Hi Ross.
Don´t know if there is any "convention" regarding use of units
it shouldn´t be needed if people use the unit system correctly.
When somebody says "a micro hydro generator puts out 2.4 kW"
it should mean that the average power is just that.
So if it is running constantly, the energy it generates
would be 24 x 2.4kW = 57.6kWh (kilowatthours) per day.
But unfortunately people tend to confuse kW and kWh and then some...
so if somebody says "a micro hydro generator puts out 2.4 kW per day"
they could be trying to say that the energy produced is really 2.4kWh/day
which means the average power is only 100W (2,4kWh/24h = 0.1kW = 100W).
2.4kWh/day could also be produced by running a 600W generator for 4 hours
each day.
("2.4 kW per day" is incorrect use of units in this example, and I must add
that
Ross did not write that, I just used the phrase because I have seen it
elsewhere).
Some theory:
_Power_ is the amount of work done or energy transferred per unit of time.
For better explanation: http://en.wikipedia.org/wiki/Power_%28physics%29
The fundamental SI unit for power is the watt [W]
( often a more convenient unit is a kilowatt [kW] ).
The fundamental SI unit for time is the second [s]
_Energy_ can be calculated as: energy = power x time.
The fundamental SI unit for energy is the joule [J]
and thus it is possible to write: joule = wattsecond [J = Ws]
One joule (or wattsecond) is a rather small amount of energy
so larger units are often more convenient and therefore
electric energy is generally given as kilowatthours [kWh]
(1kWh = 3,600,000 J or 3,600kJ)
An example:
The produced power of my microhydro generator is now 11kW
that is 11kW at this moment, some other time it could be a bit less or more.
My microhydro generator thus produces 11kW x 24h = 264kWh of energy each day.
(That could also be written quite correctly as 950,400,000 J = 950,4 MJ
(Megajoules)
which in my opinion is just a large number nobody understands :-)
The phrase "kW per day" really can only mean rate of change in the power output
(hovever, as said before it is sometimes used incorrectly in place of
kWh/day) .
For example if there has been no rain for several weeks during summer
or if there is a long time with very cold weather during winter,
my microhydro generator might for some time be decreasing its output by
0.2kW per day.
That is, each day the power output is some 200 watts less than the day before
(after 5 days it would be down to 10kW, after 10 days it would be 9kW,
after 15days 8kW,
but fortunately in my case it never gets any lower than that)
Hope this helps somewhat.
Best regards.
Thor.
At 14:22 27.12.2005, Ross wrote:
> Gentlemen, One small point - when a web site says a micro hydro
> generator puts out 2.4 kW, they mean per day don't they? Not per
> hour... It seems that a lot of these things are rated in watts and
> kilowatts but there are times when the unit of time is unclear. What
> the convention? Thanks, Ross
>
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