JOULE is the quantity of energy in SI units
WATT is the quantity of energy used or supplied per unit time (also called power) in SI units

We often encounter power and energy in our daily lives: cars, refrigerators and motors are often priced by horsepower (power); light bulbs are often priced by their output watts (power); natural gas is sold by its combustible energy content namely therms (energy) and of course the supply-demand situation of the gas; electricity by kilowatt-hours (energy); furnaces are priced by kilowatts (power) and their temperature capability in Kelvin (K); and air conditioners by tons or BTU's per hour (power).  Power multiplied by the time of use of that power gives you energy (SI units are Joules).

Power is oomph! Power is the amount of energy that is converted in a unit of time.  Generally speaking, power influences the rate at which you can do something, e.g. higher power can make an automobile accelerate faster, make a reaction go faster, produce more, and so on. 

Expect to pay more when demanding higher power! or expect to earn more when delivering with a high-power high-productivity device.   When you purchase a energy conversion device (say electrical energy to heat with a stove-top), the device will often be rated by its "power".  This enables you to calculate how much energy you will use (and pay for) by simply multiplying the rated power by the hours used.  When using electric power the commonly used units of energy are kilowatt-hours (which is an unit for energy).   Each electric unit that you pay for, is normally one kilowatt-hour (also sometimes called one unit).  When you pay for units, your US electric utility bill contains price the for the units used during normal usage, and the price for units used in a "demand penalty" condition (peak hour use).  You should always carefully watch this penalty number, and contact your electric utility provider to find ways for reducing the "penalty" charge. The calculator(s) below will help you to convert energy from one unit to another and if you know the time over which the energy is delivered, it will allow you to calculate the power ((energy-used)/time).  You can do more with the calculator.  Say you wanted to get a feel for the amount of energy used in terms of equivalent amount of gasoline burnt, or in terms of how much ice the same energy would melt; you can do that also.  Have fun!   Remember Energy is always conserved (such a conservation process is known as the first law of thermodynamics).   Entropy (a measure of the quality of the energy) is not conserved.  When you posses energy with the lowest entropy, you posses the most useful form of energy for conversion to work.  However the value of energy to you depends on what you need it for, and so it is wrong to think that disordered form of energy is not as good as ordered form of energy.  For example, if you are cold and you need heat, your objective is not for an ordered form of energy.  On the other hand when you have an automobile, it is better that you can order the energy of your motion.  When you convert energy from one form to another, you create entropy (almost always) - and when you do that, the process is called irreversible (because you generate entropy and degrade the quality of energy by using it).   In an ideal situation, you may have a process that does not cause any irreversibility, and consequently no entropy is generated.  Electrochemical processes, and processes where energy is converted to other forms of energy without any generation of disorder, are close to such ideal (reversible) processes.  But almost no processes is really reversible.   In thermodynamic terms, any gradient (i.e. the difference of a thermodynamic property with distance) in potential, including gradients in temperature, composition, height or charge can cause energy to flow spontaneously with a corresponding generation of entropy.   Whenever entropy is generated, the process is called irreversible.  Note that the word irreversible also has the connotations of the direction of time.  Both time and energy flow spontaneously only in a certain direction.

Whenever enthalpy is produced (say by combustion) and the energy now at a higher temperature is transferred spontaneously to a lower temperature object (e.g. on account of a temperature gradient) - the process is irreversible because entropy is generated.  This is why using the best energy efficient device is important - you end up degrading or wasting the least amount of energy.  MHI devices are energy efficient. 

Ask for a comparison of MHI products with others when you contact us.  Please also visit our VISION page.

Energy (used or delivered)	Time (This is the time over which the energy was used or delivered)
Joules	Seconds Minutes Hours Days Months Years	Watts (UNIT OF POWER)
Watt Hours		Kilowatts (UNIT OF POWER)
Kilowatt Hours		Horsepower (UNIT OF POWER)
BTU		BTU/hr (UNIT OF POWER)
Therms		tons (ODD LOOKING UNIT BUT USED FOR POWER ALSO)
This is the average power in full sunlight falling on a square foot surface, directed toward the sun, and collecting hours each day.

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The following table illustrates how various energy storage systems would store this amount of energy:

Chemical	gallons of gasoline ( pounds)
	pounds of natural gas ( cubic feet at psi),
	pounds of hydrogen Gas ( cubic feet at psi),
	a lead-acid battery weighing pounds
Mechanical (Kinetic)	a high-tech flywheel weighing about pounds
	a pound car traveling at miles per hour
Thermal	melting a block of ice weighing pounds
	heating a gallon tank of water by degrees F.
	evaporating pounds (about gallons) of water
	heating a pound hunk of concrete by degrees.

Note that unless a phase change is indicated, the table below does not consider latent  heat.

To convert from the unit in the first column to the unit in the second, multiply by the conversion factor in the third column. Example 1 Btu/hr = (1 x 0.0003929) Horsepower = 0.0003929 Horsepower

To Convert	To	Multiply By
Btu/hr	Horsepower	0.0003929
Btu/hr	Gram-cal/sec	0.07
Btu/hr	Foot-pounds/sec	0.2162
Btu/hr	Watts	0.2931
Btu/min	Kilowatts	0.01757
Btu/min	Horsepower	0.02356
Btu/min	Foot-lbs/sec	12.96
Btu/min	Watts	17.57
Foot-pounds/min	Kilowatts	0.0000226
Foot-pounds/min	Horsepower	0.0000303
Foot-pounds/min	Kg-calories/min	0.000324
Foot-pounds/min	Btu/min	0.001286
Foot-pounds/min	Foot-pounds/sec	0.01667
Foot-pounds/sec	Horsepower	0.000818
Foot-pounds/sec	Kilowatts	0.001356
Foot-pounds/sec	Btu/min	0.07717
Foot-pounds/sec	Kg-calories/min	1.01945
Foot-pounds/sec	Btu/hr	4.6263
Horsepower	Kilowatts	0.7457
Horsepower	Kg-calories/min	10.68
Horsepower	Btu/min	42.44
Horsepower	Foot-lbs/sec	550
Horsepower	Watts	745.7
Horsepower	Foot-lbs/min	33000
Horsepower (boiler)	Kilowatts	9.803
Horsepower (boiler)	Btu/hr	33479
Horsepower (metric)	Horsepower	0.9863
Kilowatts	Horsepower	1.341
Kilowatts	Kg-calories/min	14.34
Kilowatts	Btu/min	56.92
Kilowatts	Foot-lbs/sec	737.6
Kilowatts	Watts	1000
Kilowatts	Foot-lbs/min	44260
Watt (international)	Watt (absolute)	1.0002
Watts	Kilowatts	0.001
Watts	Horsepower	0.001341
Watts	Horsepower (metric)	0.00136
Watts	Kg-calories/min	0.01433
Watts	Btu/min	0.05688
Watts	Foot-lbs/sec	0.7378
Watts	Btu/hr	3.4129
Watts	Foot-lbs/min	44.27
Watts	Erg/sec	107
Watts (absolute)	Btu (mean)/min	0.056884
Watts (absolute)	Joules/sec	1

Energy Efficiency (First Law and Second Law Efficiencies). 

The first law states that energy is always conserved (but of course may be converted from one form to other form by heat, mass flow or work interactions).  In this context, efficiency indicates the efficiency of your conversion to the form of energy you require from the form of energy you start with.  For example you may wish to tap energy from an electric power socket and wish to convert it to the energy contained in a hot gas that you may need - with an Airtorch™.  The efficiency of this process will be defined by the power delivered by the hot gas when compared to the power from the electric source.  First law efficiencies depend on the objective of the energy conversion.

The second law can be stated in many forms e.g., the Kelvin-Planck Statement or the Clausius Statement, and has to do with the impossibility of converting disordered forms of energy completely into ordered forms of energy.   In this context, efficiency is the measure how close your process can be to a reversible process (see above for the meaning of reversible).   Generally speaking, the higher the temperature, the higher the efficiency of the conversion of disordered energy into ordered forms of energy.  There is no significant penalty going the other way i.e. changing an ordered form of energy to a disordered form.   See examples below the Quiz for examples for each type of conversion.

Quiz Q1 :  What kind of energy is converted by our digestive system.  How efficient is our digestive system compared to other living beings?  Answer.

Quiz Q2:  Is a steady state (time invariant) energy conversion process reversible?  Answer.

Rule: There is no process that can violate the first or second law of thermodynamics.

Here are some typical efficiencies encountered in everyday processes. 

Burning fossil fuel to get useable heat - about 85% (running a gas-fired water heater, or making steam to power a turbine...some heat goes up the smokestack).   Here we are speaking about first law efficiency. Burning fossil fuel to get electricity - about 33% (active world-wide research area to improve several types of efficiencies).  The big drop in efficiency comes from the second law limitation which states that not all heat (a disordered form of energy) can be converted to work (an ordered form of energy).   Here we are seaking about the second law efficiency. Sunlight to electricity in a PV (photovoltaic) cell - about 10% (active world-wide research area to improve conversion efficiency some polymer photovoltaic's are now reporting over 35%).  Can you tell if this process has a second law limitation? Hint- think if any heat is produced. Storing electrical energy in a battery (charging it) and pulling it back out -about 90% . Converting electrical energy into mechanical energy with an electric motor - about 85%. Fuel Cell about 60% or lower.  Theoretically this can be even as high as about 92%.   Electrochemical processes are limited in efficiency by how much enthalpy of the reaction can be converted to free energy (the entropy generation because of the of mixing of constituents which limits the conversion efficiency). IC engine about 30%.  Increases with temperature of the heat source. Wind energy about 59.5%..  Wind energy mills convert the kinetic energy of the wind to rotational shaft energy which can be used to create electrical energy. The limitaion of wind energy conversion limitation comes from having to cater for residual kinetic energy in the wind past the windmill.

All calculations above and in the conversion tables are approximate only, and should not be used for detailed engineering calculation requiring a high degree of precision.  Please check all values against more established calculators.  None of the values above are guaranteed to be accurate.

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