I have bought four of the very same led light bulbs two weeks ago. Their lightning characteristics appeared to be the same (brightness, angle, color). Two days ago I have realized that one of the bulbs became approx. 40% less bright than the others.

Does this decrease in lumen also demand less power? If the bulb is expecting 10 watts initially, is it just demanding 6 watts now? Does the answer for led also apply to traditional bulbs, halogen and other technologies?

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    \$\begingroup\$ IMO you could make a guess easily: Energy do not just disappear, so check the temperature and if both still consume 10watts, I expect the less bright to be at a higher temperature after few minutes. \$\endgroup\$ Jun 11, 2016 at 18:25
  • \$\begingroup\$ It does rather depend on the mode of failure luminous characteristic or VI curve change. Both options are possible. If you have individual series resistors the volt drop across them would offer clues, if they are all in series with one current source then the forward voltage would indicate if one is consuming more or less power. To use LEDs in series they need to be reasonably matched for output vs current. \$\endgroup\$
    – KalleMP
    Jun 11, 2016 at 19:26
  • \$\begingroup\$ This is just a wild guess at it, but I bet one (or more) of the LED strings inside went open-circuit, ie an LED went bad and took a bunch down with it. If you open the diffuser, you'll probably find a bunch of elements that aren't lit. If this is the case, you should see a similar drop in power consumption. \$\endgroup\$
    – Daniel
    Jun 16, 2016 at 16:27

1 Answer 1


This can occur for several reasons. My first observation would be whether the application/installation is the same for all four lamps. If one is in a recessed can, covered, or enclosed, it may produce less light. LEDs experience a "thermal roll off": as the LEDs heat up, they become less and less efficient.

Typical light bulbs experience a 6-15% roll off as they warm up and stabilize. If one bulb rolls off more than the rest, it can be due to being placed in a hotter environment OR perhaps they assembly was not consistent and did not secure the LEDs to the heat sink/thermal solution.

Lastly, there may be an issue with the driver (power supply) inside the light bulb. If certain components fail open or disconnect, there can be a drop in power consumption. If they are producing far less light as 40% suggests, they most likely are consuming much less power. What causes that drop in power could be a number of things.


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