That's a hard question exactly given the fact that the price of them vary wildly. Even an incandescent set of 100 miniature lights can run anywhere for $2-6. LED Christmas lights vary equally wildly, from anywhere from $5-20 for an equivalent 100 miniature lights.
But the cost to operate them doesn't vary that much. That's because the efficiency of them is pretty much the same over a broad set of lights. So I'll focus on the savings, and then you can draw your own conclusions about whether or not they are worth it for you.
Miniature LED lights
A typical incandescent set of miniature lights typically uses about 41 watts of electricity. An equivalent LED set is about 7 watt. That's a savings of about 34 watts per 100 lights. That savings per 1,000 hours of operation (assuming a national average of about $0.12 a kilowatt hour) is about $4.08. So even if the LED lights cost more than $4.08 more for a 100 light set, it'll pay for itself in 1,000 hours of operation, and then save you about 83% to operate every hour after that.But for most people, it'll take awhile to reach that period. Assuming 30 days of operation at 8 hours a day is only about 240 hours of operation. So it probably won't pay for itself in the first season, but it should within a couple seasons after that. Of course, the more you use them, the quicker it'll pay for itself.
C7 and C9 LED lights
These lights pay for themselves a lot quicker. C7 and C9 incandescent lights use a lot of electricity. You can tell they do because if you've ever touched one when it's been running awhile, it can actually burn you to the touch. I remember at my grandmother's once when I turned it on while it still in the box, the close proximity to each other made the bulbs explode.Typically, a C7 or C9 bulb uses about 4-7 watts per bulb, or 400-700 watts per set of 100. An LED equivalent is about 45-120 watts per set of 100. That's a savings of about $42-69 per 1,000 hours of operation. That means a set of 100 that cost $20 more for LEDs would pay for itself within about two seasons, and then cost of operation would be $10-16 less per seasons per set of 100 bulbs. That savings add up pretty quickly!
Of course, LED lights have advantages and draw backs. They will last longer, so they're a lot less likely to burn out within a couple seasons. Some people don't like the color, but even then they're getting a lot closer to the color of traditional incandescents.
But purely from an economic perspective, it is still something to consider. With the expenses of the season adding up, purchasing LED lights now can make the financial pain of the holidays a bit less painful later on.
