Solar panels for a spa or Jacuzzi is the same solar panels been used the swimming pools. Solar panels work at it’s best on a sunny day. By getting the maximum heat from the sun, you must set the pump timer to come on when the sun start to shine on the panels and to stop when the sun start to set. So will the water stay warm long after sunset.
Although the solar panels not working effectively if it is cloudy and does not work at all in the evening, there must be a back up like a heating element or a heat pump to keep the water warm.