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:

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 second.<./p>

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 still 90W.

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 heating.

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 conditioner)!

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 a film).

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 incandescent bulbs.

Sources:
https://en.wikipedia.org/wiki/Electric_heating
http://en.wikipedia.org/wiki/Heat_pump
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.