The difference, in numbers
The number is the bulb’s width in eighths of an inch. A C7 is 7/8 of an inch across and roughly 1.5 to 2 inches tall. A C9 is 1 1/8 inches across and about 3 inches tall. C7 screws into a small E12 candelabra base, C9 into a larger E17 base, so a string of sockets is one or the other.
Both are the traditional cone-shaped bulb. C9 is the one from the old photographs of houses lit in the 1950s.
Which belongs on a house
C9 is the roofline bulb. It is big enough to read from the street, and it is usually spaced about a foot apart, which gives a roofline that crisp dotted line. A house set back from the road, which describes a lot of Vermont, is a C9 house.
C7 suits the parts people see up close: porch railings, window and door frames, a walkway, bushes. It also suits a small house or a condo that sits right on the sidewalk, where C9 can look heavy.
Mixing sizes works when each size has its own job, C9 along the roof and C7 on the porch rail. Mixing them on one run does not.
LED or incandescent
Both sizes come both ways. An incandescent C9 draws about 7 watts, so 100 of them on a roofline pull around 700 watts every evening. The same run in LED draws well under 100. LED bulbs also stay cool to the touch and last many seasons, which is why nearly every installer and most stores now sell LED.
The old argument for incandescent was the glow. Newer warm white LEDs copy it closely, and the same warm white vs cool white rule applies to either size: pick one white for the whole house.
What about C6?
C6 is the small strawberry-shaped LED bulb, about an inch tall. It is a tree and wreath bulb, or a roofline bulb on a very small house. On a normal roofline it disappears from the road.
