"You left the lights on!" was a familiar complaint in my childhood, the monthly electricity bill being what it was. These days, we get advised to unplug power supplies and even television sets that use a trickle to remain alert for the remote control ON command.
I now realize that this is all nonsense in the winter but triply true during the summer.
About 90 percent of the electricity used by an old-fashioned light bulb goes to heat and only 10 percent to light. But even the light soon turns into heat, given the laws of thermodynamics. In the winter, turning off a light just means that the furnace has to work that much harder to maintain room temperature.
There are some exceptions, such as outdoor lighting, where the heat serves no secondary purpose. But as a photon of light goes bouncing around off the living room walls, it is eventually absorbed and turned into radiant heat, infrared variety.
With sufficiently well-insulated walls, you can heat a room by leaving the lights on. Or the computers. Each person present adds another 100 watts. There may be other considerations, such as having to replace the light bulbs more frequently or it being locally more expensive to heat with electricity, but otherwise watts are watts whatever the source.
In warmer months when the furnace isn’t needed to heat the house, then leaving on the computers and the lights really is a waste. If it is warm enough to open the windows, start turning things off.
And when it gets hot enough outside for air conditioning inside, the waste triples. That’s because removing 1 watt of unwanted heat requires about 2 watts of electricity, inefficiency being the rule (a law, actually; thermodynamics of entropy). So that’s 3 watts of waste that you can avoid by turning things off.
It’s all relative—to the season. If I’d only known more thermodynamics in my childhood, I’d have had the perfect comeback.