Updated on 11.29.11

Saving Pennies or Dollars? Hot Tub Usage

Trent Hamm

saving pennies or dollarsSaving Pennies or Dollars is a new semi-regular series on The Simple Dollar, inspired by a great discussion on The Simple Dollar’s Facebook page concerning frugal tactics that might not really save that much money. I’m going to take some of the scenarios described by the readers there and try to break down the numbers to see if the savings is really worth the time invested.

Nick writes in: We rent a home and are responsible for all utilities. It came with a rather nice 8 person hot tub, currently, we will turn it on (warm it up) only when we plan on using it that evening. In the winter it takes about 8-10 hours to warm up. My question is this, should we leave it warm year-round, or only heat it up when we plan on using it. I think currently we use it 2-3 times a month, if we had it warm all the time, we would probably use it 1-2 times a week. Maybe more during the winter. During the summer it retains heat thanks to the cover and the temperature outside, and during the winter, it will retain the heat for maybe a day at most.

Without running the numbers at all, I can tell you that it’s cheaper to heat the water on occasion rather than keep it constantly hot. Hot water will constantly lose heat to the environment, and the greater the difference between the environment and your hot tub, the greater the heat loss at any given time.

Think about a hot cup of coffee. It’s initially very hot, but it doesn’t take too long for it to cool down to the point where it’s drinkable. However, after that, it doesn’t cool down at a very fast rate at all. It does eventually reach room temperature, but it stays at an acceptable heat for quite a while as you’re drinking it. That’s because the closer the liquid gets to room temperature, the slower the heat loss is.

Since the exact heating and cooling of water in a hot tub varies a great deal depending on the model, the insulation, and other factors, the best I can do is look strictly at the heating of the water.

A hot tub of the size you describe holds about 500 gallons of water. It takes about 8.34 BTUs to raise one gallon of water one degree Fahrenheit. So, if you’re raising that water from 40 F to 100 F (in the winter), you’re using about 250,000 BTUs, which is roughly 75 kWh of energy. A kWh costs about $0.12 from your electric company, so your energy cost for heating that much water in the winter is about $6.25 just to heat it for one use.

Now, your major issue with the hot water is your insulation. How much of it are you losing to the environment? For most hot tubs, the hot tub cover has a much lower R-value than the hot tub itself, so that’s how most of your heat is lost.

In your example, you mention that the heat in the tub is completely lost after a day or so if you’re not running the heating. Just using extreme back-of-the-envelope math, you’d essentially be heating the tub’s water each day during the winter, costing you $6.25 per day to keep it hot all the time. That’s going to add up quite fast.

My suspicion is that either it’s retaining at least some heat for longer than that day or your hot tub cover is a pretty thin one with a low R-value – or, most likely, both. I sent an email out to a friend who owns a hot tub in the southern part of the United States and who keeps a pretty close tab on his energy use and he estimates it costs him about $30 per month to keep his tub hot around the clock, but it would cost about $50 a month with a “typical” cover.

Compare that to $18 a month heating the tub for the three times you’d use it during that month.

You might want to stop by a hot tub dealership just to get an idea as to the R-value of the insulating cover on your tub. You can get a good ballpark estimate just by the feel of the covers they sell.

While the math isn’t exact (again, there are so many variables here and the math gets quite complicated quickly), you can compare the cover you do have with some of the others that they sell. If you see one with an R-value twice as much as the cover you do have, it’ll hold in the heat for roughly twice as long as your current cover.

Is a new cover an investment you want to make? If you really desire to leave your hot tub on all the time in the winter, it probably is at least worth looking into, as you’ll repay that cover’s cost eventually in the money you save.

However, the best approach is to simply turn on the tub in the morning if you plan to use it that evening. If you’re using the tub more than six or seven times a month, you may want to consider running it all the time and investing in a very high R-value cover for the hot tub, particularly if you’re going to live there for a long time or if your landlord will help with the cost of the cover (provided it stays with the house, I’m sure).

The savings for making the right choice here is going to be on the order of $20 a month, so it pays to think this one out.

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  1. Brad Moore says:

    Love the Saving Pennies or Dollars Series. I do some comparisons like that on my own blog….but you get really technical with it when you speak of energy usage and all. In fact, the one you did on Christmas lights convinced me to stay with my non-LED lights.

    Keep it up and I’ll keep letting others know!

  2. Carole says:

    It doesn’t sound to me like using a hot tub is a very frugal thing to do, anyway you try to manage it. I suppose if you can afford it, it’s one of life’s little pleasures.

  3. Nick says:

    Thanks Trent!

    @ Carole #1 – It’s not really. But it’s along the lines of coffee for some people. We very rarely use it, so I was curious. It costs far more than I thought just to heat it up! We didn’t choose the house because of the hot tub, it was just a nice amenity that came along with it.

  4. Sun says:

    In New Zealand, I went to a few hostels and spas that heated their hot tubs using geo thermal. Except for the initial outlay of infrastructure and the cost of water, the heating was free.

    In a similar thread, I know some people use their roof and hosing to store and provide hot water. If you put this liquid in the tub, the difference between starting and desired temperature is smaller… thus costing less to heat and maintain.

  5. Vicky says:


    Using a hot tub is very frugal when it’s shared with friends and it means a social night out instead of going to a bar or out to dinner!

  6. Adam P says:

    You can save even more money if you buy $3 bathing suits in the tub too.

    Or better yet, skip the suits altogether in the hot tub! That would be a cheap night. In more ways than one.

  7. Kristin says:

    I have friends that have a hot tub at their ski house. When they arrive for the weekend (during the winter in Maine, so it’s cold!), they will turn the heater on but leave it at a lower temperature than desired, as it’s expected that people will be using it throughout the weekend. A little before people actually go in, they’ll turn it up to a higher temperature. They’ll turn it off when they leave for the weekend.

    I don’t know enough about the covers Trent recommended, but this approach, when anticipating high usage in a short period of time, reduces both the overall heating required and the lag time of starting to heat and actually using it.

  8. kristine says:

    I have no idea the breadth of the audience to which this post was deemed likely relevant. Do that many penny-pinching readers own hot tubs? Scratching my head.

  9. Jim says:

    There is a big flaw with the coffee cup example. Most hot tubs come with two thermal covers. The first one is a foam type cover you put on top of the water, it’s almost like a backpacking closed foam pad. The next is the main cover that seals the top of the tub so that any more heat does not escape. By doing this, you can turn the tub down to about 90 and it takes very little hot water to keep it hot. Then, an hour before getting in, just turn it up to 99!
    We had an outdoor hot tub that came with our house in Madison, WI. In the winter, I figured out the cost to have the hot tub, using it 3-4 times per week in extreme cold, was about $20-25 per month.
    It was a great comfort to my wife who has fibromyalgia…the relief it gave her is something I would have paid much more than the $25 per month, so in this case, this discussion is VERY relevant.

  10. TLS says:

    I use the hot tub (and the sauna) at the gym I belong to. There was a discussion a while back here as to whether gym membership was frugal. Maybe not, but I use the gym frequently and certainly enjoy the hot tub!

  11. Lori says:

    Depending where you live it may not be an option to turn off when not in use and also may not be sanitary to have sitting water. If the weather gets to cold the water can freeze. We have a programmable hot tub and often let it get very cold when not in use, but have it heat up periodically in line with the manufacturer’s recommendations.

  12. Anne says:

    I’ll preface this by saying that I have zero experience with running hot tubs; they’re very enjoyable, but I’ve not owned or maintained one. My question is: what are the health implications of the water dropping below a certain temperature? Is there a minimum temperature a hot tub needs to maintain to be clean?

  13. michele says:

    My husband and I bought a 5 person hot tub 1 year after moving into a brand new home, built in 2006. It cost us $4500 with the electrical installation and cement work needed in the back yard. We paid cash because we always dreamed of having one. Now that we are retired, we have one! Yay!

    Here’s what we noted:
    $2 a month increase in water usage
    $1 a month increase in electricity
    $12 a month (average) in Chemical purchases
    $7 a month (average) in new filters
    for a total of $22 a month after a year

    We dump the tub every three months and use it EVERY night most of the year. When my husband’s back is in bad shape, he uses it 2-3 times a day, too. We keep it at 90 during the day and it takes 1 hour to reach 104 even in a hard snow storm. We use it for 15-20 minutes a night.
    Well worth if for the relaxation and back pain relief…and it’s only two of us most nights. Several times a year, we share it with family, but that does not really count.

  14. Cheryl says:

    Teri, I read something once that went like this,” If the only birds that sang in the forest were the birds that sang well, it would be a quiet forest”.Maybe u shuld cut this man some slack. He has some really good tips that he’s willing to share with others. Ultimately it will be up to him to decide if he needs to take any writing courses to fine tune his passion of writing. Even some very famous authors need editing and grammar correction before their work goes to the publishers.

  15. Cheryl says:

    P.s. Excuse my spelling of “u” and “shuld”. I’m typing this from my cell phone in bed at 6 am with “text mode” in my groggy mind.

  16. Becky says:

    Michele – I’m curious to know a bit more about your situation, as we are contemplating purchasing a hot tub. Your numbers suggest the biggest cost is the maintenance of the tub. How do you heat the tub? Electric, solar, gas/propane? It just seems so cheap with only a $1.00 increase in electricity. Our old home had an in-ground pool and would cost us about a $1,500 on propane to heat it from mid-May to mid-October (cold climate, used a solar blanket). We swam every day, often in the morning and evening, so I know that we would use a hot tub daily too.

  17. Chuck says:

    In an attempt to get this thread back on track……

    In your cost analysis, you seem to ignore certain issues that crop up if you live in areas where the temperature drops below freezing. In order to avoid busting the piping in your outdoor hot tub you need to either keep your hot tub powered up and heated or else drain the tub, and “winterize” the system. Then, before you can use it again, you have to “de-winterize” the system and then refill the tub. The cost of the winterization/de-winterization and the refilling of the hot tub will significantly impact your economic analysis of whether to heat it all the time or just when you use it a couple of times a month.

  18. brett says:

    Man…what i wouldnt have done for a nice little hot tub to jump into this morning.

  19. Jen says:

    Awww…what happened to all the “other” comments??? Anyway, we have a hot tub, and we love it, and we use it all the time, and our kids love it too. It probably is too expensive to maintain, but we enjoy the splurge.

  20. Luke G. says:

    We’ve considered getting a hot tub in the past, but were concerned about the costs of keeping it running.

    Forgive my ignorance with regard to hot tubs, but how does not running the heat until you are almost ready to use it affect the chemical needs? For instance, how would that work if it was not actively heated except once a day on the weekends?

    I’d hate to slip into a hot, relaxing pool of toxic death because the chemicals stopped working properly… O_o

    @Jen (#19)
    Trent removed the “other” comments, thankfully.

  21. Jamie says:

    I think these calcs are way off. The heating energy seems right, but not the cooling. If you assume a 40 degree location, the temp difference is 60 deg F, and an R-value of 18 (from http://www.hottubessentials.com/cover_R_Values.htm), then the Qdot is 3.33 Btu/(hour*ft^2), which means an 8 foot square tub will lose 5120 Btu/day, or 1.5 kWh, or 45 kWh per month. That’s only about $5.50, less than the cost of heating it up. That’s total heating cost of $12/month. Leave it hot.

  22. prodgod says:

    Because running a hot tub is indeed not a frugal choice, ours has been sitting empty for nearly 10 years. While we definitely miss using it, the cost to run it was considerably higher than $12 a month for us. I’m not counting chemical maintenance – that’s a relatively negligible expense. In addition to the cost to keep it warm, the manufacturer recommended running the filter twice a day, for two hours each time (I only ran the filter for a hour, 2x a day). This increased our energy bill by an average of $80 a month, which, 10 years ago, was much more affordable for us than it is now. If we could power our hot tub for $12 a month, I would have no qualms about continuing to use it.

  23. Petra says:

    I was wondering how hygienic a hot tub is? How do you keep the water sterile-ish if you use it several times? Lots of chemicals?

  24. Petra says:

    Aaa… Prodgod just answered all my questions :-)

  25. michael bash says:

    Agree with #2. We should focus on efficiency, frugality and economic common sense. The USA (and the world) can no longer afford hot tubs. “Our house was foreclosed; we sure miss not having a hot tub in our car.”

  26. charles says:

    keeping a solar cover on the heating tub will make a huge difference in time taken. there are anumber of effective sanitizers on the market. the water should be drained every sixty to ninety days depending on bather load. Clean water is your friend. Trying to go cheap is not smart. Pumps to day are very efficient. A soaler cover is cheap,

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