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Posted by Nick Pine on October 20, 2003, 11:07 am
 


Please learn the difference between power and energy.

Nick


Posted by News on October 20, 2003, 11:22 pm
 


Oops: kW/h  Did Power and energy at skule.


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Posted by Duane C. Johnson on October 20, 2003, 11:42 pm
 Hi News;

News wrote:


Still wrong. Should be kW * hour or kWh.

Duane

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Posted by News on October 21, 2003, 10:24 am
 

Correct Energy is Power x Time, so no "/".  I should read my own posts.

I simplified the post:

POWER



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



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.



BOILERS & CAR ENGINES



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.



Gas



That just leaves gas, which nowadays in the UK 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.



Therm



Again, in the past, 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).



I think that is right.


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Posted by News on October 21, 2003, 10:30 am
 re-post as I don't know why all the spaces occured.


Correct Energy is Power x Time, so no "/".  I should read my own posts.

I simplified the post:


POWER

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

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.


BOILERS & CAR ENGINES

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.


GAS

That just leaves gas, which nowadays in the UK 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.


THERM

Again, in the past, 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).


I think that is right.



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