simply by multiplying the other two.
It's the output voltage that's critical.
So as long as a third-party phone charger has the same output voltage as the original charger you will be okay.
The output amperage isn't critical, it just determines how long it will take for the battery to charge up.