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.