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Discussion in 'Home Power and Microgeneration' started by m Ransley, Mar 12, 2005.

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  1. m Ransley

    m Ransley Guest

    How do I compare Kwh costs to Ng. I pay 0.12 Kwh and 1$ per therm. Is
    one therm equal to 100000 btu and 1 kwh hr equal to 450btu? Is my cost
    for electricity nearly 300% that of Ng?.
  2. News

    News Guest

    A BTU is a measure of ENERGY. 1 lb of water raised 1F. No power there at
    all. The POWER is energy by time hence BTU/hr for output.

    You can have an electric motor turning a compressor. The compressors input
    is power in.

    To clarify for you:


    The watt (W) is a unit of Power.

    The kilowatt (kW) is simply 1,000 watts. A one-bar 1 kilowatt electric fire
    or ten 100 watt light bulbs will consume one kilowatt.

    BTU/hr is a unit of Power


    Energy is Power x Time.

    You pay for energy not power. What you have to pay for is the product of
    power and time. This is obvious - the electric fire operating for three
    hours is going to cost three times a much as for one hour. Therefore the
    chargeable electricity 'unit' is the:

    kilowatt-hour (kWh) Which is ENERGY.

    This is by tradition in the world of electricity metering just called a
    'unit'. What you are paying for is energy, rather than power.

    kWh is energy
    Wh is energy

    BTU is energy.


    Although some people think of the watt (a unit of power) as an electrical
    unit, it's not restricted to electricity. Boilers, whether powered by
    natural gas, LPG or oil, and heat emitters (radiators) have power outputs
    quoted in watts or kilowatts. So do car engines nowadays.

    In days gone by in the UK, boilers etc. were rated in British thermal units
    (BTU or formerly BThU) per hour (BThU/hr), which is POWER.

    The BTU is a unit of ENERGY

    The BTU is not power. Hence the division by time (BTU divided by hr
    [BTU/hr]) to get power. People often speak of say, a "60,000 BTU boiler";
    when what they really mean is 60,000 BTU/hr.

    One kWh (energy) is equivalent to 3,412 BTU (energy) Note: One figure has a
    time factor and one does not.

    A 60,000 BTU/hr (power) boiler is rated at approx 17.6 kW (power). Note: The
    time factor figures are reversed for power.

    For the engine, horsepower was used, and:

    One HP is 746W.

    So a 75 kW engine is equivalent to near enough 100 HP.


    Is charged in kWh (energy), just like electricity. There is a difference
    though in that the electricity meter measures kWh directly, whereas the gas
    meter records the volume of gas used in multiples of 100 cubic feet (or in
    cubic metres on newer ones). The calculation to get from volume to energy
    in kWh (energy) is shown on the gas bill.

    The conversion factor is not constant since it involves the calorific value
    of the fuel, which varies from region to region.


    Again, in the past, and still in the USA, gas was charged for by yet another
    energy unit, the Therm. One therm is simply 100,000 BTU (energy),
    equivalent therefore to 29.31 kWh (energy).
  3. Richard W.

    Richard W. Guest

    I was told by the gas company years ago that 1 therm was equal to almost
    1 gallon liquid.
  4. daestrom

    daestrom Guest

    A 'therm' of natural gas is that amount of gas that, when burned cleanly,
    will deliver 100000 btu (about 100 ft^3). So yes, you have that part right.
    One kilo-watt-hour of energy (in any form) is equal to 3413 btu (not the 450
    number you have).

    So if you pay $0.12 for 1 kwh in the form of electricity, that is $0.12 for
    3413 btu of electricity. Scaling that up to a 'therm' would put the price
    at $3.52 per 'therm' of electricity.

    So if a particular energy need can be satisfied by burning NG, it is much
    cheaper source of energy. But it is pretty hard to pump water with NG
    without going through some kind of engine. If you burn NG in an engine to
    pump water, the engine is less than 1/3 efficient. Whereas an electric
    motor used to pump water can be 90% efficient. So in the case of pumping
    water, you would burn about 3 times the natural gas. The marginal cost for
    pumping the water would be about the same in the end.

    But if you just want to heat up water, or heat your home, or cook dinner,
    the inefficiency of a heat-engine doesn't enter into it and NG then is a
    much cheaper way to cook dinner.

  5. Gìmmìe Bob

    Gìmmìe Bob Guest

    Good stuff but some of your terminalogy is off a bit.

    Power is never "consumed" but rather the "Rate of consumption"

    What is a "one-bar"? The only bar I know of is for barometric pressure and
    the units named "bar" for about 15 PSI

  6. News

    News Guest

    Old fashioned electric heaters had "bars" of 1 kW. You could see the size
    by looking at the bars.
  7. ....long rant snipped.

    I fell asleep before figuring out if he even came close to answering the
    question, which really had nothing to do with a misuse of terms for power
    vs. energy, and simply asked if the OP was paying 300% the cost of gas for
    his electricity. In a word, no. But I couldn't say what the difference
    really is...
  8. News

    News Guest

    A rant?
    You should pay attention.
    Do you mean the above was wrong?
    All the OP had to do look at the part that was applicable to him. All
    there. Simple.
  9. When it's relevant.
    I'm sure it wasn't. But why focus on the fact that he (like probably 90% of
    the general population) doesn't know the difference between energy and
    power, instead of just (if you really had to) mentioning that they aren't
    the same and answering the question he actually asked?
    Not so simple at all. I sure didn't see the answer there, and I understood
    your point. All that would happen is he'd see you were more interested in
    flaming than answering and skip to another post - where he'd actually get a
    decent answer.
  10. News

    News Guest

    Flaming? Are you serious?

    All he had to do was look at therm.
  11. Steve Spence

    Steve Spence Guest

  12. News

    News Guest

    100 cu feet = 1 therm? Not quite. It depends on the calorific value of the
    gas. With natural gas it is approx 1000. So, 100 cu feet = 1 therm is
    ballpark. The CV can vary from region to region.
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