Heat rate depends on factors such as tub size, outdoor temperature, and heater size. The following numbers give a range for each type of heater: Electric heaters: 4–12 °F per hour Gas heaters: 10–40 °F per hour Wood heaters: 5–20 °F per hour An important note about heat rate: It is easy to get swept up in the heat rate, but note that with gas and electric heat systems, you will not frequently be waiting for your tub to heat up. This is because they are thermostatically controlled and also because there are features such as economy mode and WiFi control that allow you to plan and automate your hot tub temperature management around your hot tubbing schedule. The only time you may have to wait for your tub to heat up is after a water change, but even in that instance connecting your garden hose to your home’s water heating system when filling the tub, could potentially help you to shorten the wait time, if possible, and if necessary. The point is, don’t worry too much about having a high heat rate unless you will be using your tub infrequently, erratically, with little notice, and without WiFi connectivity. All of these factors coexisting is uncommon.