Typical 15 amp wall plug in home

Typical 20 amp wall plug in the office (notice the extra horizontal slot)

Let's use a simple electrical formula. Power (in watts) = Voltage (in volts) times Current (in amperes) or P=VI which is what engineers like to say (why "I" is an abbreviation for current is a closely guarded secret by electrical engineering purists.)

In the United States, a typical wall outlet is 120 volts and all Christmas light strings are wired to handle it. Your wall outlet is usually wired in parallel to several others and connected to some sort of circuit breaker or fuse, usually 15 or 20 amps. Your goal is to not overload the circuit breaker/fuse. It's good practice to never put more than 80% load on your breakers/fuses... strictly as a safety measure.

You will have to figure out if the outlet where you want to plug in your Christmas lights is on a 15 or 20 amp circuit breaker or fuse. The easiest way to find out is plug a radio in and turn it up pretty loud. Go to you electrical panel and start turning off circuit breakers or unscrewing fuses. When the radio stops making noise, you're on the right circuit. The breaker/fuse will tell you how much current that circuit can handle.

If you have a 15 amp circuit with 120 volts and factor in an 80% load, using the P=VI equation (P=V*I*0.8) you can have a total load of (120*15*0.8=) 1440 watts.

If you have a 20 amp circuit with 120 volts and factor in an 80% load, using the P=VI equation (P=V*I*0.8) you can have a total load of (120*20*0.8=) 1920 watts.

Use the chart below to get a pretty good idea how much power different types of Christmas lights consume. There are plenty of exceptions to the chart since there are so many different types of lights. To see if you're close, check the tag at the plug-end of a new string of lights and it will tell you how much current it consumes... in most cases, it will match up with the yellow column in the table and you can feel comfortable with the numbers

LED's typically use 80-90% less current than incandescents. There might be a temptation to connect thousands of lights to one string until you can put the maximum load on a circuit. Avoid that temptation. Most copper wire used for Christmas lights is thinner than lamp cord wire and will melt under heavy load. Best practice is to limit any continuous string of lights to just a few amps. Extension cords are cheap compared to rebuilding your home after an electrical fire.