Delta T at lower outdoor temperatures ?

[JOSHUA NAGE]

Can anyone provide a formula to calculate the delta T for an 8000 BTU air conditioner relating to various outdoor temperatures ? I have a neighbor running a noisy A/C at temps as low at 25 degrees. At what temperature, does running an air conditioner produce an inconsequestial delta T ? I imagine as the outside air gets colder and harder to compress, running a residential A/C becomes almost useless?

[tbirdtbird]

Anything below 65*F is a waste and could harm the unit. If it is that noisy it probably is an older machine and not very energy efficient anyway.