A 50-light string of incandescent midget lights uses only about 34 watts. That’s not a whole lot of energy consumed over the typical month of use per year.
If they are used 6 hours a day, that’s 186 hours for the whole month. A single string then uses about 6.3 kWh. At the national average electric rate of $0.12/kWh, it costs $0.76 for the season per string.