Christmas Lights Power Calculator
Holiday light season cost from bulb type, hours, and electric rate.
What it does
Holiday displays hide their cost in small watts times long hours: 250 glowing C9 bulbs drink like a space heater all December, while the same count in LED sips under a dollar. The bulb type decides the bill.
The tool above takes bulb count with type, hours, days, and rate. It returns season kWh with dollar cost and the LED swap savings.
The default 250 C9 at 5 hours over 35 days burns 306.25 kWh for $52.98. The LED twin costs $0.38, saving $52.60 a season.
Math follows EIA measuring pages with ENERGY STAR string guidance behind them. Both pin the 1,000 divisor and the 0.05 to 0.2 W LED band used here.
How to use
- Count the bulbs by type.
- Pick the bulb type.
- Enter nightly hours.
- Enter season days.
- Enter your $/kWh rate.
- Read cost, then LED savings.
Formula
- bulbs
- Bulb count on the display.
- watts
- Label watts: C9 7, C7 5, mini 0.4, LED 0.05.
- hours
- Nightly hours times season days.
- rate
- Marginal $/kWh from the bill.
Watts come from the string label, not memory. Timers and dimmers change real hours; metered readings beat estimates. Rates move yearly, so the bill marginal price rules.
Worked example
Example: 250 C9 bulbs
Defaults: 250 C9 bulbs, 5 hours x 35 days, $0.173/kWh.
- Bulbs: 250 C9 at 7 W each.
- Watts: 250 x 7 = 1,750 W.
- Hours: 5 x 35 = 175 hr.
- kWh: 1,750 x 175 / 1,000 = 306.25.
- Cost: 306.25 x $0.173 = $52.98.
- LED check: 250 x 0.05 W = 12.5 W, 2.19 kWh, $0.38.
- Savings: 52.98 - 0.38 = $52.60.
- Tool shows $52.98, 306.25 kWh, $52.60 saved vs LED.
Reference table
| Bulb | Watts | Note |
|---|---|---|
| C9 incandescent | 7 | 140x an LED |
| C7 incandescent | 5 | Mid size |
| Mini incandescent | 0.4 | Dense strings |
| LED | 0.05 to 0.2 | STAR max 0.2 |
Limits
- Label watts rule; old strings drift.
- Timers and dimmers change hours.
- Tiered rates need marginal price.
- Fuse and circuit limits sit outside.
Common mistakes
- Averaging mixed bulb types.
- Forgetting nightly hours add up.
- Using average instead of marginal rate.
- Chaining past fuse limits.
- Pricing LEDs at incandescent watts.
Try it yourself
Quick checks. Answers sit tucked under each question.
100 mini bulbs, 6 hr x 30 days, $0.15?
40 W x 180 hr / 1000 = 7.2 kWh x $0.15 = $1.08.
200 LEDs, 5 hr x 30 days, $0.17?
10 W x 150 hr / 1000 = 1.5 kWh x $0.17 = $0.26, already LED.
25 C7 bulbs on defaults?
125 W x 175 hr / 1000 = 21.88 kWh x $0.173 = $3.78.
Questions
How many watts per bulb?
About 7 W C9, 5 W C7, 0.4 W mini incandescent, and 0.05 to 0.2 W LED per bulb.
How is the cost computed?
kWh = watts x hours / 1,000. Cost = kWh x your $/kWh rate from the bill.
Do LEDs really save that much?
Yes, roughly 92% per ENERGY STAR: a 7 W bulb equals about 140 LEDs in power draw.
How many strings can I chain?
Follow the string label and circuit load. Fuse limits beat math every time.
What if my display mixes bulb types?
Yes. Sum each bulb group separately instead of averaging watts.
What about tiered electric rates?
Use the marginal tier price and note it, since average rates understate extra load.