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Rated Wattage vs actual wattage

  1. Jul 3, 2012 #1
    I have an electric heater which is rated 2000W, 230V .

    Is it that the OUTPUT (thermal energy/coming out as hot air) is capable of transfering 2000J of energy to the surrounding room per second ?


    The heater needs a 2000J of electrical energy per second to power it (at max) so that it then transfers some unknown ("x" amount of thermal energy which is less than 2000J ofcourse dependant on the efficiency) heat energy to the surrounding air in the room .

    Which one is True ?
  2. jcsd
  3. Jul 3, 2012 #2


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    Electric heating can be considered 100% efficient at point of use. Ok in the case of a fan heater some power is "wasted" moving air but even that heats it up. A little is also "wasted" on the power indicator LED :-)

    So the short answer is that all the electrical power used is converted to heat. If it consumes 2000W then 2000J/S is delivered to the room.


    1) the rating plate on an appliance states the maximium (not the actual) current drawn from the supply. So although the heater is rated at 2000W it might not draw 2000W from the supply. It might draw a bit less. If it draws 1950W then 1950J/S is delivered to the room air. Still virtually 100% efficient.

    2) If there is a thermostat on the heater then, once the room reaches that temperature, the heater will start switching on and off. Under those conditions the long term average output from the heater should match the temperature losses of the room. In other words if you set the stat to 22C and find the heater is on for only 50% of the time then the heat loss of the room is 1000W at 22C. If it's on 75% of the time then the heat loss is 1500W etc.

    Since all electric heaters are virtually 100% efficient this means that many sales and marketing claims for exotic heaters are bogus. If all else is equal the power required to maintain the temperature of a room depends mainly on the heat loss of the room (eg how well insulated it is) NOT the type of fancy heater used.

    PS: Thats 100% at point of use. There are significant losses turning oil or gas into electricity at the power station so overall other types of fuel might be more efficient end-to-end depending on the application.
    Last edited: Jul 3, 2012
  4. Jul 3, 2012 #3
    Just to be clear here, i am talking about something like this :

    @Cwatters, the heating is dependent on the resistance of the nichrome wire (coiled up on a rod).

    So, would there be no energy loss in this kind of a heater, would it be 100% effiecient ?
    Don't you see the bright red light (from the rod), that light is not transferring energy to the room, or heating up the room ! is it ? ergo this type of heater must not be 100% effiecient (in terms of power)
  5. Jul 3, 2012 #4


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    The red light from the rod is transfering energy to the room in the form of visible light. This gets absorbed by the walls or furniture, heating them up. They can then transfer this thermal energy to the air through conduction and by radiating IR. The light from this rod or from an LED indicator will only not heat the room if there is a window that allows this visible light to escape into the environment.
  6. Jul 3, 2012 #5
    You can quibble if it is 99.999% or 100%...it's not relevant.

    This is correct:
    On the other hand, there are I2R losses in the house and appliance wiring so the amount of power supplied is a smidgen bigger than consumed by the appliance.
  7. Jul 3, 2012 #6


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    Where is the light going? Most will strike walls, furniture or people and heat them up slightly. Could even be considered an advantage. (In fact see next post). I suppose if you pointed heater so the light went out through a glass window there would be some loss but doubt the percentage is significant. So yes safe to assume 100% efficient.
    Last edited: Jul 3, 2012
  8. Jul 3, 2012 #7


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    This does raise a good point..

    If you need to keep people warm in a building that has open doors (like a truck loading bay) then it's pointless trying to heat the air as it will all escape. Instead "direct" heaters are used. These emit energy in the form of "light" at Infra Red frequencies. It travels through the air and heats people directly. The main problem with this type of heater is that they tend to be directional and only warm the side of the body facing the heater.
  9. Jul 4, 2012 #8


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    You need 4 of them, one at each compass point.
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