What this calculator works out
Almost all of a hot tub's running cost goes on holding water at a set temperature against the surrounding air, not on the handful of soaks actually taken in it. That makes it closer to a small heating appliance left permanently switched on than to something like a kettle, used for a few minutes a day. Publishers who quote a single "it costs £X a month" figure are almost always averaging over a specific tub, a specific month and a specific tariff, which is exactly why this tool builds an honest range instead: pick a tub type, a size, how often it's used and which months it runs through, and every figure below, daily, weekly, monthly and yearly, comes out as a low-to-high band rather than one falsely precise number.
The range exists because real running costs genuinely vary this much between two households with what looks like the same tub. A sheltered spot against a warm wall, a snug, well-sealed thermal cover and a mild autumn all sit at the low end; an exposed corner, a worn cover and a hard frost sit at the high end, often for the identical model. If a smart plug or an in-line energy meter has already told you what your own tub draws in a day, the optional override field replaces the built-up estimate with a tight band around that measured figure instead, since your own reading beats any published range.
How the daily energy estimate is built
The starting point is a typical kWh-per-day range for each tub type, taken from published running-cost guidance: Lay-Z-Spa's own running-cost figures for inflatable models, and independent UK hot tub running-cost guides for hard-shell spas (see Sources below). Once up to temperature, a well-covered inflatable tub typically draws somewhere around 4 to 6 kWh a day in warm weather and 7 to 13 kWh a day once it's cooler outside, because its thin inflated wall loses heat far faster than rigid foam. A quality insulated hard-shell spa is much steadier, typically around 3 to 5 kWh a day in summer and 5 to 7 kWh a day in cooler months, since its foam-backed shell and denser cover hold heat in far more effectively.
Choosing the 5 to 7 person size nudges that baseline up by around a fifth, since a bigger tub holds more water and more surface area to lose heat from. Usage does something smaller but real: a tub used most days loses a bit more heat through the open lid and runs its pump a little more, adding roughly a tenth on top of the baseline, while a rarely used tub sits a little below it. None of these adjustments are lab-measured for your specific model, which is why the result stays a range rather than tightening into a single number.
Why cooler months cost so much more
Heat always moves from warm to cold, and it moves faster the bigger that gap is, so a tub held at 37 to 38°C against a cold, wet November loses heat far quicker than against a mild August. That is the entire reason the "cooler months" figure sits so much higher than the "summer" one. The size of the jump differs sharply by construction, though, and it's worth being honest about that rather than quoting one figure for every tub: an inflatable's thin wall gives it comparatively little insulation to fall back on, so its running cost commonly climbs two to three times higher in the depths of winter than in summer. A well-insulated hard-shell spa, with real foam behind its shell, is affected far less by the outside temperature; independent running-cost guides for well-insulated models put the winter increase at roughly 40 to 70% above a summer baseline, not two to three times over.
A worked example
Take an inflatable tub, the default 2 to 4 person size, used a few times a week, running all year round, on the default 26p/kWh UK rate. In summer, its estimated 4 to 6 kWh a day comes to roughly £1.04 to £1.56 a day. In the eight cooler months it climbs to 7 to 13 kWh a day, or roughly £1.82 to £3.38 a day. Blending four summer months (about 122 days) against eight cooler months (about 243 days) gives an estimated yearly energy use of around 2,190 to 4,015 kWh, which at 26p/kWh works out at roughly £570 to £1,045 for the year, about £47 to £87 a month once blended. Swap to a hard-shell insulated tub with the same settings and that yearly figure falls to somewhere around £250 to £450, the difference almost entirely down to insulation.
The cost of a soak, and changing the water
The heating cost above already covers holding the tub at temperature whether or not anyone gets in; sitting in it for half an hour adds comparatively little on top, since the jets and pump are the only extra load running and the water is already hot. That marginal per-soak figure, shown separately in the results, is typically a matter of pennies, not pounds, which is why frequency of use moves the daily estimate only a little rather than a lot.
Water itself is not free to maintain either. Most hot tub water needs fully draining, refilling and rebalancing roughly every three to four months, sooner for heavy use, both to keep sanitiser working and because dissolved solids from sweat, lotions and top-ups build up over time. A full change typically costs £15 to £30 in chemicals and metered water, on top of an ongoing £10 to £20 a month in sanitiser and pH balancer between changes. Both figures are built into the results panel, scaled for tub size and how often changes suit your usage level.
Cutting the cost: cover, shelter and setback
Three things move the real-world figure more than any input in this calculator. The single biggest one is the cover: research on pool and spa covers from the US Department of Energy attributes as much as 50 to 70% of potential heat loss to evaporation and convection straight up through an uncovered water surface, so a torn, warped or badly sealed lid quietly undoes a large share of a tub's insulation regardless of the shell underneath. Wind matters too: moving air across an exposed tub adds further convective heat loss, so a spot out of the prevailing wind, behind a fence, hedge or wall, measurably eases the heater's workload, even though the exact saving is too dependent on local conditions to reduce to one figure.
Temperature setback is the one with genuine, printable maths behind it. Heat loss through a cover scales roughly with the temperature difference between the water and the air trapped just beneath it, not with the water temperature alone.
Dropping the setpoint from 38°C to 35°C while away for a week, against a representative under-cover air temperature of around 10°C in a UK autumn, cuts that difference from 28°C to 25°C, an 11% reduction, which maps onto roughly an 11% cut in that week's heating energy. That's a genuinely useful saving for a short trip, and it avoids the much bigger cost of letting the tub go fully cold and reheating several hundred litres from scratch, which is covered in the FAQ below.
Questions people ask
Is an inflatable or a hard-shell hot tub cheaper to run?
Hard-shell insulated spas are cheaper to run, typically 3 to 7 kWh a day against an inflatable's 4 to 13 kWh a day depending on the season, because their foam-backed shell and denser cover hold heat in far better than an inflatable's thin air-filled wall. The gap widens sharply in cooler weather: an inflatable's running cost can climb two to three times its summer figure, while a well-insulated hard-shell tub typically rises by only 40 to 70%.
Is it cheaper to leave a hot tub running, or heat it from cold each time?
For anything shorter than a couple of weeks away, leaving it running (or on a lower setback temperature) is usually cheaper than switching it off, because reheating several hundred litres from close to ambient temperature is a large, one-off energy spike, and inflatable heaters in particular can take a day or more to bring the water back up. Turning it off and draining is only worth doing for a genuinely long break or an off-season pause, when the standing heating cost over that whole stretch would outweigh the one-off reheat.
Can an inflatable hot tub be run through a UK winter?
Most inflatable models are rated to keep working through a UK winter, but running costs rise sharply, commonly into the 7 to 13 kWh a day range used in this calculator, and manufacturers typically advise against operating one in a hard freeze, since the pump and heater are not built for genuinely sub-zero conditions. A thermal cover and a sheltered spot matter more in winter than at any other time of year for exactly that reason.
How much do the chemicals and water changes really add?
Typically £10 to £20 a month in ongoing sanitiser and pH balancer while the tub is in use, plus £15 to £30 in chemicals and metered water each time the water is fully changed, usually every three to four months for average use. Together that commonly adds up to somewhere around £150 to £350 a year on top of the electricity cost, shown broken down in the results panel above.
Why does this show a range instead of one number?
Because the exact position, cover condition, local wind and weather of any two households with the same tub can genuinely differ enough to move the real cost by a large margin, even with identical settings entered. Showing a single figure would claim a precision the underlying published guidance was never built to deliver; the range here is built from typical low and high figures for the type, season, size and usage entered.
Does a thermal cover actually make a difference?
Yes, and by a large margin. US Department of Energy research on pool and spa covers attributes as much as 50 to 70% of a tub's potential heat loss to evaporation and convection straight up through an uncovered water surface, so a torn, warped or poorly sealed cover undoes a meaningful share of even a well-insulated shell's advantage.
This calculator builds a kWh-per-day estimate from published running-cost guidance for inflatable and hard-shell hot tubs, adjusted for size, usage and season, always shown as a range rather than one falsely precise figure. It excludes standing charges, does not model your specific ambient temperature, wind exposure or cover condition, and the chemical and water-change figures are UK-typical estimates, not a quote for any specific product. If a smart meter has measured your own tub's actual draw, use the override field for a tighter, more personal estimate.