You buy string lights and trust the spec sheet. The spec sheet is a rounding, not a measurement, and that difference is where holiday runs die. I have watched good lights fail in production because nobody budgeted the real draw of the whole circuit, so let us fix the math before December fixes it for you.
The spec sheet hides your real draw
The wattage printed on the box is the rated LED chip load at 20 degrees. It does not include the driver, the current limiters, or the fact that cold weather pushes a sealed string’s actual line draw 15 to 25 percent above the printed number. A string that says 10 watts on the label has drawn 12 watts on the meter at minus ten, and I have logged that gap on a production test bench twice in one winter. Incandescent minis are more honest and more punishing: a 100-bulb strand genuinely sits near 40 watts, and a C7 string of 20 bulbs lands around 130 watts because each C7 glass burns six to seven watts.
The second hidden number is the circuit itself. North American outlets are usually 120 volts on a 15 amp breaker, and the code answer is not 1800 watts. Continuous loads get a 20 percent haircut, so your real working ceiling is around 600 watts per circuit before a breaker starts to sweat. EU and UK installs are similar in spirit: budget the circuit, not the string, and leave headroom for everything else that is plugged in at the same time.
The 80 percent rule
Here is the rule I apply to every customer install: never load a circuit past 80 percent of its rating, and never assume a single 16 amp EU outlet is a personal power grid. Six 40-watt incandescent strands is 240 watts, which feels small until you add the neighbour who is plugging three of the same into the same box and you are now 720 watts on a 13 amp circuit, and the breaker is doing the math for you. LED strings change the story entirely. A 100-bulb LED strand sits at five to ten watts, so a full house run of 40 strands is still under 400 watts, and the breaker is never in the conversation.
That headroom is why we spec commercial runs at 60 to 70 percent of circuit capacity even when the lights could physically take more. The margin covers the cold-weather draw rise, the inrush when 200 strings power on at once, and the fact that a 230-volt driver running warm inside a sealed housing has a lower effective ceiling than its nameplate suggests. The one I have seen take a storefront down was a mall display that hit 95 percent of a 32 amp subcircuit on a warm December afternoon and cooked a connector. The lights were fine. The plan was not.
What a real run costs to run
Do the arithmetic before you buy, because the difference is the difference between a December bill and a bill you can ignore. At 16.5 cents per kilowatt hour, ten 40-watt incandescent strings running six hours a day for 45 days lands near 113 kilowatt hours, about 19 dollars. The same 45 days, same hours, but 100-bulb LED strings at eight watts each comes in under 22 kilowatt hours, so close to four dollars. The LEDs cost more up front and cost less every month you own them, which is the trade that pays off the moment you run the lights for the full season rather than a single long weekend.
When you are buying, do not buy the cheapest watts. Buy the strings whose rated wattage matches a real spec you can verify against the driver housing, and if you are running more than six strands, split the load across two circuits instead of trusting one. That split is a five-minute job in the box that saves a whole season of dark eaves.
Look at the christmas string lights catalogue for 100-bulb LED runs, and if you are dressing a roofline or storefront, the outdoor christmas lights range is where the power budgeting starts to matter.