You can hang two strings of christmas string lights on the same roofline and one looks like a trade pro did it while the other looks like a bead necklace. The difference is almost never the LED. It is the gap between bulbs, and nobody prints the number on the box.
Why the gap is the real spec
Coverage is not about how bright each diode is. It is about how far the eye has to travel before it hits the next point of light. A string spaced too wide reads as a dashed line, and a dashed line on a fascia reads as cheap. Spaced too tight and you waste current, add weight to the clips, and pay for bulbs you could not see anyway. In our line runs the spacing rule we hold buyers to is 4 to 6 inches between C7 units and 3 to 4 inches between C9 units, because a C9 head is larger and the eye needs less travel.
When a buyer sends us a photo of a dark patch under a barge board, that is the gap, not a dead LED. The diodes are fine. The interval was too wide for the depth of the eave.
Festoon is a different animal
Festoon is the round, globe-type head, usually 40 to 60 mm. Because the head is bigger, the gap can be a little wider before the eye loses the line. We spec festoon at 5 to 8 inches. If you take a C7 rule and apply it to festoon you will crowd the run and the globes will sit on top of each other at the peak. Take a festoon run and force it to 3 inches and you will clip half the globes against the fascia.
The other thing nobody tells you: festoon is a sagging product. It is heavier and the cable is looser. A run that leaves the factory at a nice 6-inch gap will settle into 4 inches by the second week if the cable was cut too long. When you buy festoon, cut the cable short and trust the head size, not the box gap.
Running the numbers on a real eave
Take a 6 metre fascia. At 4-inch C7 spacing that is roughly 45 units. At 3-inch C9 spacing it is about 60. Same length of roof, forty percent more heads. Multiply that by a 500-unit order and you are not comparing two strings anymore, you are comparing two price points. We quote C7 and C9 runs on this basis and the delta is real, so ask for the gap in writing, not the length.
Bottom line: ask for the inches between bulbs, not the metres of cable. That one number is the whole quality difference on your eaves.
FAQ
What gap do I use for christmas string lights on a fascia?
4 to 6 inches for C7, 3 to 4 inches for C9. Measure the eave depth first; a deep barge board wants the wider end.
Why do my lights look sparser than the sample?
Cable sag. The gap is set at the factory on a taut line, then the cable settles and the effective gap shrinks or the heads drift. Cut the cable short.
Is festoon brighter than C7?
No, it is larger-headed. Brightness is the diode and the lumen rating; the head size only changes how the line reads. Do not buy festoon because you want more light.