A hot tub can make a great addition to your house, but it is important to know that its costs don't stop once you have purchased and installed it. You must take into consideration the ongoing price of chemicals (even with an ozonator, you'll need to use sanitizers like chlorine or bromine, albeit not so much as you would without an ozonator). There is also the matter of monthly energy consumption. Energy prices can be unpredictable in some regions, so it is important to pay attention to how much that wonderful new hot tub will increase your energy bill before you commit to buying it. The good news is that today's advances in technology are making hot tubs much more efficient than they have been in the past.
The amount of energy your hot tub uses depends on a number of factors such as the size and model you buy, how frequently you use your hot tub, the surrounding temperature, and the temperature you maintain your water at. And of course, the price you spend each month will depend on the cost of energy where you live.
The helpful book What Color Is Your Swimming Pool? lists the following as an example:
"A typical four-person spa costs just $10 to $12 per month to heat, based on $.07/kW hour, a set temperature of 102-104°F (39-40°C), and approximately 12 to 15 hours of usage during the month (or about 30 to 45 minutes every other day)" (p. 30).
The book says another study showed the industry average to be about $20 a month to heat a hot tub. Either way, it probably won't be terribly expensive; you just need to be aware of that ongoing cost when you decide to buy a spa. Remember, larger hot tubs will take more energy to heat, and hot tubs that are used frequently will also require more energy overall.
One thing you can do is talk to your hot tub dealer when you're shopping around. Let him or her know that energy consumption is a concern, and they will help you find a hot tub that is suitable for your needs. If your children have moved away, and you don't plan on throwing a lot of hot tub parties, a two-person model may be enough for your needs, and its smaller size would make it cheaper to heat.
If you have an old hot tub, and one of the reasons you haven't thought about buying a new one is because the old one used a lot of energy, you may want to take a look at the new technology being put into spas. Today's spa shells and covers are designed to be energy-efficient. The higher-quality models use "close-cell polyurethane foam," which is the same insulation material that is used in freezers.
Source: What Color Is Your Swimming Pool? A Homeowner's Guide to Trouble-free Pool, Spa and Hot Tub Maintenance