C7/C9 bulbs are the older style bulbs popular in the sixties and seventies, and still use on the exterior of homes. In the eighties, we saw the mini-lights come into fashion, and increased safety as the were cooler and cost a tenth of the cost to operate 4x as many bulbs. In the last few years we have seen LED holiday lights. The cost a tenth of the mini lights to operate, and can pay for themselves in 1-2 seasons.
Think of it this way, if you decorate with 1 string of lights that\’s 25 C7 bulbs, or 100 mini or led lights. Based on 8 hours a day for 45 days, the C7 bulbs would cost you $5.67 for the season, the Mini-Lights would cost you $1.30 for the season, and the LED lights would cost you only 49 cents for the season.