The average house with 7 strings of LED lights, left on for 6 hours a day all December, spends about £1.50 on the electricity used by those lights.
With adult supervision, plug your fairy lights into an appliance monitor plugged into the wall. If you don't have an appliance monitor keep an eye on your smart meter. If there is no other electrical items running that will cause changes in your electricity use, this is a good way of seeing how much electricity the lights use.
Looking at the display on your monitor or meter. Can you see how much energy the lights are using? What questions can you ask about the energy use? Do coloured lights use more energy? Do longer strings of lights use more energy than shorter ones? Do lights use more energy when they are flashing or still? Does the size of the individual lights make a difference to how much energy is consumed? Discuss your answers with your adult.
Note that if households are using older style non-LED lights, the costs could be much higher.