Fantastic really does save energy . 
We have a smart meter and between midnight and exactly 7am with the lazy spa at 37 degrees the meter used to read £1.56 to £1.57 used without the thermal cover .obviously this wasn’t all the lazy spa but as a comparison the meter now reads £0.95 to £0.97 pence used so my none scientific analysis is in 7 hours we are saving 50 odd pence so probs £1.50 a day ish lol