Lighting stopped being a large number, and the advice has not caught up

The size of what already happened

An incandescent lamp works by heating a wire until it glows, which is a spectacularly inefficient way to make light, because most of the energy leaves as heat rather than as anything visible. A modern light-emitting diode producing similar light draws a small fraction of the power. The ratio is close to a factor of ten, and in some comparisons better.

That change happened across roughly a decade, and it happened almost without effort on anybody’s part, because lamps fail and get replaced. Most households completed the transition simply by buying whatever was on the shelf when a bulb went.

The consequence is that a large body of household energy advice, written when lighting was a substantial share of the electricity bill, now points at a load that has largely evaporated. Switching lights off in empty rooms remains good manners and it is no longer a meaningful economy in most homes.

The measure that matters is lumens per watt

Light output is measured in lumens and electrical input in watts, and the ratio between them is efficacy. Buying by wattage, which is how everyone learned to buy lamps, is now buying by the wrong number entirely, since wattage describes what a lamp consumes and tells you nothing about how much light it makes.

Efficacy varies more between products than the packaging suggests, and it trades against qualities people actually care about. Very high efficacy tends to come with poorer colour rendering, and light that makes food and skin look wrong is a poor purchase however little it costs to run. A warmer colour temperature also costs a little efficacy compared with a cold one.

For domestic use the sensible balance is to buy for the light and stop optimising the watts. The difference between a good lamp and an excellent one, across a whole house, is now small enough that it is no longer where a household’s attention belongs.

What fails in an LED lamp is not the diode

The semiconductors themselves last a very long time, and the long life claimed for these lamps is a claim about the diode. What sits alongside it is a driver: a small power supply converting mains alternating current into the low-voltage direct current the diode needs, packed into a space designed for a filament.

That driver contains components that dislike heat, and it is sitting inside a sealed enclosure next to something producing heat, often in a recessed fitting with no ventilation. This is why cheap lamps fail long before their stated life, and why the failure is usually sudden rather than a gradual dimming.

The practical implications are small and real. Enclosed and recessed fittings shorten lamp life, so a fitting with some airflow is worth choosing. And integrated fittings where the lamp can’t be replaced turn a failed driver into a replacement of the whole fitting, which is worth knowing before buying a house full of them.

The heat that used to be a bonus

There is a genuine subtlety in the arithmetic that gets dismissed too quickly. Old lamps produced a great deal of heat, and in a heated house during a cold season, that heat was not entirely wasted, because it displaced a little of what the heating system would otherwise have supplied.

The offset is partial rather than complete, and it depends on how the house is heated and on what that heat costs relative to electricity. Where heating comes from a cheaper fuel, replacing electric heat with fuel heat is a saving even before efficacy is considered. Where the house is electrically heated by resistance, the offset in winter is close to complete.

And in a cooling climate the effect reverses and compounds. Every watt of lighting heat has to be removed by the cooling system, which costs additional electricity, so efficient lighting saves twice. That asymmetry is why lighting upgrades were always a stronger proposition in hot regions than in cold ones.

Where the remaining lighting energy actually is

A few categories survived the transition. Halogen spotlights, particularly in kitchens and bathrooms fitted out a couple of decades ago, are still numerous and still consume like filament lamps. External floodlights and security lighting run for long periods and are often oversized for the job. Specialist lighting for aquariums, plants and workshops can be substantial.

The pattern is the familiar one: what matters is power multiplied by hours, so a bright lamp used briefly costs less than a modest one left on all night. Outdoor lighting on a dusk-to-dawn sensor is the classic example, and switching it to a motion sensor usually reduces its running hours by a very large factor while making it more useful, since a light that comes on is more informative than one that never goes off.

Beyond that, the honest conclusion is that lighting is finished as a household energy topic. The attention that used to go there belongs to heating, hot water, vehicle charging and the handful of appliances that run continuously, all of which are now far larger than the lamps.