Item 05 of 10
Insulation and running cost
Keeping a swim spa warm with the cover on costs about $57 a month for a typical one, and $22 to $87 across 44 California listings.
A swim spa keeps two to seven times a big hot tub's water warm all year. Insulation decides how much of that heat leaks away.
What it is
A swim spa holds roughly 900 to 2,900 gallons; Master Spas' seven-seat LSX 850 hot tub holds 395. Heat leaves through the top, the walls and the floor, and the heater puts it back. Insulation slows the leak. Makers fill the whole cabinet with expanding foam, line the cabinet walls and floor and leave an air space, or mix the two: Hydropool blows 6 to 8 inches of foam around the shell and wraps the rest in reflective blankets. California tests every spa sold there for standby power, the watts it draws to hold temperature with the cover on.
Why it matters
Standby power runs every hour of the year, so small differences add up. Every 100 watts is about 876 kilowatt-hours a year, or about $159 at the US average of 18.19 cents for January to July 2026.
Size explains part of the spread, so the fair comparison is against California's limit, which rises with volume. The Michael Phelps Signature Pro's listing uses 56 percent of its allowance; every MAAX-built swim spa listing, sold as American Whirlpool and Vita, sits one watt under the limit. California's own analysts found same-size spas differing by up to 150 watts and blamed how the insulation was applied, including hit-and-miss spots.
Time makes insulation choices matter more. A leak inside full foam is slow and costly to dig out, which is the drawback Hydropool lists for it. Summer brings the opposite problem in hot places: warm air pushes the water too warm. Master Spas calls it heat creep and offers a heat pump that also cools on two H2X models.
How makers do it
Full foam
The good wayExpanding foam fills the cabinet around the shell and plumbing. Master Spas lists full foam on its Michael Phelps swim spas.
Foam at the shell, blankets at the walls
AcceptableFoam around the shell, reflective blankets on floor and walls. Hydropool uses 6 to 8 inches of foam this way.
Perimeter insulation
AcceptableInsulation lines the cabinet walls and floor, leaving an open cavity that pump heat warms.
Heat pump beside the heater
AcceptableA heat pump heats, and on some models cools, the water. Master Spas offers Temp+Control on the H2X Challenger 15D and Trainer 15D.
Foam sprayed unevenly
AvoidGaps and thin spots behind the shell. California's analysts blamed differences of up to 150 watts on this.
In the showroom
Check this
- Ask for the California energy listing: standby watts and the maximum allowed for its size.
- Read the spec sheet for insulation: full foam, perimeter, or a mix.
- In hot climates, look for a heat pump that cools as well as heats.
Ask this
- What are this model's standby watts in California's listing, and what's its limit?
- Which cover was it tested with, and is that the cover you'll deliver?
- Is a heat pump offered, and does it cool as well as heat?
How each brand does it
14 of 17 brands publish enough to tell. For the rest, ask the dealer.
Every brand, A to Z (14)
- PartlyAmerican Whirlpool
Passes California's standby limit on every model, but only by 1 watt.
- PartlyArctic Spas
Perimeter FreeHeat insulation and a thick Mylovac cover are standard, but Arctic publishes no standby-watt or running-cost figure for any swim spa.
- PartlyArtesian Spas
Full foam on every model; California standby runs from 35% to 99% of the limit.
- YesBullfrog Spas
Full-foam cabinets; California's tests put the S150 and S180XD at 52 to 65 percent of their standby limits.
- PartlyCal Spas
Wall-and-floor insulation, not full foam; California's tests put the swim spas at 68 to 75 percent of the limit, with a thinner cover.
- PartlyCoast Spas
Partial foam is standard and full foam costs extra. Coast's own estimate for a 1600 is $132.01-$162.93 a month on its standard heater and $66.36-$83.17 with its optional heat pump.
- PartlyDynasty Spas
Perimeter insulation (RMAX foil panels around a heated air space), with no published standby figure.
- PartlyEndless Pools
California lists Endless's single-zone models at 351 to 501 standby watts, 70 to 84% of the limit; the insulation isn't described.
- PartlyJacuzzi
Full foam plus heat-reflecting insulation on most models and heat-pump-ready plumbing, but Jacuzzi publishes no energy figure for any swim spa.
- PartlyMarquis
Full foam on all four. The V150s' 2020 energy tests use about three-quarters of California's limit; the V174 has no listing.
- YesMaster Spas
Full foam on every model; where California has a listing, standby runs about half to four-fifths of the allowed limit.
- PartlyPDC Spas
Not full foam; California lists PDC's swim spas at 360-457 W standby, about 40% under the state limit.
- PartlySwimEx
Heat is held by the balsa-cored shell and a thin floating blanket rather than foam, and SwimEx's own estimate is about 809 kWh a month. No standby energy listing exists for comparison.
- PartlyWellis
Board-and-foil insulation and a heat-pump option, but no published energy figure for any swim spa.
Hydropool, LPI and Sundance Spas don’t publish this.
Read nextSwim spa costSwim spa electrical requirementsIs a swim spa worth it?Best swim spasSwim spa sizes
Where it gets complicated
The standby figure isn't a perfect ranking. It's measured with the cover on at 60°F air, and a spa that can't reach 100°F may be tested at a cooler water temperature than one that can. Full foam against perimeter insulation is argued both ways; California's analysts said how evenly insulation is applied matters more than which kind.