You can't use the average.
Example: Let's say for half year the average temperature is 11°C and for half year it's 31°C. That gives an average temp of 21°C and a δT of 0, even though you would still need to heat for half a year.
Example: Let's say for half year the average temperature is 11°C and for half year it's 31°C. That gives an average temp of 21°C and a δT of 0, even though you would still need to heat for half a year.