What time of day is most efficient to heat your pool?

If you are using a heat pump then you want to heat during the warmest part of the day. So I run my water pump maybe 9 am to 5pm. Then the heater will cycle on and off during that time. If a heat pump has 100,000 BTUs when it is 80 degrees outside that might fall to 80,000 BTU's when it is 50 degrees outside. Now some people get a 40 or 50% break on their power at night. Then I would run the heat pump at night as the savings in power makes it worth the reduced BTU's. If you have a gas or oil heater it really doesn't matter as they both heat the same day ot night. If you have a 400,000 BTU gas heater you really are putting about 330,000 BTU's in the water as the rest goes up the exhaust. That will be the same day or night. A blanket on the water will help keep in the heat though and might cut your heating bills over 50%.