A 120v sauna heater plugs into an ordinary household outlet and is the reason a traditional sauna is possible in a rented apartment or a house whose panel is full. It is also sharply limited by physics: a standard outlet can only deliver so much power, and that sets a ceiling on the cabin it will heat.
The power ceiling
A standard household circuit can carry a limited continuous load, and a heater has to stay inside it with margin. That puts 120v heaters at the small end of the range, and it is why they are specified for compact cabins only. A maker who claims a 120v unit will heat a large cabin is describing a room that never quite arrives.
What it will heat
Roughly a one or two seat cabin, well insulated, with modest glass, indoors rather than in a cold garage. Add a large glass door, an uninsulated wall or an outdoor position and the same heater falls short. Read the maker's stated volume as a maximum for a good cabin, not as a target for yours.
Longer heat-up
Less power into the same job means more time, so expect a longer wait than a 240v heater in the same cabin. For many owners that is fine: set it going, do something else, come back. It matters if the household pattern is spontaneous use, which is where the plug-in convenience and the wait pull against each other.
When to run the 240v circuit
If the cabin is over about two seats, if it is outdoors or in an unheated space, if it has a lot of glass, or if you want steam from a generous stone load. At that point the electrician is the right spend: a bigger heater on a dedicated circuit is a better sauna than a small heater struggling, and it cannot be retrofitted cheaply later.
Questions people ask about 120v sauna heater
Can a sauna run on a normal outlet?
A small one can, with a 120v heater specified for that volume. It limits you to roughly a one or two seat well insulated indoor cabin.
Why does it take so long to heat?
A standard circuit can only deliver so much power, so the same job takes longer. It is physics rather than a fault.
When should I upgrade to 240v?
Over about two seats, outdoors, lots of glass, or if you want real steam from a generous stone load. It is not cheap to retrofit.