Is a 120-volt hot tub enough in winter?
A 120-volt plug-in hot tub heats at about a quarter of the speed of the same tub wired for 240 volts. A common Balboa spa pack is rated 4 kW at 240 volts and 1 kW at 120 volts. It costs the same to keep hot either way. You give up speed. Recovery after a soak takes hours instead of about one. In a hard freeze, the tub may never reach 100 °F.
Why is a 120-volt heater so much weaker?
It’s often the same heating element. Halve the voltage and the element puts out a quarter of the heat. That is just how a resistance heater works. The same Balboa listing gives its larger heater as 5.5 kW at 240 volts and 1.375 kW at 120 volts.
The circuit limits it too. A plug-in tub runs on one household circuit, usually 15 or 20 amps. A 240-volt tub gets its own 40 or 50 amp circuit.
Does a 240-volt tub cost more to run?
No. The heater only puts back the heat the tub loses. How fast it loses heat depends on the cover and the weather, not on the heater.
In the hot tub cost calculator, a 400 gallon tub at 100 °F, with a good cover and 30 °F air, costs about $60 a month. That’s the same with a 1 kW heater or a 4 kW one. The bigger heater does the same work in fewer hours. The winter running cost guide has the month-by-month numbers for colder and milder air.
Can the heater and the jets run at the same time?
On many 120-volt tubs, no. Coast Spas’ manual says tubs “wired for 110V only have enough power to heat or operate pumps. Turn off jets to power heater.”
So while you soak with the jets on, nothing replaces the heat you lose. The cover is off, and people are getting in and out. When you’re done, the heater has to win it all back at 1 kW.
How long does each one take to heat?
From the heat-up time calculator: a 400 gallon tub, a standard cover, 30 °F air.
| Heater | Fresh fill, 55 to 100 °F | Win back 4 °F |
|---|---|---|
| 1 kW (120 V) | about 3½ days | about 11 hours |
| 1.375 kW (120 V) | about 2 days | about 5 hours |
| 4 kW (240 V) | about 12 hours | about 1 hour |
| 5.5 kW (240 V) | about 9 hours | under 1 hour |
A good cover helps the small heater most. With one, the 1 kW tub wins back those 4 degrees in about 7 hours rather than 11. The 4 kW tub barely changes.
When does a plug-in tub stop keeping up?
Every tub has a ceiling, where the heat lost equals the heat the heater adds. A 4 kW heater on a covered tub is nowhere near it. A 1 kW heater can hit it.
Take a 400 gallon tub with a thin or old cover. At 30 °F outside, a 1 kW heater tops out around 99 °F. At 10 °F outside, it tops out near 79 °F. Waiting longer won’t change that. Only a better cover or a bigger heater will.
Which one should you get?
A 120-volt tub makes sense if your winters are mild, or you mostly use it in warmer months. It also suits you if you rent, or your panel has no room for a 50 amp breaker. Buy the best cover you can. It matters more for you than for anyone.
A 240-volt tub makes sense if it will sit outside through real winters and you plan to use it often. You’ll need an electrician, a GFCI and a 40 or 50 amp circuit. Both Hudson Bay Spas and Coast Spas list those sizes, depending on the model.
Some spa packs can switch between 120 and 240 volts. The Balboa pack above is one. If you buy a plug-in tub, ask the dealer whether you can rewire it for 240 volts later.
What doesn’t help
An extension cord. Both manuals forbid one. Hudson Bay says plug the tub straight into the outlet, and Coast warns low voltage can damage the tub, which the warranty won’t cover.
Sharing the circuit. Both manuals want a circuit with nothing else on it, no lights and no other appliances.
Leaving the jets on to “spread the heat”. On a tub that can’t run both, the heater stays off while the jets run.