Keep it covered-

“ Keep outdoor hot tubs covered when not in use. If you have a pool, use a solar cover to use the natural warmth of the sun to heat the water.”

https://www.powerhousetv.com/EnergyEfficientLiving/EnergySavingTips/101WaysToSaveEnergy?utm_source=WS&utm_campaign=Energy-EfficientLiving/Energy-savingsTips/027471