British thermal units measure heat energy. And when used to describe the power of heating systems or grills, the term "Btu" is understood to mean Btu per hour. The kilowatt is the metric unit of power. Conversion between the two requires the use of a simple conversion factor.
Write down our Btu value. For an example, suppose your furnace has a maximum output of 240,000 Btu.
Multiply your Btu value by the conversion rate of 0.0002931 kilowatts per 1 Btu. Continuing the example, 240,000 Btu x 0.000293 kilowatts / 1 Btu = 70.32 kilowatts. So the furnace in the example would have a maximum output of 70.32 kilowatts.
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Double-check your calculation by multiplying your result by the inverse of the conversion rate, or 3,412. Multiply 70.32 by 3,412 to get approximately 240,000 Btu. Because this equals your initial furnace output, you know that the conversion was accurate.
Always check that the units cancel properly when performing a conversion.