How much does 100-watt bulb raise room temperature?

["" <just lurking>]

I'm trying to find out how much a 100-watt incandescent light bulb
raises the room temperature per hour. I Googled for 10 minutes and
struck out.

Layman's terms or a very technical are both fine; sister has physics
degree. Thanks!

[N:dlzc D:aol T:com (dlzc)]

Post by "" <just lurking>
I'm trying to find out how much a 100-watt incandescent
light bulb raises the room temperature per hour. I
Googled for 10 minutes and struck out.

It depends on the room, how much surface area it has, and how
well insulated it is.

How heat leaves a space is via conduction, convection, and
radiation (assuming you don't have an exhaust fan exchanging air
with the outside). Insulation and draperies slow conduction
through the walls.

Assuming the amount of energy emitted as light is negligible,
then the amount of heat the bulb adds to the room (and hence the
temperature increase) is proportional to the number of
"watt-hours" it draws. So a "60-watt equivalent" 15-watt
fluorescent bulb will heat the room only 25% as much as the 60
watt bulb.

David A. Smith

["" <just lurking>]

On Sat, 4 Aug 2007 09:49:40 -0700, "N:dlzc D:aol T:com \(dlzc\)"

Post by N:dlzc D:aol T:com (dlzc)

Post by "" <just lurking>
I'm trying to find out how much a 100-watt incandescent
light bulb raises the room temperature per hour. I
Googled for 10 minutes and struck out.

It depends on the room, how much surface area it has, and how
well insulated it is.

I see my question was rather sloppy. Thank you for overlooking this.

Post by N:dlzc D:aol T:com (dlzc)
How heat leaves a space is via conduction, convection, and
radiation (assuming you don't have an exhaust fan exchanging air
with the outside). Insulation and draperies slow conduction
through the walls.
Assuming the amount of energy emitted as light is negligible,
then the amount of heat the bulb adds to the room (and hence the
temperature increase) is proportional to the number of
"watt-hours" it draws. So a "60-watt equivalent" 15-watt
fluorescent bulb will heat the room only 25% as much as the 60
watt bulb.
David A. Smith

[Herman Family]

Post by "" <just lurking>
I'm trying to find out how much a 100-watt incandescent light bulb
raises the room temperature per hour. I Googled for 10 minutes and
struck out.
Layman's terms or a very technical are both fine; sister has physics
degree. Thanks!

Look at the efficiency of the bulb. My guess is that incandescent bulbs are
about 10 percent efficient. That would mean that the other 90 percent of
the energy (90 watts) goes in to heating up the room.

EZbake ovens used to use two light bulbs as their heating element. They may
have changed, but it was quite effective.

Michael

["" <just lurking>]

On Sat, 04 Aug 2007 16:55:43 GMT, "Herman Family"

Post by Herman Family

Post by "" <just lurking>
I'm trying to find out how much a 100-watt incandescent light bulb
raises the room temperature per hour. I Googled for 10 minutes and
struck out.
Layman's terms or a very technical are both fine; sister has physics
degree. Thanks!

Look at the efficiency of the bulb. My guess is that incandescent bulbs are
about 10 percent efficient. That would mean that the other 90 percent of
the energy (90 watts) goes in to heating up the room.
EZbake ovens used to use two light bulbs as their heating element. They may
have changed, but it was quite effective.
Michael

Thanks, I can work with that. So we basically have a 90 watt heater
that also puts out some light. I should have guessed that, my camping
lantern heats a tent quite well.

Thanks again!

[N:dlzc D:aol T:com (dlzc)]

Post by "" <just lurking>
On Sat, 04 Aug 2007 16:55:43 GMT, "Herman Family"

Post by Herman Family

Post by "" <just lurking>
I'm trying to find out how much a 100-watt incandescent
light bulb raises the room temperature per hour. I
Googled for 10 minutes and struck out.
Layman's terms or a very technical are both fine;
sister has physics degree. Thanks!

Look at the efficiency of the bulb. My guess is that
incandescent bulbs are about 10 percent efficient.
That would mean that the other 90 percent of the
energy (90 watts) goes in to heating up the room.
EZbake ovens used to use two light bulbs as their
heating element. They may have changed, but it
was quite effective.

Thanks, I can work with that. So we basically have
a 90 watt heater that also puts out some light. I
should have guessed that, my camping lantern
heats a tent quite well.

More like you have a 100 watt heater, because the light produced
is also trapped in the room, and is scattered to be essentially
heat anyway. Passing some through a window or through the walls
of a tent would be a very small fraction of the total energy
usage.

Compare the lightbulb to a human being:
http://www.primusweb.com/fitnesspartner/library/weight/calsburned.htm
1800 *food* calories per day, is 75 kilocalories per hour. This
is 87 watt-hours (an 87 watt bulb for 1 hour), most of which (for
an adult male), is waste heat. This neglects the humidity a
person adds to the room, that makes it seem hotter.

One 100 light bulb takes more energy than a human being at rest,
unit time per unit time. Fluorescent bulbs are more efficient,
and LEDs are more efficient still.

But none of this answers your question "how hot does this light
bulb heat my room". But is "more than you do being a couch
potato" come close enough to an answer?

David A. Smith

["" <just lurking>]

On Sat, 4 Aug 2007 19:15:57 -0700, "N:dlzc D:aol T:com \(dlzc\)"

Post by N:dlzc D:aol T:com (dlzc)
More like you have a 100 watt heater, because the light produced
is also trapped in the room, and is scattered to be essentially
heat anyway. Passing some through a window or through the walls
of a tent would be a very small fraction of the total energy
usage.
http://www.primusweb.com/fitnesspartner/library/weight/calsburned.htm
1800 *food* calories per day, is 75 kilocalories per hour. This
is 87 watt-hours (an 87 watt bulb for 1 hour), most of which (for
an adult male), is waste heat. This neglects the humidity a
person adds to the room, that makes it seem hotter.
One 100 light bulb takes more energy than a human being at rest,
unit time per unit time. Fluorescent bulbs are more efficient,
and LEDs are more efficient still.
But none of this answers your question "how hot does this light
bulb heat my room". But is "more than you do being a couch
potato" come close enough to an answer?
David A. Smith

Thank you, David. I believe your first sentence best answers my
question. I am neither a scientist nor an electrician. My situation is
that it is 80 degrees F at 9:00 PM and we run an air conditioner in
the kitchen all night, but my sister (with the physics degree) insists
on keeping a 100-watt kitchen lamp on, too. Our conversations usually
become a scientific debate, so I am preparing to ask her why we need
both A/C and a heater running.

[N:dlzc D:aol T:com (dlzc)]

Dear ***@dow:

"***@dow" <just lurking> wrote in message news:***@4ax.com...
...

Post by "" <just lurking>
Thank you, David. I believe your first sentence best
answers my question. I am neither a scientist nor
an electrician. My situation is that it is 80 degrees F
at 9:00 PM and we run an air conditioner in the
kitchen all night, but my sister (with the physics
degree) insists on keeping a 100-watt kitchen lamp
on, too. Our conversations usually become a
scientific debate, so I am preparing to ask her why
we need both A/C and a heater running.

Are you prepared for an emotional debate?

I would research a safe middle ground, prior to starting the
discussion. Or even a pre-emptive strike. Settle on a
fluorescent replacement bulb, that emits the light of a 100-watt
bulb but draws (and adds to the room) 25 watts.

This way you both can win.

See, I have a single, bright light on in my house all night too.
There are no hallway receptacles to put nightlights in... so it
is that, or turn on lights that get in everyone's eyes during the
night.

David A. Smith

[Herman Family]

Post by "" <just lurking>
On Sat, 4 Aug 2007 19:15:57 -0700, "N:dlzc D:aol T:com \(dlzc\)"
Thank you, David. I believe your first sentence best answers my
question. I am neither a scientist nor an electrician. My situation is
that it is 80 degrees F at 9:00 PM and we run an air conditioner in
the kitchen all night, but my sister (with the physics degree) insists
on keeping a 100-watt kitchen lamp on, too. Our conversations usually
become a scientific debate, so I am preparing to ask her why we need
both A/C and a heater running.

You are going to argue this point with a woman? And you are trying to win
it with our advice? Good grief. You need a bit more help than with simple
physics if you think you'll pull that off.

Any bets on whether the light stays on?

Michael

[PD]

Post by "" <just lurking>
I'm trying to find out how much a 100-watt incandescent light bulb
raises the room temperature per hour. I Googled for 10 minutes and
struck out.
Layman's terms or a very technical are both fine; sister has physics
degree. Thanks!

100 W light bulb generates 360,000 joules/hr, or 360 kJ/hr.
For a perfectly insulated room of dimensions 4 m x 3 m x 3 m (36 cubic
meters), and for static dry air with a heat capacity of 1.0 kJ/kg/degC
and density 1.2 kg/m^3, this works out to a temperature rise of
360 / (1.0 * 1.2 * 36) = 8.3 degC/hr. (This number seems high to me,
but those are the numbers.)

To remove that with an air conditioner of efficiency of 15% would
expend about 670 watts of AC line power, or 0.67 kW. Running that for
an hour will expend 0.67 kW-hr of energy per hour, costing about 7
cents per hour, plus the penny it costs to power the light bulb.

Presuming that someone wants the light bulb on at night during the
summer, a period of about 10 hours, the privilege of not stubbing your
toe on the way to the bathroom in the middle of the night is costing
about $0.80 a night, or about $24 a month. If the time period is only
2 hrs/night, this cost goes down to less than $5/month, about the cost
of two Starbuck's coffees per month.

For some people, that is an acceptable cost, for others not. That part
of the battle is something a physicist is not going to get enmeshed
in.

PD

[N:dlzc D:aol T:com (dlzc)]

Dear PD:

"PD" <***@gmail.com> wrote in message news:***@22g2000hsm.googlegroups.com...
...

Post by PD
100 W light bulb generates 360,000 joules/hr, or
360 kJ/hr. For a perfectly insulated room of
dimensions 4 m x 3 m x 3 m (36 cubic meters),
and for static dry air with a heat capacity of 1.0
kJ/kg/degC and density 1.2 kg/m^3, this works
out to a temperature rise of 360 / (1.0 * 1.2 * 36)
= 8.3 degC/hr. (This number seems high to me,
but those are the numbers.)

That sounds right. I think air is 1295 gm/ m^3... so closer to
"1.3" than "1.2".

Usually some or all of the mass of the walls is containing the
insulation, so the "room side" of that heats up also. Which is
probably what doesn't feel quite right.

Then you have to decide whether you let the room air pressurize
(to keep constant density), or let gas escape to keep a constant
ambient pressure...

David A. Smith