Christmas lights use very little power. Mini-bulbs, small incandescent bulbs, have been in use for years and a string of them use about the same power as a normal light bulb. LED lights are becoming more common as well, and they use as much as 98% less power than mini-bulbs. To calculate the amount of power used, it is necessary to know a little about electricity measurements and terms.
Volts, Amps, and Watts of Christmas Lights
In the US, the standard power from an outlet is 120 volts. Traditionally, a string of 48 mini-bulbs uses 2.5 volts. 48 times 2.5 equals 120, and so it logically follows that a single strand will equal the standard number of volts from an outlet. Now it is common, however, for there to be 50 bulbs, which just means the voltage is spread a little thinner. Each one uses a tiny bit less power and is imperceptibly dimmer. Voltage is like water pressure in a pipe; the more area it’s required to cover, the less there will be for each fixture.
The amount of power of an electrical object is measured in watts. Multiply the wattage of each bulb by the number of bulbs on the string to get the total power required. Dividing that number by 120 volts results in the amount of current, or amps, needed for the lights. Amps are the amount that flows through the pipe.
Calculating the Voltage of a Strand of Lights
Most strands should be labeled with what their voltage is, but over the years, tags can fall off or the information can get worn away. After all, Christmas lights are not particularly expensive products. But now that you have the basic idea of how electricity works for Christmas lights, you can use the principles of calculating voltages on any strand you might have.
First, you’ll need to know how many lights are in your strand. This should be easy since most strands come in even numbers like 50 or 100. So, if your outlet is 120 volts, and you have a strand of 100 Christmas lights, all you have to do is divide 120 by 100 to get the approximate voltage of about 1.2. As we saw in the earlier example, this isn’t entirely exact all the time, but it will definitely give you a good starting point.
These lights use so little power that worries about the amount used aren't really necessary. However, the same principle as above applies.