Probably the most common question that is asked when someone is considering the purchase of a hot tub is “what are the running costs”.
It is a difficult question to answer definitively as there are a large number of variables that can affect the figures, where the electricity cost (kW/hour) is just one. These include the following:-
the size and capacity of the hot tub
the temperature that the hot tub is operated at
the mains water temperature when the hot tub is filled up
the amount of time that the hot tub is used with the cover off
the number of bathers
the outside air temperature
the insulation properties of the hot tub
regularity of water change
Most manufacturers of hot tubs and spas do not carry out specific tests for each of their models, partly due to the time and costs involved in doing so. Also, they view the variables listed above would render the results useful as only a rough guide. One manufacturer conducted a test in a controlled environment which produced the following results when electricity costs were around 7 pence per kW hour.
Two hot tubs were tested:-
SMALL- 198cm diameter round spa with 2 speed pump, ozonator, 3 hours per day filter cycle, 922 litres capacity
LARGE- 209x198cm rectangular spa, 2 speed pump, air blower, circulation pump, 6 hours per day filter cycle, 1265 litres capacity
These hot tubs were sited in an unheated room with an ambient air temperature of 14°C and the spa temperature set to 37°C.
The results achieved were as follows:-
Electricity cost per day 27p 55p
To heat up after a water change £2.16 £2.27
At the time of writing this article, electricity costs are approaching 14p per kW hour which would mean that the running costs for the above criteria would be in the region of £3.78 per week for the small hot tub and £7.70 for the large one.
An increase of 10% in energy costs increases the typical running costs of a family hot tub by around only 80 pence per week!
Once a hot tub is up to temperature the running costs per day are quite low. This is because the insulation properties of many spas are very good these days, with thick hard top covers also providing further protection from heat loss. Once up to temperature the spa is maintained at this level automatically and uses little energy to do so if the top cover is left on at all times when the hot tub is not in use.
A typical family usage of a spa is for around thirty minutes per session 3-4 times a week and during this time the water temperature only normally drops by 1-2°C. Again, the electricity used to bring the hot tub back up to temperature is quite low and is achieved quickly.
The test conditions were with a room temperature of 14°C. If the hot tub is used outdoors throughout the year then this ambient temperature will vary quite considerably, with the running costs being higher in the winter months and lower in the summer.
In a domestic situation it is often suggested that the water in a hot tub is changed every 2-3 months, but again this very much depends on a number of factors. The usage, proper chemical treatment and cleanliness of the bathers are some of the most important factors in determining how quickly the water should be changed. If the water is changed 4-5 times a year then the cost of the electricity to reheat from cold would be in the region of £20 p.a. The cost of the water would depend on whether the supply is metered or not.
With regards to consumables the weekly cost is pretty minimal as the water content of around 1000 litres is significantly lower than an above ground swimming pool. The water quality should be tested at least once a week and when indicated by the test strip results a small amount of the relevant chemicals will need to be added such as chlorine, pH Plus, pH Minus, Shock treatment and Spa Clarifier.
It is recommended that the cartridge filter is cleaned once a fortnight, depending on usage. Many hot tub owners find that having a spare cartridge on hand is ideal as it ensures that the spa can be used constantly. The dirty cartridge is soaked overnight in a filter cleaning solution and then should be allowed to dry out. It can then be brushed out using a brush to remove any loose particles that remain. Having a spare cartridge will allow one to be placed in the hot tub straight away when the other requires cleaning.