Leaving the bathroom light on all morning or forgetting to switch off the kitchen lights before going to bed might not feel like an expensive mistake.
With modern LED bulbs, in fairness, it usually isn’t.
Lighting has become considerably more efficient than it once was, which means the old warnings about a forgotten light sending the electricity meter racing are increasingly out of date. But that doesn’t mean leaving lights on unnecessarily costs nothing.
At current electricity prices, a single LED left on for a few extra hours each day may add only a few pounds to the annual bill. Multiply that across kitchens with several spotlights, bedrooms, hallways, bathrooms, and outside lighting, and the figures become more noticeable.
So what does leaving the lights on actually cost?
Start with the wattage, not the brightness
To calculate the cost of running a light, you need to know how much electricity it uses.
This is measured in watts.
Modern LEDs produce far more light per watt than older technologies. Energy Saving Trust says most domestic LED bulbs use less than 10 watts and describes LEDs as the most energy-efficient type of household light bulb currently available.
For a straightforward example, take a 10-watt LED bulb.
If that bulb is left on for five hours when nobody needs it, it consumes:
10 watts × 5 hours = 50 watt-hours
That is 0.05 kilowatt hours (kWh).
Ofgem’s average electricity rate for a standard variable tariff customer paying by Direct Debit between 1 July and 30 September 2026 is 26.11p per kWh.
At that rate, those unnecessary five hours cost about 1.3p.
Hardly enough to cause alarm.
But repeat it every day for a year and that one forgotten 10-watt light costs approximately £4.77.
The individual mistake is cheap. The habit is what costs money.
What if several lights are being left on?
Most homes don’t have a single light bulb operating in isolation.
A kitchen might have six or eight LED spotlights. There may be several fittings in a living room, lights in the hallway and landing, bedroom lamps and outdoor lighting.
Consider five 10-watt LED bulbs being left on unnecessarily for five hours every day.
Together, they draw 50 watts.
Over five hours that’s 0.25kWh per day, or 91.25kWh over a year.
At 26.11p per kWh, the annual cost comes to approximately £23.83.
Ten equivalent bulbs left on for those same five unnecessary hours each day would cost about £47.65 a year.
Again, these aren’t estimates of what the typical household wastes. They’re examples showing how the calculation works. Actual costs depend on the number of lights, their wattage, how long they are used and the electricity tariff being paid.
Leaving an LED on all day still isn’t hugely expensive
Another useful way of putting the numbers into perspective is to calculate the cost of one light running continuously.
A 10-watt LED running for 24 hours consumes 0.24kWh.
At 26.11p per kWh, that costs approximately 6.3p a day.
Leave it running continuously for an entire year and the electricity cost would be around £22.87.
Few households intentionally run the same indoor light 24 hours a day for a year, of course, but it illustrates an important point.
Modern lighting is efficient enough that individual lights don’t generally dominate household electricity bills. Energy Saving Trust estimates that lighting accounts for around 5% of the average home’s energy bill.
That doesn’t make unnecessary use free; it simply means the savings need to be kept in proportion.
Older bulbs change the calculation
The figures become much less forgiving when older lighting is involved.
Energy Saving Trust says LEDs use substantially less electricity than halogen lighting while producing comparable light levels. Although LEDs now dominate the market, older halogen bulbs and spotlights still appear in British homes.
Imagine leaving an older 50-watt bulb on unnecessarily for five hours each day.
That would consume 91.25kWh over a year and cost approximately £23.83 at the current average capped electricity rate.
That’s the same electricity cost as leaving five 10-watt LEDs on for the same amount of time.
If a home still contains numerous older bulbs, replacing them with suitable LEDs will likely have a greater impact than becoming obsessive about switching a single low-wattage LED off for ten minutes.
Energy Saving Trust estimates that replacing remaining halogen spotlights with LEDs could save a household around £45 a year in Great Britain.
Why do we leave lights on in the first place?
Sometimes it’s simply forgetfulness.
More often, it’s because the way a house is used doesn’t fit neatly around the position of its switches.
Someone goes upstairs intending to return in a minute and leaves the downstairs light on. Children leave bedroom or bathroom lights running. Outside lights stay on long after they’re needed. A hallway light remains illuminated because nobody remembers who switched it on.
This is where controlling lighting becomes as much about convenience as energy use.
Timers, occupancy sensors, and smart light switches can help households control when lighting is on without relying on someone to remember to walk back to the switch.
A schedule might switch outside lighting off at a particular time each night, while remote control lets someone check and operate compatible lighting without being in the room.
But smart controls shouldn’t be presented as a guaranteed route to dramatically lower electricity bills.
Savings depend on how the household currently uses its lighting. If people already switch lights off when they’re not required, there may be very little unnecessary consumption for automation to eliminate.
Smart equipment can also consume a small amount of power to remain available for remote commands.
What about dimming the lights?
Dimming can reduce the power used by compatible LED lighting, but the relationship between brightness and electricity consumption isn’t necessarily as simple as setting a light to 50% brightness and assuming it will use exactly half the electricity.
Performance varies between lamps, drivers and dimmers.
Compatibility matters too. A bulb needs to be designed as dimmable, and the dimmer needs to suit the type and load of lighting being controlled.
The main reason to install dimming is usually that different light levels suit different times and activities. Any reduction in energy consumption is an additional benefit rather than a reason to make sweeping savings claims.
Should you switch lights off every time you leave a room?
With LEDs, there is little justification for deliberately leaving an unwanted light running simply because it uses relatively little electricity.
Energy Saving Trust’s current advice is straightforward: switch lights off when they aren’t being used or when leaving a room. It estimates that doing so can save around £8 a year in Great Britain.
That won’t transform a household budget.
But it is also one of the easiest energy-saving habits to adopt because it costs nothing.
The bigger gains are likely to come from combining sensible behaviour with efficient lighting: replace inefficient older bulbs where appropriate, avoid leaving empty rooms lit for hours at a time, and use controls where they solve a genuine problem.
The real cost is in repetition
One forgotten LED light isn’t an energy crisis.
A 10-watt bulb left on unnecessarily for five hours costs little more than a penny at the current average capped electricity rate.
But homes contain many lights, and habits repeat themselves.
Five equivalent lights unnecessarily burning for five hours every day add almost £24 to the annual electricity bill at today’s rate. Ten bring the figure close to £48.
Homes with older, higher-wattage lighting can spend considerably more.
The lesson isn’t that householders should walk around in darkness worrying about every minute a bulb remains illuminated. Modern LEDs are remarkably efficient.
It’s simply that you still pay for electricity when a light illuminates an empty room.
Switching it off remains the cheapest smart lighting decision of all.