A hot tub is a lovely thing to own and a quietly alarming thing to run. Unlike most appliances, which cost something only while you use them, a hot tub spends most of its life keeping a large body of water hot and gently circulating whether you are in it or not. That round-the-clock duty is what makes it one of the most expensive items a household can keep.
Why it costs so much
Two things run up the bill. The first is heating: keeping a few hundred litres of water at around 38 degrees, day and night, in the open air, means the heater fires regularly to replace heat lost to the cold around it. The second is the circulation and filtration pump, which runs for hours every day to keep the water clean, and which on many tubs also helps move heat about. Heating water is energy-hungry at the best of times, and a hot tub does it continuously, which is the opposite of how you keep an energy bill down.
What it adds up to
The honest answer is that it varies enormously, from a manageable monthly sum for a well-insulated tub used in summer to a genuinely painful one for a poorly insulated tub kept hot through a cold winter. Rather than trust a single scary figure, find your tub's heater and pump wattage and put them through the running cost calculator with a realistic estimate of how many hours a day each runs. The result is usually a wake-up call, and it tells you your own number rather than someone else's.
The factors that decide your bill
A handful of things separate a cheap-to-run tub from a ruinous one. Insulation is the biggest: a well-insulated cabinet and, above all, a thick, well-fitting, undamaged cover make a huge difference, because most of the heat escapes from the surface. The temperature you hold matters, since every degree costs more to maintain. So does the weather, as a cold, windy, exposed location loses heat far faster than a sheltered one. And of course how often and how long you use it, since opening the cover and reheating after a soak both add up.
How to cut the cost
Start with the cover, because it is the cheapest big win: keep it on whenever the tub is not in use, replace it when it becomes waterlogged and heavy, and consider a thermal blanket on the water surface underneath it. Set the temperature to the lowest you actually enjoy. Site the tub somewhere sheltered from wind. If you only use it at weekends, work out whether dropping the temperature between uses saves more than the energy to reheat, which depends on your tub and climate. And if you are on a tariff with cheap off-peak electricity, heating mainly in that window helps.
The honest bottom line
A hot tub is a luxury, and run as one it carries a luxury running cost that no amount of tweaking makes trivial. The sensible approach is to go in with eyes open: know roughly what it costs you using the calculator, keep that cover on religiously, hold a sensible temperature, and treat the bill as part of the price of the pleasure rather than a surprise. The single most effective thing you can do is also the simplest, which is to never leave it uncovered.