Heat pump- impact of ambient temperature on operation

[cleancloths]

Does anyone have some information on the relative efficiency of a heat pump based on ambient temperature? I have an 80,000 BTU/HR rated unit that I have been operating at night as I have cheaper electric power at night. I am wondering if I am actually saving money or if I would be better off running it during the day when the temperature of the air is warmer. Anyone have some insight into this?

[Poolsean]

Cleancloths,

Although the efficiency of the heat pump will be reduced at night, it is still very efficient. The best thing you could do is to put a solar cover over your pool. Night time is when you loose the most amount of heat and a solar cover will help retain most of what is lost to the ambient air temperatures.
Most Heat Pumps will maintain sufficient amount of efficiency even when ambient air temps fall into the 60's. Below that and some systems, if they do not have the hot gas defrost cycle, will shut down when the outer evaporator fins ice up.
When you get down into the 50's or colder, efficiency starts to drop.

[cleancloths]

Mine will go into defrost mode if it gets too cold. What I am talking about is running the system in April/May Sept/Oct when outside temp at night can be in the 30's and 40's. It sounds like I might be better off running it during the day at those times when the temperature is in the 60' nd 70's.