I bought a used spa and the cover doesn't look like it's in the best shape. I'm concerened that it's not as effiecent as it could be. We have a CalSpa PS4 from 1995. When we get out at night the temp is 101. If the heater doesn't run for 24 hours we drop to 98 degrees. Is this normal? The lows at night right now are in the mid-40's. It takes between 30-60 minutes of heating to get those 3 degress back.

loosing only 3 degrees in 24 hours with NO HEAT is what I would consider EXCELLENT. The time it takes to heat back up is right on par. What you should check into is why the heater isn't comming on, and fix that. Some spas have an economy mode and a standard mode. Economy will only run the heater during filter cycle, and is adjusted by a timer. If you are in economy, and have a mechanical timer, add time so your filter cycle runs 4 times a day, for about 15 to 20 minutes a shot.