1Btu (15.6°C) = 0.99988Btu (15.0 °C)
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Btu (15°C) = Btu (15.6°C) / 1.00011813
Btu (15°C) = Btu (15.6°C) / 1.00011813
Btu (15°C) = 1 / 1.0001181381013
Btu (15°C) = 0.99988
The Btu (15.6°C) specifically refers to the energy needed to raise the temperature of water from 60°F (15.6°C) to 61°F.
The Btu (15°C) is a specific variant of Btu to reflect the energy needed to raise the temperature of water at 15 degrees Celsius by one degree.