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Typically, 2/3 to 3/4 of the heat in the coal is wasted before it gets to the electric kettle. An instantaneous gas-fired water heater would be 80-95% efficient. A heat-pump based heater could be powered with gas or electricity for higher overall efficencies. For small quantities, a Kelly kettle (with internal chimney) on a gas ring can be very good, and gas gives off much less carbon per therm than coal.
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