Calculating maximum wattage
Assuming the wiring is up to standards then you can multiply the voltage (220V) with the current written on the circuit breaker. You will get the apparent power, measured in VA. How much real power you get depends on the device (load) you are trying to connect - a purely resistive load (like a heater or an incandescent light bulb (not entirely resistive, but close enough) then the real power will be equal to the apparent power. Otherwise you have to multiply the apparent power with the power factor.

Real power = apparent power * PF 

The power factor may be written on the device or it may not. PC power supplies without PFC (Power Factor Correction) have power factor around 0.6, power supplies with PFC have power factor around 0.99. Purely resistive load has power factor of 1, purely reactive (ideal inductor or capacitor) load has power factor of 0.