|Some defenders of incandescent bulbs claim that
the waste heat generated will lessen their heating
bill. If 90% or 90w of an 100w incandescent bulb
generates x amount of heat how much heat could a
90w Nichrome heater produce? |
There are a couple of important questions to
answer here. First is "how much heat do
appliances produce and why?" Second is "Is it a
good idea to use light bulbs as space heaters?"
Heat is a measure of energy. Some people would
say that heat is the "waste" energy of a process.
Whenever energy is transferred, some heat is lost
to the environment. When you say a light bulb
uses 100 watts, you are talking about the power of
the bulb; power is an energy rate. The metric
unit for energy is the joule and a watt is equal
to one Joule per second. Your 100W light bulb
uses 100 joules of energy every second.
But how much of those 100 joules every second is
converted to heat? The answer is a little shocking:
All of it.
Granted, the energy isn´t all transferred to the
environment by heat all at once. Some of it
(about 10% according to Wikipedia) is converted to
light while the rest of the energy is dissipated
as heat. But the light from the light bulb hits
the walls and objects in your room and scatters.
Every time the light bounces off something, some
of it is absorbed as heat. (Which wavelengths of
light an object absorbs determines its color).
Eventually all of the energy from the light is
absorbed by objects in your room and ends up as
heat. This is why rooms get dark when you turn
off a light switch.
This means that a 100W light bulb produces 100
joules of heat every second. Consequently, a 90W
Nichrome heater produces 90 joules of heat every
Now on to the second question: Is it a good idea
to use light bulbs as space heaters? The answer
varies depending on your personal preferences, but
in general is a resounding "Meh...not really." As
you may have learned in school, hot air (and
therefore heat) rises upward and cool air falls
downward. Most people use light bulbs near their
ceiling, but if you wanted to use them efficiently
for heating you would need to install them near
the floor. With your lights high up, the space
near the ceiling would be nice and warm, but you
would be stuck on the ground, cold but very well
lit because the hot air cannot float down to the
floor. Additionally, people don´t always want
heat and light at the same time. If I´m reading a
book on a warm summer night, I certainly don´t
want to be bothered with extra heat from my lamp
if I don´t have to be.
So, will using incandescent light bulbs lower your
heating bill? Maybe a little (when you actually
want heat) but there are much more efficient ways
to warm yourself up. Blankets, natural gas
heaters, and cuddling are among my favorite ways
to get warm, and they are all more effective than
an incandescent bulb.
Let's talk about units first of all. Energy is
usually described in units of Joules, and 4.18
Joule of energy is enough to heat one gram of
water by 1 degree Celsius. The flow of energy is
a "power", which is usually described in units of
Watts. 1 Watt is one Joule per second. So if you
run a 90W heater for 60 seconds you get 90*60=5400
Joules of heat energy transferred to the
surroundings. It doesn't matter whether that
heat came from a light bulb or a heater, 90W is
And all forms of energy turn into heat
eventually. So of the 100W that an incandescent
light bulb uses, 90W turns immediately into heat.
10W flows out as visible light, but when that
light strikes surfaces in your house some of it
reflects and some of it turns into heat and heats
up the surface. Some of the light may escape
through a window, but most of that 10W of light
power turns into heat in your house eventually as
it bounces around. Fluorescent lights are about
40% efficient so the same amount of light can be
produced by a 25W fluorescent light.
So a 100W heater produces the same amount of
heating as a 100W incandescent bulb. But there is
an important difference: 100W from a heater is
targeting the heat at the people in the house.
Light bulbs in a ceiling fixture may be sending a
lot of their heat into the ceiling where it is not
as effective. Or a light bulb may heat up one
corner of a room, but doesn't effectively spread
its heat around.
So if you have more efficient lights you will
spend less money on lighting but during the
winter, especially if you live somewhere very
cold, you will need to spend more energy and money
on heating. But in terms of the comfort level of
people in the house fewer watts are needed from a
heater than would be required from a light bulb,
because a heater directs the heat where people
need it. So on the whole it is more efficient to
have efficient lighting and spend a bit more on
All of the electrical work used by an appliance
is radiated as some form of energy: In this case,
the 100W bulb generates 10W of light and 90W of
heat, and the 90W heater generates 90W of heat.
Therefore the statement that an incandescent bulb
will help heat the house is certainly true.
The reason why energy efficient lighting is a
good idea is tied to the reason electrical heating
is a bad idea: In terms of total energy required
to heat your house, it is much more efficient to
transport gas or oil to your home. Electricity
produced at a power plant and transferred over the
power lines typically represents only ~30% of the
heat released by burning coal.
Additionally, we use the lights more often than
we use the heat: In the summer, you're paying
double to heat your house (with an inefficient
light) and cool it back down (with an air
You'd be just as well off trying to heat your
house with the waste heat generated by desktop
computer, television, or electric toothbrush.
Very interesting question! It is true that an
incandescent light bulb has very low
electricity-to-light efficiency, typically around
5-10%. The continued use of these types of light
bulbs may partially be due to their highly
accurate color reproduction for applications where
color temperature is important (i.e. lighting for
The heat generated from incandescent bulbs is
sufficient to help heat an environment. However,
when it comes to the math of heat generated, 90W
of heat from an incandescent bulb is the same as
90W of heat from a Nichrome heater. I would
imagine though that a device specifically designed
to emit heat is, on the whole, more efficient at
dissipating that heat to the surroundings than a
light bulb would be. For example, sources of heat
adsorption in a light bulb might be the glass, the
stem of the bulb, and the cap and its contact with
the light socket. This means that a Nichrome
heater would be better at outputting the 90W of
heat than a light bulb would.
This is a really good question! Both the
incandescent bulb and the Nichrome heater will
produce 90 watts of heat. Actually both the bulb
and the heater will eventually convert 100% of the
electricity going into them to heat. In the case
of the bulb some of the energy becomes light
energy first, then heat energy. But the bulb and
heater may differ in where they distribute the
heat. For example, instead of turning on the
central heat you could turn on the bulb only in
the room you are in, so you don't have to heat the
whole house. But you probably don't want to have
the heat on the entire year, and if you use
incandescent bulbs that's basically what's
happening! This is why incandescent bulbs usually
waste energy overall. They are especially bad if
you turn on the AC, because it's like having the
heat and the AC on at the same time.
Also, if your heater is a heat pump, it's much
more effective than an incandescent bulb or
Nichrome heater. In heating mode, heat pumps use
electricity to move heat from a cold space
(outside) to a warmer space (inside).
(Refrigerators and AC are heat pumps in cooling
mode, and work in the opposite direction.) For
each watt of electricity going into the heat pump,
it can move 2 or more watts of heat. So if you use
this for heating, there's no reason to stick with
Hope this helps!
Power is the expenditure of energy per second,
and temperature is the average kinetic energy of
molecules. Power is power.
This means that a 90W heater will produce 90W
of power. A 90W light bulb will produce 90W of
power. They will be equally effective as heaters
in a closed space.
Now, the light bulb does produce SOME (namely,
10%) of its energy as light, instead of as heat.
If that light is not absorbed (windows, for
example, are transparent to light but not to
heat), then the energy contained within the light
will escape. Light emitted in a closed space
however will get absorbed and turned into heat, so
that means that, for the most part, your 90W light
bulb will be heating up your room equally
effectively as your 90W heater.
Click Here to return to the search form.
Copyright © 2015 The Regents of the University of California,
All Rights Reserved.