LED vs CFL vs incandescent: the complete comparison

For about a decade, changing a light bulb meant choosing between three technologies that each had a loud constituency. That argument is largely settled now, but the reasoning behind it is still worth understanding — because the same trade-offs reappear every time you specify a lamp for a particular room, and because “LED is better” is not a specification.

Three completely different ways of making light

The three technologies do not differ by degree. They make light by unrelated physical mechanisms, and almost every practical difference between them follows from that.

An incandescent lamp passes current through a tungsten filament until it reaches roughly 2,500 °C and glows. This is thermal radiation: the filament emits across a broad, smooth spectrum, most of it in the infrared. Only about 5% of the energy leaves the bulb as visible light. The rest is heat, which is why a 100 W bulb will burn your fingers and why an old cinema marquee needed its own ventilation.

A compact fluorescent lamp strikes an arc through low-pressure mercury vapour. The excited mercury emits ultraviolet, which strikes a phosphor coating on the inside of the tube, which re-emits in the visible range. Two conversion steps, each with losses, but far more efficient than heating metal. The catch is in the first step: the lamp needs mercury to work at all.

An LED is a semiconductor junction. Electrons crossing it drop into holes and release the energy difference as photons directly, with no intermediate heat stage. White LEDs are usually blue emitters with a phosphor layer converting part of the blue to yellow; the eye integrates the mix as white. Because emission is direct, efficiency is much higher and the heat that is produced stays at the junction rather than radiating forward.

Efficacy: lumens per watt

Efficacy is the number that matters most, and it is routinely confused with brightness. Lumens measure light output. Watts measure power drawn. Efficacy is lumens per watt — how much light you get for the electricity you pay for.

For decades we bought bulbs by wattage because, within incandescent technology, wattage was a reliable proxy for brightness. That proxy broke the moment a second technology arrived, and it is the single largest source of confusion at the shelf.

The efficacy gap between incandescent and LED is roughly seven to one. That ratio is why lighting went from around 20% of a typical household electricity bill in 2005 to under 10% in most efficient homes today, without anybody using less light. In fact most people use rather more light than they did, which is a well-documented rebound effect — but the total still falls.

Lifetime, and what “rated life” actually means

Rated life is not the moment a lamp stops working. For LEDs it is the L70 point: the number of hours until light output has fallen to 70% of its initial value. The lamp is still lit. It is just measurably dimmer, and 30% is roughly where a person walking into the room starts to notice.

This matters because the failure modes differ:

• Incandescent lamps fail abruptly. The filament thins by evaporation until it breaks, usually at switch-on when the cold filament has low resistance and draws a surge.
• CFLs degrade at the electrodes and lose output gradually, then fail. Their life is strongly affected by switching: a CFL rated for 10,000 hours in continuous burn may manage a fraction of that in a cupboard switched twenty times a day.
• LEDs mostly fade. The semiconductor degrades slowly with junction temperature, and the phosphor yellows. When an LED lamp dies suddenly it is nearly always the driver electronics, not the diodes — which is the subject of a separate article.

Colour quality: the argument CFLs lost

The fastest way to make people hate an efficient light source is to give them bad colour. CFLs did this to themselves in the 2000s, and LEDs inherited the reputational damage.

Two numbers describe colour. Correlated colour temperature (CCT), in kelvin, describes whether the white is warm and yellowish (2700 K) or cool and bluish (5000 K). Colour rendering index (CRI), on a 0–100 scale, describes how faithfully the lamp renders colours compared with a reference source.

Incandescent lamps score CRI 100 by definition — they are the reference. Early CFLs managed 80 at best, and their spectrum was spiky rather than smooth: strong emission at a few phosphor peaks, gaps in between. Objects whose colour fell in a gap looked drained. Skin tones and red fabrics suffered most, because the deep-red region (measured as the R9 value) was where the spectrum was weakest.

Modern LEDs fixed this, but only if you buy for it. A 2700 K, CRI 80 LED is fine for a hallway. For a kitchen where you judge whether meat is cooked, a bathroom mirror, or any retail display, specify CRI 90+ with R9 above 50. The premium is small and the difference is immediately visible.

A CRI figure with no R9 value behind it tells you that the lamp renders pastels acceptably. It tells you nothing about whether red looks like red.

Our room-by-room colour temperature guide works through which numbers to specify where.

Mercury, disposal and the environmental balance

A compact fluorescent lamp contains between 3 and 5 milligrams of mercury. That is a small amount — roughly what would fit on the tip of a ballpoint pen — but it is enough that CFLs are classified as hazardous waste in most jurisdictions and must not go in household bins.

There was, for years, a genuinely interesting counter-argument: coal-fired generation emits mercury too, and a CFL saves enough electricity over its life to prevent more mercury emission at the power station than it contains. That argument held while grids were coal-heavy. As generation has decarbonised it has weakened considerably, and it never applied to the disposal problem itself — a broken CFL releases its mercury into the room, not into a regulated stack.

LEDs sidestep this entirely. They contain no mercury. They do contain electronics, and they should go to electronics recycling rather than landfill, but a broken one is a cleanup problem rather than a contamination problem.

Dimming, switching and cold starts

Incandescent lamps dim perfectly because dimming them is trivial: reduce the voltage, the filament runs cooler, it emits less light and shifts warmer. That warm shift is the effect people mean when they say a dimmed incandescent feels cosy.

CFLs dim badly. Most cannot dim at all; the dimmable models have a limited range, often flicker at the bottom of it, and do not shift warm. Their life is also shortened by frequent switching, which rules them out for motion-sensor and cupboard applications.

LEDs dim well or badly depending almost entirely on the pairing of lamp driver and dimmer. A trailing-edge dimmer rated for LED loads, with a suitable minimum load, will usually give smooth dimming to 10% or lower. A leading-edge dimmer designed for 300 W of incandescent load, presented with 30 W of LEDs, will flicker, buzz or refuse to start. This is the single most common complaint about LED lighting and it is a compatibility problem, not a quality problem — the full diagnosis is here.

On cold starts, LEDs win outright. An LED in a −20 °C garage reaches full output instantly and in fact runs slightly more efficiently when cold. A CFL in the same garage may take three minutes to reach useful brightness, if it starts at all.

Ten-year cost of ownership, worked through

Purchase price is the least important number in lighting, and it is the only one printed on the shelf edge. Here is the ten-year arithmetic for a single fitting burning three hours a day at 16 cents per kilowatt-hour — 1,095 hours a year, 10,950 hours over the decade.

Three observations about that table.

First, the lamp price is noise. It accounts for 14% of the incandescent total and 20% of the LED total; the electricity dominates in every column. Choosing a bulb on shelf price is like choosing a car on the price of the key.

Second, the LED advantage over CFL is real but modest — about $8.50 over a decade for one fitting. The decisive advantages of LED over CFL are not financial: instant start, dimming, switching tolerance, colour quality and no mercury.

Third, multiply by the number of fittings. A house with forty lamps saves roughly $4,000 over ten years by not being incandescent. That is the number worth acting on, and our savings calculator will run it on your own tariff and burn hours.

So which should you buy?

For almost every application in 2026, an LED. The remaining exceptions are narrow and worth naming honestly:

• Oven and appliance lamps. Sustained high temperature destroys LED drivers. Use the special-purpose incandescent the appliance was designed for.
• Existing dimmer circuits that cannot be changed. If you genuinely cannot replace a leading-edge dimmer — a listed building, a sealed panel — a halogen lamp on that circuit may be less annoying than a flickering LED. This is a temporary answer, not a good one.
• Heat-lamp applications. If you are keeping food warm or brooding chicks, the infrared output is the point.

CFLs have no remaining category where they are the best choice. They were an important transitional technology and they are now dominated on every axis except a shrinking price advantage that the electricity bill erases within months.

When you do buy LED, buy on four numbers rather than one: lumens for brightness, kelvin for colour, CRI for colour quality, and the enclosure rating if the lamp is going into a sealed fitting. Wattage is now only useful for working out the bill.

Common questions