If the circuit itself can support 20 amps of current, you should install plugs which can handle at least that same current. Otherwise you'll eventually forget what you did, and plug a 20 amp load into the 15 amp plug, and get more warmth in the wall than you might like.As for how many amps a given load pulls, the formula is easy (and even applies to AC): watts = volts × amps. Your wall puts out 120 volts. Look on your curling iron or hair dryer for it's wattage, and just divide: amps = watts/120. A 1500W hair dryer uses 12.5A. A 100W light bulb uses 0.83A. and so on.