The power ratings of bulbs vary quite a lot. One single bulb in a small LED light chain will consume power of 0.4 to 0.5 Watts. This seems almost insignificant. But this adds up depending on how many bulbs you have on a string. This can range from a couple of dozens to hundreds of them depending on the length of the light chain. On the other hand, vintage bulbs consume many times more power than this. How much actual electricity it will consume depends on how long you use it. Among all light chains, LED string lights consume the minimum amount of electricity. If you are worried about the running cost of it, you will be better off using LED lights.