Leaving a single LED light on overnight costs about 2p, and running it round the clock for a whole year comes to roughly £23. Swap that LED for an old-fashioned 60-watt bulb and the same year costs about £137. So how much it costs to leave a light on depends almost entirely on the bulb: for modern LEDs the figure is small, and for the halogen and incandescent bulbs still lurking in some homes it is not. This guide puts real pennies on every scenario, from a forgotten landing light to a security floodlight blazing all night, so you can see which habits are worth changing and which are not.
The short answer. A typical 10-watt LED bulb costs about 0.26p an hour, so leaving it on for an eight-hour night is around 2p, and leaving it on all year round the clock is about £23. The same sums for an old 60-watt incandescent bulb are roughly 1.6p an hour, 12p a night and £137 a year. At the current Ofgem price cap of about 26p per kWh, lighting is cheap when it is LED and surprisingly dear when it is not. If you still have old bulbs anywhere, that is where the money sits.
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How to work out what a light costs
Every lighting cost comes back to one sum, the same one that prices any appliance. Take the bulb's power in kilowatts, multiply by the hours it is on, then multiply by your unit rate. Bulbs are rated in watts, so divide that figure by 1,000 to get kilowatts: a 10-watt LED is 0.01kW, a 60-watt bulb is 0.06kW. The kilowatt hour guide explains the unit in more detail, but you do not need the theory to get a number.
A worked example makes it concrete. Say you leave a 7-watt LED bulb on in the hall for five hours a day. That is 0.007kW × 5 hours = 0.035 kWh a day, which at 26p is about 0.9p. Over a year it adds up to roughly £3.30. The wattage is printed on the bulb or its box, usually next to a brightness figure in lumens, so you can price any fitting in your home in about thirty seconds.
Leaving a light on overnight, bulb by bulb
Here is the one table that answers the question for the bulbs you are most likely to have. The middle column is a single eight-hour night; the last is the cost if you somehow left it on every hour of the year, which is the worst case and a useful upper limit.
| Bulb | Per hour | 8-hour night | On all year (24/7) |
|---|---|---|---|
| 7 W LED | ~0.18p | ~1.5p | ~£16 |
| 10 W LED | ~0.26p | ~2p | ~£23 |
| 15 W energy-saving (CFL) | ~0.39p | ~3p | ~£34 |
| 50 W halogen spotlight | ~1.3p | ~10p | ~£114 |
| 60 W incandescent | ~1.6p | ~12p | ~£137 |
Worked at about 26p per kWh, the current Ofgem price cap. Your own unit rate is on your bill; swap it in and the pattern stays the same.
The takeaway is that a forgotten LED is close to a rounding error. If you leave the landing light on all night by accident, and it is an LED, you have spent about 2p. Feeling bad about that is not worth your evening. The old bulbs are a different matter, and they are the reason the advice to turn lights off ever carried any weight.
Why the bulb type decides everything
All five bulbs in that table can throw out roughly the same amount of light, around 800 lumens, which is a normal bright room bulb. The difference is how much electricity each wastes as heat. An old incandescent bulb is really a little heater that happens to glow, turning most of its 60 watts into warmth you do not want. A halogen is a slightly better version of the same idea. An LED does the same job for about 8 to 10 watts because it makes light directly rather than by heating a wire until it glows. That is a six- to eightfold difference in running cost for identical brightness, which is why switching to LED is one of the few changes that pays back quickly, as the LED lighting guide sets out.
So the real question is rarely "should I turn this light off" and more often "is this still an old bulb". A house fully converted to LED can leave a fair few lights on and barely notice it on the bill. A house still running halogen downlighters can save more by swapping the bulbs than by nagging anyone about switches.
The lights that still cost you: spotlights and floodlights
Single bulbs are cheap to leave on. Clusters of powerful ones are not. A kitchen or bathroom fitted with eight 50-watt halogen downlights is drawing 400 watts whenever they are on, the same as a small heater. Run that for four hours a day and it is 1.6 kWh, about 42p a day, or roughly £150 a year. Replace those halogens with 5-watt LED equivalents and the whole set drops to 40 watts, cutting the yearly cost to around £15. That is a swap that pays for itself inside a few months and keeps paying every year after.
Outdoor security lighting is the other big one. A 500-watt halogen floodlight left on from dusk to dawn, say ten hours, burns 5 kWh a night, about £1.30, which is close to £475 a year if it runs every night. An LED floodlight of 30 watts doing the same job costs under 8p a night, around £28 a year. Better still, pair it with a passive infrared sensor so it only comes on when something moves, and the running cost falls to almost nothing. If any light in your home is both high-wattage and often left on, it is worth pricing with the running cost calculator before you decide whether to change the habit or the hardware.
Does switching a light off and on waste more than leaving it on?
This is the myth that keeps lights burning. The belief is that flicking a light on draws a big surge of electricity, so for a short absence you are better leaving it on. For the bulbs in a modern home it is simply wrong. An LED or an incandescent bulb draws its normal power the instant it comes on, with no meaningful surge, so turning it off always saves energy, even for a two-minute trip out of the room. Off for a minute is a minute you did not pay for.
The grain of truth behind the myth belonged to old fluorescent tubes, the long strip lights in garages and kitchens. Switching those on did cause a small amount of wear to the starter, so very frequent switching could shorten the tube's life a little. Even then the effect was about the tube's lifespan, not the electricity, and it never applied to ordinary bulbs. With LEDs there is no downside to switching at all. The simple rule holds: if you are leaving the room for more than a moment, turn it off. The energy saving myths guide runs through the rest of the half-truths that cost people money.
Smart bulbs, dusk-till-dawn lamps and standby
A few lights never really switch off. A smart bulb stays connected to your wi-fi even when it is "off", drawing a small standby load, usually under half a watt. That costs on the order of £1 a year per bulb, so a houseful of them is a pound or two, not something to lose sleep over, though it is part of the wider standby power picture. Plug-in timers and dusk-till-dawn sensors draw a similar trickle. None of these is a reason to avoid smart lighting; they just are not the saving some people imagine them to be either.
What I would actually do
Stop worrying about LEDs left on, because the numbers show it is pennies, and put the effort where the watts are. First, hunt down any remaining halogen or incandescent bulbs, especially clusters of spotlights, and swap them for LED; that single move does more than a lifetime of diligent switching, and the bulbs cost very little, as the cheap quick wins guide covers. Second, put outdoor floodlights on a movement sensor so they are off unless needed. Third, if you want to see where lighting sits against everything else you run, the what uses the most electricity guide ranks the whole house, and lighting, once it is LED, is usually near the bottom. In short, fix the bulbs and the switches stop mattering.
Common questions
How much does it cost to leave a light on all night?
For a typical 10-watt LED bulb, about 2p for an eight-hour night at the current Ofgem cap of roughly 26p per kWh. An old 60-watt incandescent bulb doing the same costs around 12p a night, and a 50-watt halogen spotlight about 10p.
Is it cheaper to leave a light on or turn it off?
Turning it off is always cheaper. The idea that switching on draws a big surge is a myth for the bulbs in a modern home; an LED or incandescent bulb uses its normal power the moment it lights, so even a short absence is worth switching off for.
How much does a light bulb cost to run per year?
An LED bulb used a few hours a day costs only a few pounds a year. Left on round the clock, a 10-watt LED is about £23 a year, while a 60-watt incandescent bulb is about £137. The bulb type, not the hours, is what decides the figure.
Do LED lights really save that much?
Yes. An LED gives the same brightness as an old bulb for roughly a sixth to an eighth of the electricity, because it makes light directly instead of heating a wire. Swapping remaining halogen or incandescent bulbs for LED is one of the fastest-paying energy changes you can make.