Nederland Renewable Energy Project

National Renewable Energy Lab (NREL)
Home Power - www.homepower.com/home
DIY Projects - www.builditsolar.com/Projects

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Power versus Energy:
 

From the start, a person should understand the difference between power and energy.  These two terms are mistakenly used interchageably.

Unless a novice educates themselves on these two fundamentals much confusion can arise later.

For example; a solar panel will typically be rated in units of watts.  This is often called the name plate power rating.  So you might see a name plate or sticker on the panel which with other specifications, shows for instance 150 watts.  This is the maximum power that this panel is capable of producing under ideal conditions; i.e. at noon on a summer day at a favorable latitude under a cloudless sky.  And this is only for a given instant in time.

Energy however is the amount of power generated (or used) over a period of time.

Energy = Power multiplied by some period of time or   

      E = power x time

Typically, energy in the U.S. is expressed in units of a thousand (kilo) watt-hours or kilowatthours (abbreviated kWh)

So lets say you leave an oven on that uses 1,000 watts of power for one hour.  That is (1 kilowatt x 1 hour) one kilowatthour of energy that has been consumed, abbreviated 1kWh.  If you were to leave that same oven on for 2 hours then you've just consumed 2kWh worth of energy and your Watthour meter on the outside of your house and electric bill at the end of the month will reflect that.

Now let's say you are fortunate enough to have a stream that runs through your backyard and you have a small hydro-electric generator that supplies energy instead of consuming it.  In this case you have a big stream and a big water turbine/generator that puts out a nice steady 1 kilowatt of power for one hour.  You've just generated instead of consumed 1 kilowatthour (1kWh) worth of energy.

Unfortunately, most renewable energy systems don't provide a steady stream of energy so what would be more telling is to have an idea of what the energy output of a solar array might be over a given year.  This depends on many things such as the weather at your location, your latitude, the angle the array is installed etc.  The energy output (in kWh) for a given year can be approximated and it is a good idea to find out from the company who you contract with or if you plan on a do-it-yourself (DIY) project, to calculate it yourself.  There are all kind of resources on the Internet and plenty of books on the subject.  If there is enough interest within NedREP perhaps we can pull together some sort of 1 or 2 hr. course that covers the basics in more depth.

John Ott
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