A bulb rated 25,000 hours is not promising to work for 25,000 hours. It is promising that at 25,000 hours it will still emit at least 70% of its original brightness — assuming it has not failed for some other reason first.
That standard is called L70, and understanding it explains most of the gap between the number on the box and what happens in your ceiling.
What L70 means
LEDs do not usually burn out the way filaments do. They fade. The phosphor degrades, the chip's output drops, and the light gets gradually dimmer and often slightly cooler in color.
L70 is the point at which output has fallen to 70% — chosen because that is roughly where the average person starts noticing a room has got dimmer. The fade is slow enough that most people never notice it happening, only that a room that used to feel bright no longer does.
| Rating | At L70, running 3 h/day | Running 8 h/day |
|---|---|---|
| 15,000 h | 13.7 years | 5.1 years |
| 25,000 h | 22.8 years | 8.6 years |
| 50,000 h | 45.7 years | 17.1 years |
Those numbers are also why the rating is not very useful as a purchasing input. Nobody keeps a bulb for 22 years.
What actually kills LED bulbs
The driver fails first, almost every time.
The LED chip may well last its rated life. The electronics in the base — particularly the electrolytic capacitors — will not, because they sit in a small sealed compartment directly beneath a heat source, and capacitor life falls sharply with temperature.
This is why a bulb rated 25,000 hours can fail in two years, and why the failure is usually sudden rather than gradual: it is not the LED giving up, it is the power supply.
The single biggest factor is heat
Everything that traps heat around the bulb shortens its life:
Enclosed fittings. A sealed glass shade with no ventilation can double the internal temperature. Use bulbs explicitly rated for enclosed fixtures — the rating exists precisely for this and most bulbs do not carry it.
Recessed downlights in insulated ceilings. Insulation packed against the back of a fitting is the worst common case. Use IC-rated fittings and keep the clearance the manufacturer specifies.
Base-up orientation. Heat rises into the driver compartment rather than away from it. A bulb pointing downward from a ceiling runs hotter internally than the same bulb in a table lamp.
Overdriving. Very high lumens from a physically small bulb means high current and high junction temperature. A 1,600 lumen bulb in a standard envelope runs much hotter than an 800 lumen one, and lasts accordingly.
Other things that shorten life
Frequent switching matters far less than for fluorescent, but it does stress the driver's inrush circuitry. Not a reason to leave lights on.
Dimming on an incompatible dimmer. A mismatch can put the driver into a repeated restart cycle that is much harder on it than steady operation. See dimmer compatibility.
Voltage spikes. Cheap drivers have minimal surge protection. A bulb that fails during a storm did not fail from age.
What to look for instead of the hours figure
The hours rating is the least informative number on the box. More useful:
- Enclosed fixture rated, if the fitting is enclosed
- Warranty length — a manufacturer offering five years is making a claim they have to honour, unlike an hours figure
- Physical size of the heat sink — a bulb with real aluminum finning is designed to shed heat; a light all-plastic body at high lumens is not
- CRI 90+ and low flicker, because the brands cutting those corners are cutting driver quality too
If bulbs keep failing in one fitting
Suspect the fitting, not the bulbs. Enclosed shades, insulation contact, a loose connection producing arcing, or a dimmer that the load does not suit will each kill bulb after bulb.
If they fail only on one circuit, and especially if they flicker before failing, the problem is upstream of the bulb.