How scoring works
Cold plunge running costs: the electricity bill brands leave off the box
A self-chilling plunge is a refrigerator you sit in. It runs a compressor around the clock to hold the water cold, and that electricity is a real recurring cost almost no brand puts next to the sticker price. It is the cold-plunge version of the spec that gets hidden, so we surface it.
Why a chiller costs money every day
A built-in chiller is a small refrigeration unit. To hold water at, say, 40 F in a warm room, it cycles its compressor on and off all day, every day, whether or not you plunge. The bigger the temperature gap between the water and the room, the harder and longer it runs. So the running cost is not a one-time number; it is a monthly line item that depends on the chiller's wattage, your climate, and how cold you keep the water.
The number that is missing from the box
Most brands publish the chiller's horsepower or its amp draw, but not an estimated annual electricity cost, because it is unflattering and varies by user. We estimate it from the chiller's power and typical duty cycle and score it against the median for self-chilling units, so a more efficient or better-insulated plunge is rewarded and a power-hungry one is not. It is the same move as crediting irradiance only at a usable distance: we put the real operating number where the buyer can see it.
Insulation is the lever
The single biggest factor you control at purchase is insulation. An insulated shell and a tight-fitting lid hold the cold so the compressor runs less; an uninsulated tub bleeds cold into the room and makes the chiller work constantly. That is why our Build dimension rewards insulation and our Value dimension rewards a low running cost, and why an insulated plunge can cost less to run than a cheaper, uninsulated one. A lid is not a luxury; it is a running-cost device.
Ice-only tubs: a different cost, not a free one
A tub with no chiller has a zero electricity bill, but it is not free to run. You pay in ice, every single session, plus the time and hassle of buying, hauling, and adding it, and the water still warms as you sit. So the honest comparison is not "free versus expensive electricity." It is a recurring ice-and-effort cost against a recurring electricity cost. We score an ice-only tub as the different product it is: capped on cooling, with no chiller running cost but no temperature hold either.
What to actually look for
Ask three questions about ongoing cost. First, does the unit have a built-in chiller at all, or are you signing up to buy ice forever? Second, if it chills, is it insulated with a real lid, which is what keeps the running cost down? Third, what is the chiller's power draw, and how big is the gap between your target temperature and your room. Those three predict the bill the box does not print.
Browse the scored cold plunges, see the self-chilling shortlist, or read the chiller-vs-ice guide. This is general information, not medical advice.