# How can I calculate the cost of Christmas lights?

This year I've put up Christmas lights up around the living room and found that they're nice to leave on overnight in case someone needs to come downstairs for some reason.

They are LED lights, so I don't think I'm adding creating a lot of extra costs for electricity, but how can I measure the additional cost associated with leaving those lights on 24 hours a day?

• The most accurate way is to measure their actual usage via a kill-a-watt or power meter type device. Commented Jan 5, 2018 at 21:37
• Any energy you're spending keeping internal lights on is turning into heat, which lowers your heating bills. So in the winter, apart from leaving lights onwhen you're not home, it's pretty much free. Commented Jan 5, 2018 at 21:43
• @Acccumulation, it's only free if you're using electric resistance heaters. If your heating comes from natural gas, you need to account for the price difference between electricity and gas; if you're using a heat pump, you need to account for the efficiency difference (electric lights are only 100% efficient; a heat pump under good conditions can hit around 400% efficiency).
– Mark
Commented Jan 5, 2018 at 23:21
• @HartCO Unfortunately LEDs tend to wreck havoc with smart meters. I've seen multiple homes where most of the lighting was switched out from incandescent to LED where the energy bill didn't drop as much as it should. But only on those with smart meters. Old meters were unaffected.
– Mast
Commented Jan 6, 2018 at 12:17
• @DanNeely Fun story... had a company come out a pitch us on installing a solar hot water heater. As a bonus they would throw in a power factor correction system to save on electricity costs. Didn't bite, but later discovered our power company doesn't charge residences for power factor relative (reactive) loads, they only charge industrial / commercial users!
– user12515
Commented Jan 6, 2018 at 17:27

One way is by using a plug power meter. Simply plug the meter into your outlet, then plug whatever you're measuring into the meter, and let it go for a while. It will tell you the cumulative power usage (usually in kWH which is what your electric bill uses too). From there you can extrapolate the actual cost per hour/day/etc.

Warning: since your lights are LEDs and presumably use very little power, it's very likely that the cost of the meter will be more than the cost of what you're measuring. But it's still a fun tool to have around the house. Learning how much it costs to dry your hair or vacuum the house can be somewhat enlightening.

• The meter might be not precise enough to measure low loads.
– n0rd
Commented Jan 6, 2018 at 1:00
• Well, if it’s that low, the cost is minimal to begin with. Commented Jan 6, 2018 at 2:43
• @n0rd: even in the "10 € class" you can find devices that are rated to measure down to 0.2 W with an error of ± 0.2 W which corresponds to some 10 ct uncertainty for a month of 24/7 lighting. IMHO quite sufficient for practical purposes. Commented Jan 6, 2018 at 15:19

There are two costs:

1. Upfront costs and replacement
2. Electricity consumption

I'll go over these costs for this product, but you can adapt them for whatever kind you bought.

If these kinds of light last for 75,000 hours (this might be a stretch) and you are running them 24/7, then they will last for 8.56 years. It will cost an average of \$2.34 a year to replace each set of lights.

The lights I linked to have power consumption of 30.6W, or 0.0306KW. In one hour, a set of lights will consume 0.0306KWh of electricity. Electricity will cost around 12 cents per KWh. In one hour, the lights will cost \$0.003672 to run. The electricity will cost around \$32.17 a year to power one strand.

Altogether, it will cost about \$35 a year to power each strand of Christmas lights.

• Your "this product` link gives me an "access denied". Commented Jan 5, 2018 at 22:25
• @MikeWise the link works fine for me. Just takes you to a Home Depot product page Commented Jan 5, 2018 at 22:39
• Interesting. I am coming from Germany, maybe they only allow US IP addresses. Out of curiousity, how many lights were on that string? Commented Jan 5, 2018 at 23:18
• @MikeWise imgur.com/a/qRQpF Commented Jan 6, 2018 at 0:04
• That always happens with Home Depot links, I don't know why. If you go to the homepage you can usually find the product just fine. Commented Jan 6, 2018 at 10:04

A nice rule of thumb is that 1 Watt costs approximately 1 € or \$1 if you run it 24/7 for a whole year, or 0.1 € or \$0.1 if you run it for a month. This is a quite rough estimate that could be wrong by +-50%, but it is easy to remember so you can check power usage already at the store.

So find the Watt reading on the type label:

(source: ukchristmasworld.com)

Here the rating is 5.94 W. From this it is quite easy to figure even without a calculator that it will cost about \$0.60 to run these lights for one month.

• Just be sure you take the input side rating if there's voltage conversion - power supplies can be shockingly inefficient... Commented Jan 6, 2018 at 15:07
• Is it really independent of electricity price?
– user45830
Commented Jan 8, 2018 at 16:07
• @9ilsdx9rvj0lo The actual cost of course depends on electricity price, the rule-of-thumb assumes ~\$0.12 per kWh which works for many countries. Though there are exceptions, e.g. Germany has quite expensive electricity nowadays, and 3 EUR/watt-year would be more correct there.
– jpa
Commented Jan 9, 2018 at 15:13

So, Nosrac's answer is mostly correct in terms of how to do the calculation, but the actual results can vary quite a bit.

Personally, I bought several 100 LED strands this season for about \$7 USD each. Each strand is rated at 6.5 W (which seems much more reasonable than 30+ W for an LED strand.) Where I live, power costs about 9.6 cents/kW-Hr.

With these numbers, using the same 8.56 year projected lifespan, we get these results:

Purchase Cost

\$7 / 8.56 yr = \$0.82 / yr

Energy Usage

6.5 W * \$0.000095 / W-Hr = \$0.00062 / Hr

So, for a year, that would be 365 * 24 * \$0.00062 = \$5.43

Total Cost of Owning and Using the Lights per Year of Operation

Given the above figures, this means that, if you ran the lights 24/7 all year, you'd have a projected total cost around \$6.25 per strand per year if you used the lights I purchased and paid a power cost similar to mine.

HVAC

If you want to be extremely precise about your calculations, you need to consider the factor that Acccumulation mentioned in the question comments: heating and air conditioning. If you live in a climate that requires heating in the winter and you only use these lights inside in the winter, your marginal cost of operating the lights is actually reduced, as nearly all of the 6.5W used by the strand will end up getting dissipated as heat into your home.

This is effectively the same thing as if you were using a 6.5 W resistive heating coil. Thus, if you were using resistive heating to heat your home, you'd actually break even on the power cost of operating the lights vs. the power you'd have otherwise used from your heater. However, electric heat pumps can provide heat significantly more efficiently. If you'd normally be heating your home with an electric heat pump that moved 4 times its power consumption into your home, then the effective cost of power the LED light strand would only be reduced by a quarter rather than being eliminated entirely.

On the flip side, if you're running the lights during the summer in a climate that requires air conditioning, then you'll be spending additional electricity in your air conditioning system to remove the heat produced by the lights from your home. If you live in a temperate climate, the effects of reduced heating and increased air conditioning usage may mostly cancel each other out if you run the lights all year long.

Conclusion

Since I use an electric heat pump to heat my home during the winter, using my example numbers from above, my effective cost of operating each strand of lights within my home if I operated them only during the winter months would be:

\$0.82 + (0.75 * \$5.43) = \$4.89 per year of operation