Calculating the cost of Christmas lights

 

I saw a claim in a supermarket advertising magazine that a 1000 LED fairy light array would only cost 44p for the whole of December. The small print clarified that it is 6 hours a day and a Unit cost of 34p as a basis for the calculation. 44p/34p = 1.3kWh used. 31 days x 6 hours = 186 hours. That means power of the array = 1300Wh/186h =7W. For 1000 LEDs, that's 7mW each. Using data from here, a white LED running on about 3V would draw 20mA and have a power of 60mW. But if these are really dropping 230V across 1000 in series, each would get 0.23V. That would be below the threshold for most diodes and they shouldn't light. The LEDs would need to have roughly 75 groups of 12 wired, with each 12 in parallel, to get the desired voltage. This needs some thinking.