How efficient is an electric heater? How efficient is an electric heater? 
My guess: greater than 95%. Possibly even 99%. I say this because most energy is converted into heat; some is converted into light and kinetic energy, and possibly other forms of energy.
Anyone other opinions? (This is not homework. I am just curious and I'm having a discussion with a friend who says an electric heater is horribly inefficient, less than 5%.)
 A: It was a good answer Mark. Of course by drawing a lot of current some Joule heating will happen outside the house as well, in the transmission lines and transformers especially. So the efficiency will get lower depending upon where you draw the (electrical box). Some of the energy from the TV and refrigerator will also escape from the house before being degraded to heat (harmonics in
the electric lines, noise, and light from the picture tube (I'd bet these loses are well under one percent)).
In any case air sourced heat pumps are far less efficient than the theoretical limit. The figure I've seen is that a quality heat pump might give you about 3.5 times as much heat as the electrical power input. Not bad compared to the alternatives, but a very long way from Carnot efficiency. Using real (im)perfect working materials is really a drag. 
A: It depends on what you mean by efficiency.  
Suppose you want to heat your house.  An electric heater like you're considering would do this by converting electrical energy directly into heat.  Pretty much all the electrical energy does get converted to heat, as you suggest.  The energy used to get a certain amount of heat into the house is simply equal to that amount of heat.  In that sense, the electric heater is 100% efficient, since energy not directly turned into heat will be turned into heat soon.  That isn't a very useful way of thinking about efficiency, though, because any form of energy in your house will probably decay into heat energy pretty quickly.  Your computer, television, and refrigerator are 100% efficient at heating your house from this point of view, because although they do things other than generate heat, the energy they use to do those things becomes heat in short order.
By contrast, a heat pump would heat your house by taking heat from the outside and moving it inside.  The energy it needs to do this depends on the outside and inside temperatures.  If the temperatures inside and outside are $T_i$ and $T_o$, an ideal heat pump (i.e. a Carnot engine) would require 
$(1-\frac{T_o}{T_i})*dH$
Joules of work energy to move $dH$ Joules of heat energy from outside to inside (if the outside temperature were greater, this number is negative, meaning the heat pump can extract energy).
The efficiency of the electric heater, compared to the idealized heat pump, is 
$1-\frac{T_o}{T_i}$
for given inside and outside temperatures.  When the inside and outside temperatures are the same, the electric heater is zero percent efficient.  If it's 0C outside and 25C inside, the electric heater is about 8% efficient.
A: The efficiency is 100%, which is considered a poor heating efficiency. It is 100% because all light, motion, etc produced by the heater at some point becomes dissipated to heat. 
Same heating efficiency have all other completely-in-door devices: heating with a TV set is as efficient as an electric heater.
However, there is a better way to heat. A typical air conditioner can heat your house by 3 joules consuming only one joule of electric energy, making it more than 300% efficient. This is done by actually cooling the outside world.
A: Are you also interested in cost effectiveness and not just efficiency of the device? 
Heating with natural gas can be more financially efficient because it costs less per energy unit for natural gas delivered to your property/location than it costs per energy unit of electricity in some reasons making natural gas fueled heating more "efficient" than electric heating in terms of $ cost to heat a building while this may have nothing to do with actual efficiency of the device converting the energy to heat. In the same way a heat pump under the right environmental conditions can be very effective while in other applications waste oil furnaces (ie if you have lots of used motor available for low cost) can be relatively efficient in terms if energy conversion and cost effectiveness without having the theoretical efficiencies of a heat pump
A: I've wondered this question myself.  Efficiency is calculated as: output/input X 100 & yields a percentile answer.  For example in an electric power supply, it is easy to measure the input & output power, then calculate a corresponding efficiency.  I believe this is what the original writer is looking for.  However, the problem with the electric heater is that the input is measured in WATTS (ie, about 1200 or 1500 Watts for a typical electric space heater), but heat output is measured in BTUs. According to Wikipedia, 1 BTU = 0.293071 W·h, or conversely, 1 W·h is about 3.4 BTUs.  Given that, a 1500 Watt heater operated for 1 hour should produce about 5120 BTUs. So.... here's the rub;  Does anyone publish or know the actual BTU output of any electric heater?  I haven't seen that number, and since the efficiency calculation requires the same units of measure for both input & output,  I don't think the question will be answered until someone measures & publishes the BTU output.  But..... All that said, I feel the heater is 100% efficient because the ultimate goal is to put energy into a space, and with an electric heater, all the energy that is delivered to the heater is delivered as energy into the space.....regardless of how we measure it. That's 100%.
A: The correct answer is the difference between how many BTU's supplied to the electric heating device and the output in BTU's then the percentage can be calculated.The formulas above are for heat loads which are not only effected by indoor and outdoor temperature difference alone.You also need to take into consideration the effects the two have on resistive values of the insulation product used.If you can answer the first sentence I would be interested in the answers and values of various forms of electric heat.
A: In as much as "waste" energy in any system is generally converted to heat, directly or indirectly, heating systems are all close to 100% efficient in converting their direct fuel into heat. [The exception is a heat pump as Mark covered fully, which exceeds 100%.]
The main issue with efficiency and electric heating is that using electricity as the fuel may be an inefficient use of the original power source. 
It is therefore inefficient as a use of the gas or what have you that was originally the source of the electricity.
Typically a power station might have an efficient of say 34%, although can be over 50%, in converting the fuel energy into electricity.
See eg: Wikipedia on combined power
Some of that is less lost in tranmission, but this is a small factor relative to the energy lost during generation.
Gas heating will however convert 100% of the fuel energy into heat directly in the home. 
It is in this sense that electrical heating is not efficient, even though the electrical heater as a device is efficient.
It follows that electrical heating uses up more fossil fuel and generates more greenhouse gases than gas heating.
