iPhone - 5.0V DC, 1A
iPad - 5.0V DC, 2.1A
Watt = Volt*Amp, so 5.0V * 500mA = 2.5W
Although according to what I've read, 5V DC, 500mA may charge your iPhone (slower than 1A), I would recommend 1A for your iPhone and 2.1A for your iPad. Also note that I have used an iPad charger (2.1A) to charge my iPhone with no problems.
If you are referring to how many watts, you would need 5 watts (5.0 volts * 1 amp) for an iPhone and 10.2 watts (5 volts * 2.1 amps) for an iPad. In the case of the iPad, 10 watts should work since the Apple charger for the iPad is rated at 10 watts. I would suggest buy a charger that says it's compatible with an iPhone or iPad depending on your needs.