Run hours are the input people get wrong, not watts
Everyone knows to look at the wattage. Almost nobody works out the hours properly, and the hours are half the product. A display on a timer set to dusk-to-eleven does not run six hours a night for six weeks — it runs 6.17 hours on the first night, 6.33 in the middle when the evenings are darkest, and 5.92 by early January. On the default schedule that comes to 259.8 run hours across 42 nights.
The shortcut of taking the first night and the last night and averaging them gives 253.8 hours — six hours short, 2.4 percent under. That is small in dollars and it is entirely systematic: the shortcut draws a straight line between the two ends and every night in between is longer than that line, because the darkest evenings sit in the middle. The more asymmetric the season, the worse it gets.
Why dusk moves the way it does
Sunset gets earlier through late autumn, reaches its earliest point somewhat before the winter solstice rather than on it, then starts getting later again while the mornings are still getting darker. That is why a dusk-triggered timer produces a V shape rather than a straight line, and why the middle of a holiday season has the longest run nights.
The size of that swing depends on latitude and on where in the season you are. This page does not compute it. It asks you for dusk on the first night, dusk at the darkest point and dusk on the last night, and interpolates in a straight line between them, because the alternative is a page that pretends to know where you live and how much tree cover is west of the house. The default times here swing 25 minutes from the first night to the last, which over 42 nights is the six hours the endpoint shortcut misses — real, and still much smaller than the difference between guessing 4.8 watts per string and looking it up.
Zones exist because displays are not one thing
An inflatable with a blower running all night is a different creature from a roofline of LED strings. The blower runs continuously and draws real power; the strings draw almost nothing. Splitting the display into zones makes that visible, and it also lets you handle the zone that only goes on at weekends, or the one that came out of the box three weeks into the season.
On the default zone list — 144 W of roofline, 58 W of porch, 96 W of wrapped trees and a 65 W inflatable that only runs 30 of the 42 nights — the whole display comes to 363 W with everything on, and 3.03 A at 120 V. The season total is 89.5 kWh, or $14.32 at 16 cents. The roofline is the biggest single share at $5.99; the inflatable, despite being the largest single item most people notice, costs $1.93, because it runs 30 nights rather than 42 and 65 W is 65 W.
An hour is worth more than a zone
The last line on the calculator is the one worth looking at: what one extra hour every night would cost. On the default display it is $2.44 for the season. That is the honest scale of the whole exercise. Turning a modern LED display off an hour earlier saves two and a half dollars; running it all the way to seven in the morning instead of switching off at eleven adds eight hours a night, about twenty dollars, and takes the season past thirty.
The picture is entirely different with incandescent strings, where the same display might draw ten times the watts and the same schedule costs over a hundred dollars. That comparison, including how long the bulbs last, is properly handled on the LED conversion savings calculator. If you want the season cost in the context of a whole bill with its fixed charges, tiers and taxes, that is the electricity bill calculator. For what the peak draw means once it has to be plugged into something, the chain and circuit calculator takes your own breaker rating and working figures — and reports rather than judges.
Questions people ask
How much does it cost to run Christmas lights?
Watts times hours, divided by a thousand, times your rate. A 363 W LED display running dusk to eleven for 42 nights is 259.8 run hours, and with one 65 W zone only running 30 of those nights it comes to 89.5 kWh — $14.32 at 16 cents per kWh. The same schedule with incandescent strings drawing ten times the power lands over a hundred dollars. The wattage on the boxes is the number that decides it, and it varies by a factor of ten between products.
Why do the run hours change from night to night?
Because a dusk-triggered timer follows sunset, and sunset moves. It gets earlier through late autumn, bottoms out shortly before the solstice, then starts getting later again. A display that comes on at dusk and goes off at a fixed clock time therefore runs longest in the middle of the season. On the default schedule the nightly length goes 6.17 hours, up to 6.33 at the darkest point, then down to 5.92 — and averaging just the first and last night undercounts the season by six hours, about 2.4 percent.
Does the calculator know when sunset is where I live?
No, deliberately. It asks you for dusk on the first night, dusk at the darkest point of the season and dusk on the last night, and interpolates between them. That way the answer reflects your latitude, your horizon and whatever the timer sensor actually does behind the tree in your front yard, rather than a computed sunset for a location the page had to guess at.
How do I handle an inflatable that only runs at weekends?
Put the number of nights it runs as the third value on its line. The calculator spreads those nights across the season at the average nightly length. If the zone only runs on the darkest weekends in mid-December, its true hours are a little higher than reported, since those are the long nights — the page says so rather than pretending otherwise.
Is it cheaper to leave lights on all night or switch them off?
Switching off is cheaper by exactly the hours saved times the watts, and the calculator shows the cost of one extra hour a night so the scale is visible. On a 363 W LED display that hour is worth $2.44 over the whole season. There is no standby saving to weigh against it and no start-up cost worth counting for LED strings. The size of the number is the point: on a modern display this is a small decision, and on an incandescent one it is not.