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 | £ 23.45 each year | 31 days | 62 days |
| 12 hours | £ 46.9 each year | 16 days | 31 days |
| 18 hours | £ 70.48 each year | 10 days | 21 days |
| 24 hours | £ 93.93 each year | 8 days | 16 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.
This is where I go , "No looks fine to me!" But only because I have changed it! My off line spreadsheets were right but there was a function error in the scripts, thanks for spotting it, please shout if you find any more on the site, much appreciated
Richard
e.g. you say 6 hours/day of a 100w Filament Bulb uses "21.9 kWh per year".
But I make it 219 kWh per year: 6 * 100 * 365 / 1000 = 219
Just a rough in-head calc of about half a kWh per day for a year gives around 182 kWh/year.
Likewise for 6 hour/day use you have running cost saving of "£ 2.07 each year".
But I make that about £21 savings/year: 6 * 80 * 365 / 1000 * 0.12 = 21.02
Also in your payback time you don't seem to allow for the price of the filament bulb. With 7 times the life an energy saver bulb at £2 is about the same cost as the 7 Filament Bulbs used over the same period.
Nowadays energy saving bulbs are pretty much money savers now for any suitable regularly used lightbulb.
Richard