Ambient air temperature, set water temperature, cost of electricity in your area, frequency of use, hot tub insulation value, and electrical draw from the heater/circulation pump (standby wattage) are the six variables that determine daily/monthly energy costs of a hot tub.
It wasn’t until recent times that hot tub manufacturers were subject to energy regulations or efficiency standards in the United States. It used to be that a manufacturer would self-report data on how much their hot tubs cost to run per month or per year. Much of this data was biased and unreliable because it came from sources in a particular part of the country where the ambient temperatures had little fluctuation and were warmer. The cost per kilowatt hour was also cheaper in that particular part of the country as well which made for appealing operating costs that were misleading to unsuspecting consumers. That all changed around 2009 when the California Energy Commission (CEC) decided that it was going to require all hot tub manufacturers and dealers in the state of California to comply with specific guidelines and regulations regarding energy consumption. The CEC created standard testing parameters and every manufacturer was responsible for testing every one of their models using those same parameters and reporting their findings. All of the testing was to be done by independent laboratories. Two of the more important parameters measured both R value and standby wattage. R value by definition is the capacity of an insulating material to resist heat loss. Basically, the higher the R value the better the insulating effectiveness (holds heat better). The CEC’s test made certain that things like ambient temperature was constant (60 degrees) so that every manufacturer got a fair shake. They also measured standby wattage which was the amount of electricity the hot tub draws to maintain a set temperature (102 degrees) while given a constant ambient temperature (60 degrees) . In a nut shell, the goal was to set a bar to which manufacturers had to comply and also provide a resource for consumers to find neutral and unbiased data on the different brands available in the market place. Manufacturers also would face penalties if their spas were being sold in the state of California and found to be non compliant. Many other states have followed the CEC’s example and the industry as a whole is working to improve energy efficiency. However there are still spas out there that are as much as 50% more efficient than CEC standard. The CEC is currently working on making labels on hot tubs at the point of purchase mandatory so that consumers can make a more educated buying decision when shopping for a new hot tub. We would recommend asking your local hot tub dealer if the brand and model they sell is CEC compliant and for documentation supporting their claims.
Another more recent trend being adopted by many hot tub manufacturers is the utilization of heat pumps. A heat pump works by transferring heat from one location to another. In the case of a hot tub, the heat pump extracts heat from the outside air and transfers it to the water in the tub. Heat pumps are efficient because they use electricity to move heat, rather than generating it directly, which is less energy-intensive.
The heat pump contains a refrigerant that absorbs heat from the outside air. The refrigerant then travels through a compressor, which compresses it and raises its temperature even higher. The hot refrigerant then passes through a heat exchanger where it transfers its heat to the water in the hot tub. The cooled refrigerant then passes through an expansion valve, which reduces its pressure and temperature, allowing it to absorb more heat from the outside air and start the process again. This process is known as a refrigeration cycle, and it can be used to heat or cool a space or a fluid like water.