Question

The wall charger for an iPhone is 900mA. If you have it plugged into the wall for 2 hours, how much power have you consumed? 

Answer 1

The power is current times voltage, the energy is power times time this power was used. So, the power is (the charges has a voltage rating of 5V):

$$P=IV=0.9·5=4.5\text{ W}.$$