There's a great answer about heating water with electricity, where anyone can easily get an idea how much would it cost to heat water just by applying one's price for kWh.

I'm looking for the same calculation, but for gas.

EDIT: My gas is billed in 15kg gas cylinders, but feel free to allow for liters of gas.

I also think the efficiency of gas furnaces needs to be taken into account. Whereas the electric heater converts close to 100% of the heat into heating the water just by the virtue of having the rods surrounded by water, I am seriously afraid that is not the case with gas furnaces, that sip a significant amount of heat into the surrounding air.

  • Welcome to sustainability.SE! Thanks for the question. Can you tell us where you're located and/or what units your gas is billed in (cubic feet, therms, BTUs, etc)?
    – LShaver
    Commented Feb 18, 2017 at 4:01
  • I've updated that linked answer so that it works with gas or electric.
    – 410 gone
    Commented Feb 18, 2017 at 6:43
  • You only need to look up the caloric content of the gas you are using and divide the amount of energy calculated in that other question by it. I'm tempted to flag this as a 'duplicate of'.
    – user2451
    Commented Feb 18, 2017 at 16:57

1 Answer 1


The amount of energy needed

From EnergyNumber's answer to the electricity version of this question:

...the energy needed is 180,000 x 45 x 4.2 = 34 MJ

1kWh = 1000W x 3600s = 3.6 MJ

Hence, the energy needed is 34/3.6 = 9.5 kWh.

Types of gas and units of measure

Two types of gas are typically used for heating and cooking purposes in homes:

  • LPG or liquefied petroleum gas, which is typically a mixture of propane and butane
    • Stored compressed in bottles or tanks, typically sold by the kilogram (kg) or liter (L)
  • Natural gas, which is mostly methane with some mixture of ethane
    • Often piped directly to homes, and sold in units of hundred cubic feet (ccf) or the nearly equivalent therm, defined as the quantity of gas containing 100,000 BTU of heat content


FUEL            UNIT        MJ / UNIT   QUANTITY FOR 34 MJ
----            ----        ---------   ------------------
Natural gas     therm       105.5       0.322
                ccf         105.5       0.322
LPG             kg          46.0        0.739
                L           25.8        1.318

Accounting for heating efficiency

Again quoting from EnergyNumbers:

If you want to account for the heating efficiency, just divide by it. So if your heater is 100% efficient, you divide by 1 - which is why we ignore it in the above calculation. If your heater is 95% efficient, you divide by 0.95, to get 10 kWh. This calculation works whatever the type of heater it is.


  1. Wikipedia: Liquefied petroleum gas
  2. Wikipedia: Therm
  3. The Engineering Toolbox: Gross and net heating values for some common gases
  • Thanks. I've updated the question to emphasize that I'm looking for SI units, and to account for furnace inefficiency.
    – MaratC
    Commented Feb 19, 2017 at 6:03
  • @MaratC updated to include SI values.
    – LShaver
    Commented Feb 19, 2017 at 19:21
  • It's perhaps worth noting that in some places with mains gas (example: UK) it's actually billed in units of kWh.
    – Flyto
    Commented Feb 20, 2017 at 9:19

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