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How much energy to heat 100 liters of water?

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  • How much energy to heat 100 liters of water?

    I apologize in advance if the topic is too basic. If administrators think that it should be deleted, I would understand.


    There is something that has been confusing me for a while, and it is related with "types" of energies when it comes to heating water.

    Here is an example:
    volume of water to heat: 100 liters (26.4 gallons)
    required water temp = 45°C (113°F)
    mains water temp = 10°C (50°F)

    So to heat 100 liters to upper temperature of 35°C, with an electric heater:

    100 liters * (45-10) = 3500 kcal

    1 kcal is equal to 1/860 kWh, so:

    3500kcal / 860 ≈ 4 kWh (13.6 kBtu/h)

    But this is an electric energy needed to heat upper 100 liters by 35°C.


    What If I wanted to heat the same amount of water with gas? How would this example look like then?
    And what would be the result then? Thermal energy in kWh?

    How would I compare the thermal energy and electrical energy, even though they have the same units (kWh)?

    Thank you for the reply.

  • #2
    Electric water heating is 100% efficient, all the BTU generated are absorbed by the water being heated.
    With gas there are losses due to venting of the flue gases created.
    So you would have to add an additional percentage for the losses up the flue.
    So if you have a gas water heater that is 80% efficient you would have to add 20% to the amount of BTU'S required to heat the same amount of water to the same temperature.

    Conversely if using a heat pump water heater with a COP of 3 only 1/3 of the amount of KWH of electricity would be required to heat the same amount of water to the same temp.
    The input BTU'S to the water would still remain the same just less electrical energy consumed.

    Comment


    • #3
      Thank you for the reply LucMan.
      But I still do not understand it.

      If efficiency factor (EF) of electricity is 100% (or 1.0), then in order to calculate the amount of electricity needed to heat 100 liters of water by 35°C, we have:

      a) electricity:
      volume * (deliveryT-mainsT) / EF
      100 * (45-10) / 1.0 = 3500 kcal
      3500kcal / 860 ≈ 4 kWh (13.6 kBtu/h)


      Let's take the efficiency factor for natural gas to be 80% (0.8) as you suggested.
      In that case the amount of energy needed to heat 100 liters of water by 35°C would be:

      b) natural gas
      volume * (deliveryT-mainsT) / EF
      100 * (45-10) / 0.8 = 4375 kcal
      4375kcal / 860 ≈ 5 kWh (17 kBtu/h)


      But 5 kWh of what? Not an electric energy, but what? Thermal energy?
      How can I compare 4 kWh of electric energy with 5 kWh of Thermal energy?

      Comment


      • #4
        Just to mention that I am not interested in costs. For example:

        4 kWh of electric energy * 1$/kWh = 4 $
        5 kWh of thermal energy (?) * 0.5$/kWh = 2.5 $

        This is not what I am interested in.
        I just want to know how to compare 4 kWh of electric energy with 5 kWh of these other type of energy (thermal?).

        P.S.

        I can not edit my reply for some reason (the edit form is empty both in simple and advanced window), so I apologize for posting a new reply.

        Comment


        • #5
          They would be equal as the measurement comparison in both cases uses kwh as the measurement.

          The only way to compare is to use BTU's.
          One BTU is the energy required to raise one pound of water 1 degree farenheit
          So if you had 100 pounds of water that you needed to raise the temperature 50 degrees it would take 5 KWH of resistance heat to do that
          A kwh = 3413BTU in an electrical resistance heater

          Similar to that would be using gas or a heat pump but efficiencies here come into play

          Same water with a gas water heater at an 80% efficiency would take 5000 / .8 = 6250BTU

          Same water with a heat pump at a COP of 2 would take 2.5KWH
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          • #6
            Originally posted by Naptown View Post
            They would be equal as the measurement comparison in both cases uses kwh as the measurement.

            The only way to compare is to use BTU's.
            One BTU is the energy required to raise one pound of water 1 degree farenheit
            So if you had 100 pounds of water that you needed to raise the temperature 50 degrees it would take 5 KWH of resistance heat to do that
            A kwh = 3413BTU in an electrical resistance heater

            Similar to that would be using gas or a heat pump but efficiencies here come into play

            Same water with a gas water heater at an 80% efficiency would take 5000 / .8 = 6250BTU

            Same water with a heat pump at a COP of 2 would take 2.5KWH
            So I am confused with your math and the OP's math.

            Did you mean that 5000 kwh / .8 = 6250 kwh or BTU?

            Next if it takes 5kWh to heat 100 pounds of water 50 degrees why did the OP come up with only 4 kWh to heat 26.4 gal or 220 lbs 63 degree F?
            Last edited by SunEagle; 10-13-2015, 11:46 AM. Reason: spelling

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            • #7
              Bernard: energy is energy, whether it's in the form of electricity, CH4, wood, animal fat, peat, or anything else. Electricity when used as resistance heating is nominally 100% efficient. That is, every bit of energy goes to heating the water (and the tank).

              CH4 when burned in, say, a conventional tank type heater may be ~ 70% efficient, meaning that for every 1,000 units of energy liberated in the combustion process, about 700 go to heating the fuel, and the rest mostly goes up the stack. A lot of wood burning devices may have about the same combustion efficiency as CH4, more or less.

              Get yourself a couple of basic texts: One on thermodynamics and the other on heat transfer. Or, maybe better, a text on HVAC for residences. A few pages in and all will be clear.

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