It costs less money to leave the light on than it does to turn the light on and off because you don’t have to replace them so often.

Granny

When you turn a light on – any electrical circuit for that matter – then there’s a short term ‘surge’ of energy flowing as it starts up.

For a traditional incandescent bulb, this initial loading (the ‘inrush current’) is very significant. The cold element has a lower resistance and draws more power than normal in order to warm up to its glowing temperature – so if the filament is getting close to failing then all that extra energy passing through it at once is usually enough to tip it over the edge (ever noticed how the old bulbs almost always blew just when you turned them on? That’s why). In that respect, leaving the light on all the time will prevent these ‘shock loads’ and, yes, an incandescent bulb would almost certainly last longer than one that’s turned on and off.

But in terms of cost? Well, the power of the bulb is going to have a massive impact on the calculation. The inrush current lasts for milliseconds and is entirely insignificant, in all practical considerations. Once the light on then it’s drawing a steady current and we can measure that very easily. The difference between the two use cases, therefore is simply down to the cost of a new bulb vs. the power consumption it uses. Using 2022 prices, a standard 60W bulb – the kind Granny would have in her sitting room – would cost about £0.50 a day if left on for 24 hours…so a week of that and we’re going to be losing money if we listen to Granny, that’s for sure.

Of course, this assumes we can still buy an incandescent bulb – most shops don’t sell them these days! If we’re talking about LED bulbs then there’s a slightly different story: they don’t suffer anything like as badly with inrush current stresses so are less likely to fail on start-up; they also use much less power AND a replacement costs more than the old style incandescent ones did. Despite all this we should still break even after a couple of months (a 5W LED running for 24 hours will cost about 0.5 Pence).

Not your brightest idea, Granny!