Standard voltage through most of your house is 120 volts. If you multiply 15 amps times 120 volts, you get 1,800 watts. If you have a circuit that goes to 9 outlets, you can have eighteen lamps plugged into those outlets, each with a single 100-watt light bulb. Anything beyond that exceeds the capacity of the circuit. This can lead to a blown breaker or possibly greater damage to the electrical system.
Light bulbs use a relatively low amount of watts.
However, a hairdryer uses about 1,200 watts. A refrigerator uses about 350 watts and a large microwave uses about 1,800 watts. The point here is that each and every circuit in your house should be constructed to handle the lighting or appliance that will be using it. Every circuit should be built with each component having the same amp rating. As discussed above, 14-gauge wire has an amp rating of 15.