Depends on a lot of factors such as square footage, average relative humidity in your area ETC. A typical 2000+ square foot home generally has at least a 30000 - 35000 btu unit. Bigger than you need is better in my opinion. I believe it uses less energy by running less to do the work and kicking off when home gets cool. If its a life time home and a two story having two units and programmable thermostats will pay for itself by creating two living spaces for different times of the day. During the night bedroom areas run at 71 and downstairs kick up to 80. During the day if no one is home both kick to 80. An hour before you get home downstairs kicks to 71 an hour before bedtime they switch again.
This is a theory only and would need programed just right to be cost effective over YEARS of service.
This is a theory only and would need programed just right to be cost effective over YEARS of service.