The short version
A 100-bulb incandescent string draws about 40 watts; an equivalent LED string draws about 4–5 watts. At 6 hours a night through December at the US average rate of ~$0.17/kWh, one incandescent string costs about $1.26 for the season and one LED string costs about $0.16. The bulb type decides almost everything; the string count decides the rest. Price your exact display in the Christmas lights cost calculator.
The one formula: watts → kWh → dollars
Your electric bill charges for energy — kilowatt-hours — not for lights. A kilowatt-hour is 1,000 watts running for one hour. So price any display in three steps:
Step 1: add up the watts. Count your strings and multiply by the watts printed on each tag: (incandescent strings × watts each) + (LED strings × watts each). No tag? Use 40W for a standard 100-bulb incandescent string and 5W for a 100-bulb LED string.
Step 2: convert to kWh. watts × hours on per night × nights in the season ÷ 1,000. So 450 watts × 6 hours × 31 nights ÷ 1,000 = 83.7 kWh.
Step 3: price it. kWh × your $/kWh rate. 83.7 × $0.17 = $14.23 for the season. Use the supply line on your electric bill for the rate — that's the number the utility actually multiplies.
LED vs incandescent: the only comparison that matters
This is the whole ballgame. Same display, same hours, ten-to-one price gap:
One 100-bulb string, 6 hours a night, 31 nights, $0.17/kWh: incandescent (40W) ≈ $1.26 per season · LED (5W) ≈ $0.16 per season.
A 10-string display: incandescent ≈ $12.60 · LED ≈ $1.60.
A 20-string display: incandescent ≈ $25.20 · LED ≈ $3.20.
The gap widens on big displays and in high-rate states — the same watts cost roughly twice as much where electricity runs $0.30+/kWh. LEDs also run cool (a fire-safety plus near a dry tree) and last 10+ seasons, while incandescents burn out and run hot.
Worked example: the classic house
Six incandescent strings at 40W (240W) plus six LED strings at 5W (30W) = 270W total. On 6 hours a night for 31 nights: 270 × 6 × 31 ÷ 1,000 = 50.2 kWh, costing $8.54 — about $0.28 per night.
Swap the six incandescent strings for LED ($9 each = $54): the display drops to 60W, the season costs $1.90, and the $6.64 per season savings pays the new strings back in about 8 seasons on energy alone. On a 20-string all-incandescent display, the same math saves ~$22 a season — the payback drops to about 4 seasons, and the longevity means you may never buy strings again.
The timer question: the cheapest upgrade is a schedule
Runtime is money. A display left on from dusk to dawn in December can run 13–14 hours a night; a 5 PM–11 PM timer window runs 6. That one change cuts the season cost by more than half — on the classic-house example above, from roughly $19.90 all-night to $8.54 on a timer.
A basic mechanical outdoor timer costs a few dollars and pays for itself the first December. Use an outdoor-rated, GFCI-protected timer outside — never an indoor-only timer. And skip dusk-to-dawn sensors if you can: in December's short days they keep the lights burning far longer than the evening window you actually enjoy them.
Why your December bill still jumps
Here's the twist: most December bill spikes aren't the lights. Heating (or a heat pump working harder in the cold), shorter days with more indoor lighting, extra cooking, and guests taking hot showers usually move the bill far more than the display. Lights are the visible suspect because they're new in December — but they're usually the smallest new line item.
The other hidden costs: inflatable yard decorations, whose fans draw 60–150 watts continuously — two or three inflatables can out-cost the entire light display. And on time-of-use rate plans, running lights through the 4–9 PM peak window can price the kWh higher than overnight power. Price the display in the calculator first, then look at heating and hot water for the rest of the jump.
Frequently asked questions
How much does it cost to leave Christmas lights on all night?
At 14 hours a night instead of 6, everything scales by 14/6 ≈ 2.3×. The classic-house display above jumps from $8.54 to about $19.90 for the season — and the timer that fixes it costs less than the difference.
Is it worth replacing working incandescent strings with LED?
On energy alone the payback is slow — about 8 seasons for a small display — because the per-string savings are ~$1.10 a season. It pays back faster on big displays, and the real argument is longevity (10+ seasons vs. annual burnout) and cool, safer operation.
Do Christmas lights use a lot of electricity compared to everything else?
No — a typical display is 50–500W, while a space heater is 1,500W and a water heater thousands. Lights are a tiny, highly controllable slice of winter demand, which is exactly why they're worth pricing: you can shrink them to almost nothing with LEDs and a timer.
What rate should I use — the whole bill or one line?
Use the supply charge line (the $/kWh the utility multiplies), roughly $0.17/kWh on average in the US. Dividing the whole bill by kWh overstates the rate because it folds in fixed monthly fees that don't change with the lights.
Are solar-powered Christmas lights free to run?
Yes for the electricity — they charge by day, even on cloudy ones, and draw nothing from the grid. The trade-off is dimmer output and dependence on daylight; they're best for accent spots, not the main display.