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DHW fuel question

Jim Brill | Posted in Energy Efficiency and Durability on

Has anyone run across a decent online calculator that can compare the cost of DHW using various types of fuel? electric, propane, natural gas are potential choices at this point. Would be terrific if the calculator could factor in an efficiency rating too, even better if it was able to output the net cost of a gallon of water at, say 120 degrees. Thanks in advance for your suggestions!

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  1. Expert Member
    Zephyr7 | | #1

    Easy enough to calculate this out the old fashioned way. If the water heater lists it’s energy consumption, you can compare by converting everything to kilowatt hours or BTU:

    1 kWh = 3,412 BTU
    1 gallon of propane = 91,542 BTU
    1 cubic foot of natural gas = 1,000 BTU

    The actual energy content of the fuel gases can vary a little bit, but will be very close to the above numbers.

    For efficiency, divide the energy consumption by the efficiency (which will give you a BIGGER number). Electric water heaters are 100% efficient since there is no wasted heat going out the exhaust.

    To work out how much energy it takes to heat the water a particular amount, you use specific heat. For water, it takes 4.186 joules to heat one gram (one mL) of water one degree C. That works out to 15,844 joules to heat one gallon one degree C. A joule is a watt second, so 15,844 joules is about 4.401 watt hours to heat that gallon of water one degree C, or 2.455 watt hours per degree F. Note that those numbers are WATT hours, not KILOwatt hours.

    If you want to go from 70 degree F water to 120 degree F water, that would take 122.25 watt hours per gallon. You’d need to multiply that out by the size of the water heater, so a little over 6.1 kWh (kilowatt hours) for a 50 gallon water heater. You can scale those values to determine how much more energy it takes to keep water at a higher temperature once you have a baseline. To calculate it out from scratch as an absolute number you need to know energy loss through the water heaters insulation which isn’t typically specified, but a relative difference is probably all you need.


    1. Expert Member
      Dana Dorsett | | #3

      >"To calculate it out from scratch as an absolute number you need to know energy loss through the water heaters insulation which isn’t typically specified..."

      Standby losses typically ARE factored in for units sold in the US by the UEF number, derived under a standard set of test conditions that include room temp, water temp, and water volume use per day (all of which may differ from yours), and that number also factors in the raw combustion efficiency of fossil burners (since there is no way to factor it out in the test.)

      >"For efficiency, divide the energy consumption by the efficiency... ''

      What is NOT specified is the raw combustion efficiency of most fossil-burners, so there's no way to do that without digging deeper. With tankless water heaters the combustion efficiency even varies a bit with flow, firing rate, and incoming water temperature- it's not a single number- it's a range.

      Typical center-flue atmospheric drafted tank type water heaters operate at about an 80% (+/- 2%) combustion efficiency, but that's not found in the sales literature or manuals.

      For most purposes using the UEF is close enough to an apples-to-apples comparison.

      >"Electric water heaters are 100% efficient since there is no wasted heat going out the exhaust"

      Electric heat pump water heaters are more than 100% efficient when viewed solely from the perspective of the water heating efficiency, ignoring the impacts on heating & cooling energy use for the building.

      In most markets propane is substantially more expensive than natural gas, and often more expensive than resistance electric heat. In most markets heat pump water heaters have a lower operating cost than propane (and definitely lower than resistance electric), but can be comparable to natural gas, depending on rates, of course.

      1. Expert Member
        Zephyr7 | | #7

        Thanks for that info Dana, I’ll have to check out the UEF number. I was not aware that was commonly being specified for residential stuff.

        I’d only intended to include electric resistance water heaters since heat pumps are quite a bit different than resistance or fuel gas units. The same goes for tankless water heaters.

        Propane for at least the last few years seems to be priced around the same as gasoline in per-gallon terms. By actual unit of energy (BTUs), I think propane may actually cost MORE than gasoline now, although I haven’t run the numbers to be sure.


        1. Expert Member
          Dana Dorsett | | #8

          LPG runs about 91,600 BTU/gallon, gasoline about 112,000 BTU/gallon, so at the same price per gallon gasoline is cheaper per source-fuel BTU.

          The difference between the US DOE's prior EF standard for rating water heater efficiency and the new UEF efficiency standard is primarily about test parameters. It includes factors not previously used in the EF, such as first-hour gallons and capacity in an attempt to make better apples-to-apples comparisons in real-world residential applicatations.

    2. Jim Brill | | #4

      that’s fantastic; really appreciate that you’ve taken the time to explain it so well.

    3. Jim Brill | | #6

      thanks Bill!!

  2. GBA Editor
    1. Jim Brill | | #5

      bingo! thanks so much for this. it’s going to be a huge time saver as we plow through the choices.

  3. Jon R | | #9

    You might as well add low flow shower heads (1 GPM is possible) and possibly drain water heat recovery into the analysis.

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