The British thermal unit (BTU or Btu) is a measure of heat, which is measured in units of energy.It is defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit.It is also part of the United States customary units. However, NIST makes no warranties to that effect, and NIST We note that since 4.184 J is required to heat 1 g of water by 1 C, we will need 800 times as much to heat 800 g of water by 1 C. The initial temperature of the copper was 335.6 C. Assume that no heat is transferred to the surroundings.