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Lighting

Change Your Light Bulbs

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Question: What is the payback for replacing a single 100W filament light bulb, with an 20W energy saver with the same light output?

Answer: Energy saver bulbs use about 5 times less energy than a filament bulb and last about 5 times longer, but they are more expensive.  Payback varies and depends upon how much you use the bulb, an expensive bulb used for just a few hours a day may not be worth replacing on purely financial grounds, but in most cases changing light bulbs to energy savers will save you a lot of money.  The tables below shows the payback time for a cheap and a more expensive energy  saver bulb for various use times per day, base only on electricity use.  It is worth noting that over the lifetime of an energy saver (about 5 years) you will also use about 5 filament bulb costing about 40pence each.

comment on how much saved with lots of bulbs

Use Per Day
 Difference in running cost per year between filament bulb and energy saver
 Payback time if the new energy saver costs £ 2
 Payback time if the new energy saver costs £ 4
6 hours £ 20.65 each year  35 days  71 days
12 hours £ 41.3 each year  18 days  35 days
18 hours £ 62.07 each year  12 days  24 days
24 hours £ 82.72 each year  9 days  18 days

Some additional information: For those interested the the energy use each year is as follows:-

Use Per Day Filament Bulb Energy Saver Differnce in Energy Use
6 hours 219 kWh per year
 44 kWh per year
 175 kWh per year
12 hours 438 kWh per year
 88 KWh per year
 350 kWh per year
18 hours 657 kWh per year
 131 kWh per year
 526 kWh per year
24 hours 876 kWh per year
 175 kWh per year
 701 kWh per year

There is also a persistent myth with energy savers that you should leave them running because it takes so much energy to get them started, this is not true, please read more on this.

Read more on this myth...

 

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.

 

 

Leaving Energy Saving Bulbs On

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A myth regarding turning on and off fluorescent lights became apparent to me in conversation with a PhD Chemist from Liverpool a few years ago. 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.

 
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All calculations on this site are based on current fuel prices they are checked regularly and were last updated on:-
                 28th December 2008
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The costs calculated for energy use based on these fuel prices should be regarded as 'good estimates' given that there are a variety of different fuel prices on offer in the country.
The calculations in the site also have different levels of accuracy depending on the nature of the calculation. For example calculating the energy use of a known power output TV is very easy compared when to calculating the effect on household costs with insulating your cavity walls.

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