While deciding the unit for heat, which temperature interval is chosen ? Why ?

Explanation

While deciding the unit for heat, the temperature interval chosen is ` 14.5^(@)C "to" 15.5^(@)`
Reason : If we heat 1 kg of water by `1^(@)C` in a temperature range different from `14.5^(@) "to" 15.5^(@)C` the amount of heat required is slightly different.