The units are heat pumps, not a simple air conditioner. Responsible to cool in the summer, and heat in the winter.
When it is mid 30's outside, it has no trouble keeping my house 40 degrees warmer without powering up the aux heating element.
When it's 100 outside, no trouble keeping my house 30 degrees cooler.
The unit is no where near running 100% of the time, so plenty of available capacity left.
A big factor is the home construction. Newer houses are built with better insulation and better insulated windows. Much less heat transfer in modern builds.
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