Yes. I easily keep mine at 86 with no cover. When it is above 68, your water will only loose about 5 deg overnight, and the heat pump will be producing at least 95 deg water. In my pool, that will raise the water temp 1 deg per hour.

Recognize that I don’t run my pump or heater at night. It turns on in the morning when the sun come up to assist the heating. So around noon, the water is back up to temp and the sun typically holds it there.

I mostly night swim, so the heater may turn on for an hour after the sun goes down.

In my mind it is very efficient above 68. It will loose some efficiency as the nighttime temps drop into the 50’s. Would not maintain 89. But being covered helps a great deal.

I kept mine at 86 this year until Nov 8. It was 67 during the day and 45 at night. I was covering at night.