A watt (W) is a unit of measurement of power. Today, this information appears on the technical descriptions for all electrical appliances. 1 kilowatt (kW) = 1,000 watts (W).
1 kilowatt hour (kWh) is the energy consumed by a 1,000W or 1kW electrical appliance operating for 1 hour. See more at “What Do Watt and KWh Mean?” (Energuide, 2019).
So in order to calculate the energy consumed by an appliance, you would multiply its power (in KW) by the number of hours it runs.
According to Moss T and Atansah P, “US Holiday Lights Use More Electricity Than El Salvador Does In a Year” (Center For Global Development, 2015), decorative seasonal lights account for 6.6 billion kWh of electricity consumption every year in the United States, which could run 14 million refrigerators and is more than the annual electricity consumption of many developing countries.
Christmas lights at home - Harding J, “How Much Are Your Christmas Lights Costing You to Run?” (Which? News, December 13, 2017):
Christmas lights are on for about 30 days, 6 hours every night, making it about 180 hours during the season.
A string of 100 incandescent fairy lights has a power of 40W.
This brings energy consumption of a fairy light for the season to about 7kWh.
A household uses 7 strings on an average, consuming about 50kWh.
To get an approximate comparison, a 1000W microwave oven, operating for 1.5 hours/week, consumes 60kWh in a year.
An average outdoor home Christmas lighting display consumes 12.5 times more energy than a string of 160 incandescent fairy lights.
Christmas lights on the high street - “Who Takes Care of the Christmas Lights? And How Much Power Do They Actually Use?” (Energuide, 2019):
On an average, Christmas lights are on from early December until mid-January, for about 8 hours every night, and some nights until the morning, making it about 330 hours during the season.
A traditional chain of lights with incandescent bulbs has a power of between 600W to decorate an average street and 1,500W for a major route.
This brings total consumption to between about 200 and 500kWh, an average of about 350kWh.
Lighting up a large Christmas tree uses 1,000W, which makes its consumption 330kWh.
The total for the season for a high street would be about 680kWh. An inefficient combi fridge-freezer (energy rating C) switched on throughout the year uses less than that at 500kWh.
Energy can be saved by turning Christmas lights off before bed.
If the incandescent lights are replaced by LED lights, it can reduce energy consumption by 90%!
Haq G, Owen A and Dawkins E, “The Carbon Cost of Christmas” (Stockholm Environment Institute at the University of York, 2007)
“How Much Energy Do My Household Appliances Use?” (Energuide, 2019)