Hot Tub Heat Machine...

“If you have a hot tub or swimming pool, keep it covered when you’re not using it. You can buy solar covers that use the sun’s warmth to heat the water.”

https://www.ovoenergy.com/guides/energy-guides/120-ways-to-save-energy.html?fbclid=IwAR1wQh1FPGcj1n_MkLYySqEDAnP8vMEcVQr_RL0sM8HWblsTeXH7j2WWfdw