Error with consumptions and outside temperature

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For non-leap years, February has 3 less days than December. Based on your results, the average daily heating load in February (28 days) is 11.42 kWh/day, while the average in December (31 days) is 11.47 kWh/day (at 20 cents/kWh, that's roughly 1 cent/day).

In addition, February typically has more sunlight than in December (Northern Hemisphere):

This would be enough to tilt the balance towards less heating in February than in December in many locations and for many building types. This would not be the case everywhere; in my (much colder) neck of the woods, it is on average ~5K colder in February than in December; despite being sunnier, it is not unusual to heat more in February (than in December). This is just scratching the surface ...