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The performance of an electric heater in terms of heating output is most accurately measured in BTUs, which stands for British Thermal Units. One BTU is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. This unit is specifically used to quantify the amount of heat energy that can be delivered by a heating system, making it a standard measure in the heating and cooling industry.

While watts can also represent energy, particularly in the case of electric heaters (since watts equal volts times amperes), they are generally used to describe the amount of electrical power consumed rather than the heating effect produced. Joules, another unit of energy, can similarly describe energy transfer but is less practical in everyday heating applications compared to BTUs, which offer a direct connection to heat output in a way that is more intuitive for consumers and professionals in heating technologies.