Conversion Calculator for Units of
POWER
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Btu/sec
milliwatts [mW]
Btu/min
watts [W]
Btu/hour
kilowatts [kW]
therms/hour
megawatts [MW}
calories/sec
gigawatts [GW]
calories/min
terawatts [TW]
kilocalories/sec
joules/sec
kilocalories/min
kilojoules/min
kilocalories/hour
megajoules/hour
horsepower(electric)
kgf m/sec
horsepower (metric)
foot lbf/sec
Care needed with definitions of 'Btu'; 'therm'; 'calories'. (See background notes)
'kgf m' is kilogram-force metre'lbf' is foot pound-force'Btu' is British thermal unit.
Unit shown in red is the SI 'preferred' unit.
Very large and very small numbers appear in e-Format and unspaced.
Unvalued zeros on all numbers have been suppressed.
Caution
NO guarantee as to the accuracy of these values is given.
And they should be checked against some other source.
POWER
Power is a measure of the rate of doing work (or using energy) in relation to time. The SI unit of power is the watt [symbol W] which is a rate of 1 joule per second.
What does it mean?
Imagine a bucket of water being raised from a well which is 40 metres deep. The water weighs 1 kg. This is equivalent to a force of about 9.8 newtons (keep it simple, call it 10). The work that has to be done (or the energy needed) to raise that water through that height is given by
Work = Force × Distance moved
In this case that is 10 newtons × 40 metres = 400 joules
So, 400 joules of energy are needed, and nothing can alter that.
What can be altered, is the time taken to do that amount of work, or deliver that amount of energy. It could be done in 1 second (at least in theory!) or 1 hour or 1 day or any other unit of time.
Done in 1 second it needs a power of 400 watts (= 400 joules/sec).
Done in 1 hour it needs a power of about 0.111 watts (= 400J/3600 sec)
Notice: Power used × Time taken must equal 400 joules.
Putting it another way.
A litre of petrol contains about 33,000 kilojoules of energy, and a car travelling at 50 kilometres per hour might use 1 litre of petrol in about 10 minutes.
This is a power of 55 kW (or about 70 horsepower). Of course, due to the inefficiencies of the system only about 25% of that is actually delivered to the wheels. (Unfortunately!)
Whereas if 1 litre of petrol were burnt in just 1 second, as in an explosion, then it would produce 33,000 kW of power (over 44,000 horsepower).
Incidentally, it would be almost 100% efficient!
The watt is named after James Watt, a Scottish engineer (1736-1819). Ironically, he himself devised his own unit of power. In 1783 he found a 'strong' horse could raise a mass of 150 pounds through a height of 4 feet in 1 second. From this he defined 1 horsepower as a work rate of 550 ft-lbf per second.
Caution
Some of the units named here have more than one definition!
First there are: calories (and kilocalories), British thermal units (and therms).
Generally, the values used in this calculator will serve for most purposes but, for extremely accurate work, care needs to be taken.