There are a few notable differences between C7 and C9 Christmas light bulbs that you should understand when deciding which one would work best for your intended holiday lighting application. Here is what you should keep in mind, along with a few decorating tips.
C7 bulbs are smaller than C9s. They have an E12 candelabra base, the same size as a night light bulb. They are around 1 1/2 inches long and are generally 5 watts for incandescent bulbs. Although that wattage can vary, for most of our clients I use 4 watt C7 bulbs. There are also newer, brighter C7 options on the market that use 7 watt incandescent bulbs. These are used for applications like outlining roofs, stringing on pergolas and outlining driveways.
C9 bulbs are larger, over 2 inches long, and fit an E17 base. They are usually available in 7 or 10 watts for incandescent bulbs and are generally used in the same ways as a C7 bulb.
If you are using C7s or C9s incandescent bulbs, bear in mind that they will get hot with use. For this reason, it is recommended to keep them away from flammable decorations. They should especially be kept away from trees, which present a particular danger of fire. For most applications, C7 & C9 lights are screwed into a socket and that socket is attached to a stringer line that has multiple sockets. In the United States the stringers are typically required to have a fuse in the male plug end of the line. These fuses usually do not allow for more than 700 watts of power per connected line. It is possible to remove the fused socket and get more lights on one strand, but this is dangerous unless you know the true power capacity of the copper line.
C7 LED light bulbs use 90% less electricity, but are the same size, and can fit into the same size base as incandescent lights. They generally have 3 LEDs per base and come in smooth, faceted, and dimmable finishes. C9 LEDs have the same basic attributes as the C7s but usually have 5 LEDs to the base.
If you wish to use either variety in animated light shows, be sure to opt for dimmable bulbs. The LED varieties also have a wider variety of effects and finishes than their incandescent counterparts. The LED versions of the C7 & C9 strings utilize approximately 1/10th of the wattage of the incandescent versions, so you can usually utilize about 10 times as many bulbs on one run of LEDs vs. incandescent options.
Even though these bulbs do not get hot to the touch, we don't recommend utilizing the C7 or C9 bulbs in a tree unless the bulbs are fused into the socket and therefore not removable. Many people like to hang these types of lights in pine or evergreen trees, but it is possible if you are using a non-fused light string that one of the pine needles can get into a socket and cause a fire.
Decorating Ideas for C7 Bulbs
There is a variety of LED bulbs that are made to look like vintage incandescent bulbs. These are especially good for a high-end look for residential applications. They are good for outlining roofs and gazebos all year round, for edging gardens, and wrapping and hanging in trees. Disney World utilizes these bulbs for wreaths, garland and around many of their buildings at the Magic Kingdom.
For parties, they create a festive retro look, strung along walkways on stakes or hanging from trees as a kind of canopy. Their vintage look is also a good fit for holiday displays for historic buildings. The opaque bulbs even look good unlit.
Decorating Ideas for C9 Bulbs
This larger size of bulb is more often used commercially, for larger scale decorations in malls and stores. The LED C9 bulbs are often used on larger outdoor Christmas trees, and for outlining roofs and doors on commercial property. They are often chosen by churches to light their outdoor Nativity scenes, by businesses to outline fences, and for other holiday displays.
A Word About Cords
Whichever bulb you decide works for you, having the right cord is important. While both incandescent lights and LEDs work with the same lighting strings, it's important that you keep in mind the maximum wattage of 700 watts per stringer, and run no longer than 250 feet of 18 gauge wire no matter how many sockets you have installed. Most light stringers are fused so this will not become an issue as the fuse will pop prior to any line melting, but if you are utilizing a non-fused stringer you need to do the math to know the total amount of bulbs you can use per strand.