# The Naked Scientists Forum

### Author Topic: How much energy does a 60W bulb use?  (Read 23172 times)

#### Frik Lombard

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##### How much energy does a 60W bulb use?
« on: 02/10/2009 11:30:04 »
Frik Lombard  asked the Naked Scientists:

Dear Chris,

I have a question for you.

Lets say that I have a 60 watt globe in my room.

1) When I switch on that light and keep it on for an hour I will use 60 watt of electricity, or does the light use extra electricity to switch on?
What will the total electricity consumption be for that hour?

2) If I switch on the light and keep it on for 3 minutes, switch it of
when I leave the room, come back and switch it on, lets say ten times. What will my total consumption be in that 30 minutes.

What I actually would like to know is, is it better to leave a light burning or switch it on and of when needed..

Frik Lombard

What do you think?

#### Vern

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##### How much energy does a 60W bulb use?
« Reply #1 on: 02/10/2009 12:16:40 »
You will use less electricity if you switch the light on and off each time you need it. There is some thermal shock associated with the turn-on and turn-off process that may reduce the life of the bulb. My guess is that you are better off to turn the light off when not in use, but I don't have any numbers to quantify that at the moment.

#### syhprum

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##### How much energy does a 60W bulb use?
« Reply #2 on: 02/10/2009 13:42:07 »
There are very few devices where it is better to leave them running than to switch them on and off as required from a power consumption point of view.
something like a computer is probably best left running if you are going to need it in the next 10 minutes or so as they have a quite complex start up procedure but certainly not lamps that get going in milliseconds.
As Vern points out startups impose more strain on equipment than normal running so you must make a compromise between power consumption and reliability, filament lamps are normally cheap and easy to replace and only fail on switch on when they are coming to the end of their life anyway so my advice is to switch them on and off as needed.
PS a sixty watt lamp consumes 1 charge unit ( \$0.1 ?) per 16.7 hrs.
« Last Edit: 02/10/2009 15:19:35 by syhprum »

#### graham.d

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##### How much energy does a 60W bulb use?
« Reply #3 on: 02/10/2009 14:13:37 »
As above. If you switch it on for an hour you will use 60 watt.hours. You will probably pay for units of kilowatt.hours so this will be 0.06 kWHrs. If you switch it on for 10 x 3 minute intervals, the total is 0.03 kWHrs.

#### techmind

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##### How much energy does a 60W bulb use?
« Reply #4 on: 02/10/2009 21:49:43 »
Lets say that I have a 60 watt globe in my room.

1) When I switch on that light and keep it on for an hour I will use 60 watt of electricity, or does the light use extra electricity to switch on?
What will the total electricity consumption be for that hour?
You will use 60 watt.hours , or 0.06kWh.

It doesn't take any significant extra to "switch on".

2) If I switch on the light and keep it on for 3 minutes, switch it of
when I leave the room, come back and switch it on, lets say ten times. What will my total consumption be in that 30 minutes.

It's just power times time, so if it's burning for 30 minutes in total (half an hour) then it'll use 30watt.hour of energy.

What I actually would like to know is, is it better to leave a light burning or switch it on and of when needed..

As far as energy use is concerned, you may as well turn the light off and on as required, minimising the time that the light is on.

However it is understood that the thermal shock of switching the bulb from OFF to ON does shorten the life of the filament. I'm not quite sure how much loss of life that might equate to, but I probably wouldn't deliberately switch off a lamp if I knew I needed it again in less than 5 minutes.

Of course if you have a dimmer switch (and fade the bulbs up from very low) then the thermal shock is probably a non-issue.

#### RD

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##### How much energy does a 60W bulb use?
« Reply #5 on: 02/10/2009 22:01:23 »
The 60 Watt figure is nominal. The electrical resistance of the filament is proportional to its temperature, so for a fraction of a second immediately after it is switched on the power consumption will be greater than 60W because the electrical resistance of the room-temperature filament is lower than when it is white-hot. There will be a momentary current spike when the incandescent bulb is switched on from cold.

http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/restmp.html
« Last Edit: 02/10/2009 22:07:39 by RD »

#### techmind

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##### How much energy does a 60W bulb use?
« Reply #6 on: 03/10/2009 23:10:21 »
... The electrical resistance of the filament is proportional to its temperature, so for a fraction of a second immediately after it is switched on the power consumption will be greater than 60W because the electrical resistance of the room-temperature filament is lower than when it is white-hot. There will be a momentary current spike when the incandescent bulb is switched on from cold.

I accept this, but this current spike will only last 50~100 milliseconds or something, and we should re-iterate that in terms of energy used this is utterly negligible compared with that energy used by the bulb simply being illuminated for a minute or so.

Actually it might be a step too far to say the "electrical resistance of the filament is proportional to its temperature". The resistance will certainly go up with increasing temperature, but over the range we're considering, I wonder how close it really is to a strict proportionality...?
Any data anyone, for resistance of tungsten, in range 300K to 2800K?
« Last Edit: 03/10/2009 23:25:31 by techmind »

#### RD

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##### How much energy does a 60W bulb use?
« Reply #7 on: 03/10/2009 23:42:27 »
Actually it might be a step too far to say the "electrical resistance of the filament is proportional to its temperature".
The resistance will certainly go up with increasing temperature, but over the range we're considering,
I wonder how close it really is to a strict proportionality...?

Linearly proportional to the change in temperature ... http://wiki.myelectrical.com/index.php?title=Temperature_Coefficient_of_Resistance

#### graham.d

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##### How much energy does a 60W bulb use?
« Reply #8 on: 04/10/2009 09:50:58 »
Hmm, how many scientists does it take to change a light bulb?

#### Bored chemist

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##### How much energy does a 60W bulb use?
« Reply #9 on: 04/10/2009 20:50:45 »
A good scientist knows that the act of observing the lightbulb causes it to change.

#### syhprum

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##### How much energy does a 60W bulb use?
« Reply #10 on: 07/10/2009 15:10:29 »
In the days before semiconductors small filament lamps where a useful tool for constructing sound volume compressors, a simple circuit of a lamp in series with a loudspeaker will produce some degree of compression while more complex bridge type negative feedback circuits will do a very good job.

#### Bored chemist

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##### How much energy does a 60W bulb use?
« Reply #11 on: 07/10/2009 18:56:10 »
If my memory serves me well a 60W incandescant lamp uses about 3% of the energy deliverd to it; it wastes the rest as heat.

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##### How much energy does a 60W bulb use?
« Reply #11 on: 07/10/2009 18:56:10 »