Hot tubs are made in many different ways using many different materials. The least expensive designs will cost about £5 a week to own, up to £30 for other designs. Why the huge difference? Does the spa come with a Thermal cover or just a thin floating cover made of bubble wrap? Most heat escapes from the top so a good quality thermal cover will save you loads of money.
Is the spa insulated with foam or reflective materials? If it is not, then your expensive heat is just floating off into the garden. If the spa has an air blower, this will look nice, but have no therapeutic value and cost a fortune in waste heat. As the air travels from the bottom of the spa to the top it absorbs the water heat and takes it straight out in to the air.
If you can see air vents in the cabinet then this means valuable heat is being lost to the outside, instead of being recycled back into the spa water. Hot tubs are designed to be kept hot 24/7. If they are built correctly, an average 300 gallon hot tub used 1/2 hour per day, every day, with an average 15p per kW energy cost will cost the owner about £260 a year. Larger hot tubs with more surface area and longer use periods, in colder climates like Scotland, and with higher energy costs, will cost more.