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How Much Hot Water?

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David Borton (RPI Physics teacher and solar advocate) asked, “How many of you know how much hot water you use?” He smiled his mischievous, knowing smile. No one raised their hand but me. He looked at me and I said, “We used 2500 kWh on average for the last three years.”

I was on a solar home tour of David’s home and he was making a point about energy use. He stated that few people know how much hot water they use. He said, “Dan, that is how much energy you use for hot water, not how much hot water.” I was puzzled, not having thought about the distinction. Over the next couple of years I came to understand. While we may have been using a “reasonable” amount of energy for a family of four, for our hot water (I had it sub-metered, because previously I had no idea about how much energy or money we spent on hot water), the 2500 kWh might have been a lot or a little when the amount of water we actually used was considered.

It takes one BTU to heat 1 pound of water 1 degree F. Therefore, 2500 kWh means we used enough energy to heat 15,045 gallons of water to the 120 degree set-point. [The Math: Entering water temperature averages about 52 degrees. Our hot water heater was set at 120 degrees (68 degrees rise in temperature). There are 8.34 pounds of water in a gallon. There are 3413 BTUs in 1 kWh. 2500 kWh X 3413 BTU/kWh = 8,532,500 BTUs of energy used. If it takes 567 BTUs (68 Degrees X 8.34 Lb/Gal) to raise one gallon from 52 to 120 degrees, then we used enough energy to heat 15,045 gallons of water!]

So, the question about how much hot water we use is important. If we used 15,000 gallons we were heating water super efficiently; if we used 5,000 gallons, then horribly so!

The truth was, I didn’t know how much hot water we used – I had no idea. I had never thought of it in those terms. I had just thought of it in terms of energy and dollars. We were using 2500 kWh a year, about half what the “average US family of 4 used, and I thought that was pretty good. But if we were using only ¼ of the hot water an average family used, than we were heating it inefficiently. I found varying data about how much hot water the “average” family uses, depending on number in the household, their location in the US, but not on age of the kids. I also found that the “average” amount of energy the average family uses is also not very clear. So, I decided to estimate how much we “actually” used and forget about comparing our family to some mythical average.

I measured, counted and estimated what we used in a week:

  • Number and length of showers per family member
  • Number of loads of clothes washed with hot water and how much hot per load
  • Number of loads of dishes washed and gallons of hot water
  • And I threw in a number of gallons for miscellaneous and hand washing

Then I built a spreadsheet. I also estimated each item a little high and a little low to give me a range, where I was pretty sure our actual use was within the range. In the end, I estimated we used somewhere between 8,000and 12,000 gallons of hot water, but still it may be more - I may have overlooked or under estimated some aspects. It is impossible to say without a meter.

How much hot water do you use?

If you have good data, please share it. I expect there are very wide variations and no answer is correct. Here are a couple of examples where meters were installed for both incoming cold and hot.

Carl McDaniel reports that he and his wife, with occasional visitors, in their Positive Energy House, Trail Magic, use only about 9,000 gallons of cold water a year (80% less than average US family of two) and about 2,600 gallons of hot water a year (70% less). See his four-year performance data for Trail Magic.

Larry Burkes, architect and homesteader in Greenwich, NY reports that in their new Net Zero home he and his wife use about 17,000 gallons cold (about 60% less than average) and 7,300 hot (about 15% less than average) annually. See his excellent blog, with all sorts of insight and data.

Clearly the best way to find out how much hot water you use is to install a water meter on the water that goes into you water heater and into the cold water port of your tempering valve if you have a hot water solar system. Water meters for ¾” line can be purchased online and cost just a little over $100 delivered  ( These meters are accurate to within 1.5%) and can be installed by the handy homeowner or your favorite plumber.

So why is quantity so important? Well, if we actually used 8,000 gallons of hot water then we are heating water at 53% efficiency (47% of our energy is being lost to the surrounding space of the water heater). If we are actually using 12,000 gallons, then our water heating efficiency is 80%. That’s a big range, where the actual cost/efficiency can only be determined by knowing how much hot water was actually used.

In our area, water heating for the “typical” family of four is the second single biggest use of home energy, second only to heating the house. So how much energy we use to heat water is an important issue. After the basic energy conservation issues are addressed: no hot water leaks, dishes and clothes are being washed with Energy Star appliances, and we use cold whenever we can for clothes washing, then the bulk of what is left is hot water for our showers!

While we need to learn to live well while using less energy, taking tepid showers is not living well. Taking super quick showers (what is it 3 minutes in the military?) is not living well. I can’t say what living well is for you, but to me an 8 shower, shampoo and shave is usually adequate. However, when I have aching muscles or I’ve been out in the cold for hours another 5 or even 10 minutes or so “luxuriating,” as my wife says, is great!

So, if you are like me, you probably have no idea how many gallons of hot water your family is using. Since water and the energy to heat it are so valuable, it makes sense evaluate this area of your home operations.

One way to start is to work up an informal estimate the way I did with a spreadsheet, but that is fraught with peril and a lot of hard work. A better and easier way is to use an estimating tool that takes more details into account and does the math for you. Using an  estimating tool, especially when completed with you by a person familiar with hot water usage, is a fairly quick way to reasonably estimate your actual hot water usage.

Knowing how much hot water you use, is the first step to determining the best way to heat that water!

If you know how much hot water you use, please comment and let us know. If you know how much energy you use to heat it, that is even better! If you have a hot water estimator and experience using it, I’d love to hear from you too.


Dan Gibson is the Reporter and Chief Coordinator of Our Energy Independence Community ( Previously he performed home energy audits for five years in NYSERDA’s Home Performance program and new home ratings in the New York ENERGY STAR Home program. He is currently building a 100% Solar Home. He can be reached at

Comments on "How Much Hot Water?"

  1. OEIC default avatar David Borton January 15, 2013 at 3:49 pm

    Recently I spent $400 to understand hot water better.  I had a water meter added to both my main cold water input and the cold water input to the hot water heater.  It turns out on average, my wife and I (and 10 guest overnights) directly use approximately 40 gallons of cold water a day which includes the 15 gallons of hot water each day.

    My only gas appliance, my water heater used 14 therms over the 45 days or 3100 BTUs per day.  It is 7 years old and was efficient by those day’s standards.

    I did not invest in a data acquisition system that could give me good average temperature measurements for incoming and outgoing temperature at the hot water tank.  Using a reasonable temperature rise of 60 F and a conservative tank efficiency of 75% then only 1-in-3 of the BTUs of hot water in my tank actually got out of the tank as hot water, that’s 1-in-4 of the gas BTUs.

    Certainly an on-demand hot water heater would be better for me energy wise.  My measured usage of hot water also makes it hard to justify a solar hot water system.


  2. Dan Gibson's avatar Dan Gibson February 17, 2013 at 9:48 pm

    Peter Skinner responded to this article, not with a comment but rather a more indepth blog on this website. See it at:

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