Logo - Confused About Energy

Please use twitter to ask a question   

A myth regarding turning on and off fluorescent lights...

It became apparent to me in conversation with a PhD Chemist from Liverpool a few years ago that he seriously misunderstood electricity. This particular chap happily declared that to save energy he keeps all his fluorescent light on all the time. This cannot be correct I thought.

As an example, for a 10w bulb 5 seconds of normal operational power equates in energy to 0.0000139 kWh. If you were to leave that bulb on for one hour this would equate to 0.01 kWh. This means that by leaving it on for just one hour you will have consumed 720 times more energy than that consumed in one switching cycle.

It is actually a quite common misconception based on the fact that it a lot of power is required to start up an energy saver bulb. When you turn on a fluorescent light bulb there is a very brief jump in current demand. This inrush of current can be many times greater than the normal operating current. This spike, normally lasts no longer than 1/10th of a second, and draws the equivalent energy of about 5 seconds of normal operation. So, if you turn your fluorescent lamp on and off more frequently than every 5 seconds, you will use more power than normal. Switching lights on and off normally will have almost no effect on the amount of energy you consume.

So it makes no sense at all in terms of energy saving to leave them on all the time! So please ignore those who tell you this story, no matter how educated they are.

Some notes on this page

  • Payback is calculated by assuming that you scrap the old bulb so only the cost of the energy saver is considered. It could also be argued that you should include the cost of 5 filament bulbs in this calculation as they last no where near as long as energy saver bulbs.
  • KWh is a unit of electrical enegy it stands for kilowatt hours.
  • A watt is a unit of electrical power.

Comments and Questions

The ability to post comments and questions has be temporarily suspended, owing to large volumes of spam, for now please direct messages to us on twitter instead (at the top of the article there is a button). A solution is being sought.

Leaving Energy Saving Bulbs On - fb graph search  2016-10-20 06:01:03
Нi theге! Thiѕ article could not be written much better!
Looking through this post reminds me of my previous roߋmmate!
Hе constantly kept preaaching aboսt this. I am gοing tօ forward tһis aгticle to him.

Рretty sure he'll haѵe a great read. Thank you for sharіng!
Not the whole story! - Organnyx  2014-05-20 18:01:15
Your chum may well be correct, but not because of your simplistic explanation - energy saver bulbs are pricy in comparison to tungstens - the time they usually fail is at startup, due to the high current surges, so it may well be cheaper to minimise the on/off cycles, and balance the consumption against the diminished life if you're batting them on and off all the time.
The same thing goes for such things as TVs, the old advice was if you were going to switch it off for under an hour - don't!