How to Calculate Christmas Lights Electricity Cost in 5 Easy Steps

Christmas lights electricity cost in one line

Christmas lights electricity cost is counted in five moves: count the strands, read the watts off the label, multiply strands by watts by hours by days and divide by 1000 for the kilowatt-hours, multiply the kilowatt-hours by your rate for the bill, and divide by the days and by the strands for the per-day and per-strand figures — then sanity-check the total against the bill actually received. Twelve strands at 40 watts a strand, lit 6 hours a day for 30 days at $0.16 a kWh, use 86.40 kWh and cost $13.82. The rest of this guide is about getting the strands, the wattage, the hours, the days and the rate honest, because the formula is simple and the inputs are where lights maths goes wrong.

Step 1: Count the strands

Count the strands actually hung: the example counts 12. This is the judgement-free half of the whole calculation, because a strand either was hung or it was not — but it must be strands hung, not strands bought, because a strand still in its box draws nothing and a strand borrowed from a neighbour draws just like your own. Count every set on the display, including the tree, the porch and the fence, since the meter sees them all together and a forgotten set is a forgotten slice of the bill.

Step 2: Read the watts off the label

Read the watts printed on each strand: the example rate is 40 watts a strand. This step exists because a wattage is not one fixed figure — an old incandescent set and a modern LED set can differ tenfold, and a wattage remembered from a different set is not the wattage of the set actually hung. Judge by the label on the box or the tag near the plug, not by brightness: a bright LED set can draw very little, while a dim old set can draw a great deal. If the display mixes sets, work them separately and add the kilowatt-hours together, because one average wattage hides the hungriest set.

Step 3: Multiply strands by watts by hours by days, then divide by 1000

Multiply the strands by the watts a strand, then by the hours a day, then by the days, and divide by 1000, for the kilowatt-hours: 12 strands at 40 watts a strand for 6 hours a day over 30 days give 86.40 kWh. Work the multiplications in full, not rounded, because rounding early carries into the bill. The order does not matter — strands times watts times hours times days is the same whichever pair you multiply first — but the hours must be hours actually lit, not hours the house is awake: a timer that runs dusk to midnight is not the same as lights left on all night. If the display runs longer — forty days instead of thirty — the same multiplications hold, and the kilowatt-hours grow in step with the days, which is exactly why a miscounted day is the dearest mistake in the whole calculation after a wrong wattage.

Step 4: Multiply by your rate, then divide for a day and a strand

Multiply the kilowatt-hours by your rate for the bill, then divide by the days for the per-day figure and by the strands for the per-strand figure: 86.40 kWh at $0.16 a kWh is $13.82, $13.82 divided by 30 days is $0.46 a day, and $13.82 divided by 12 strands is $1.15 a strand. The rate must be the all-in rate actually paid per kWh, including delivery charges and taxes, and on a tiered tariff the rate on the last kilowatt-hour is the honest one, since the display may sit on a dearer tier than the rest of the house. The per-day figure is what each evening fairly costs, and the per-strand figure is what each set fairly adds.

Step 5: Sanity-check the total, and use the calculator

Sanity-check the total before you trust it: $13.82 for 12 strands at 40 watts a strand for 6 hours a day over 30 days at $0.16 a kWh should sit near the rise actually seen on the bill, and a total far from that rise tells you a strand count, a wattage, an hours figure, a days figure or a rate is wrong. Check the wattage first, because it multiplies into everything; the hours second, because a timer left on all night is the commonest phantom charge; the days third, because a display kept up into January is not a December display; the rate last, because it is the easiest figure to verify. Then use the Christmas lights electricity cost calculator to rerun the maths with your own numbers — change the strands, the wattage, the hours, the days or the rate and watch the total, the per-day figure, and the branches move, so the display you take into December is built from your house, not from this example.

Worked example, start to finish

Twelve strands, 40 watts a strand, 6 hours a day, 30 days, $0.16 a kWh. Electricity: 12 × 40 × 6 × 30 ÷ 1000 = 86.40 kWh. Cost: 86.40 × $0.16 = $13.82. Per day: $13.82 ÷ 30 = $0.46. Per strand: $13.82 ÷ 12 = $1.15. At 6 strands the total would be $6.91, at 8 hours a day it would be $18.43, and at $0.20 a kWh the 30-day total would be $17.28 — the branches show why the wattage and the hours deserve the most care, and why the rate, though it moves the bill, never touches the kilowatt-hours.

How Christmas lights compare with LED strands and all-night lights

A display is priced by its wattage, not by its brightness, so compare the 12-strand display with its LED twin before judging it dear: at 5 watts a strand the same 12 strands for 6 hours a day over 30 days use 10.80 kWh and cost $1.73, which is why swapping old sets for LED sets is the single biggest saving most displays can make. Compare it with the holiday lights cost calculator for a smaller display, and with the electricity cost calculator for any other appliance on the same meter. And compare it with the all-night version of itself: left on 24 hours a day, the same 12 strands over 30 days use 345.60 kWh and cost $55.30 — four times the timed display — which is the figure that settles whether a timer is worth buying.

Three mistakes to avoid

First, guessing the wattage instead of reading the label: the wattage multiplies into everything, and a guessed 40 watts on a set that actually draws far more or far less breaks the total by the full difference on every strand and every hour. Second, counting December only when the display runs longer: a display lit from late November into January is not a 30-day display, and the extra days bill at the same daily rate whether anyone remembers them or not. Third, using the supply-only rate instead of the all-in rate: the supply figure leaves out delivery charges and taxes, so the bill it predicts always lands short.