Dear Solar Guy,

        Recently I heard someone talking about "sun-hours".  I don't
understand what this means because they were talking about it as energy ?
Could you explain. 

 P.M. in Meridian.

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Dear P.M.,

        Yes, such terms are confusing at first, but once one understands the
meaning, its an easy way to say something with one word that otherwise would
take a few sentences to explain.  So here are the sentences !

        On a very clear sunny day the solar radiation level will be about
1000 Watts per square meter.  This will vary with location, for example,
clear conditions in Houston exhibiting somewhat less than this (due to
humidity) and clear conditions in El Paso being somewhat higher due to low
humidity and higher elevation, but the 1000 value is somewhat typical clear
conditions.  So, during one hour of clear sky conditions the amount of solar
energy received is about 1000 Watt-hours of energy (1.0 kW-hr, "one
sun-hour").  During one hour of somewhat overcast conditions exhibiting
solar radiation of 500 Watts per square meter, there would be 500 Watt-hours
of energy (0.5 kW-hr or "1/2 of a sun-hour").

        Solar radiation has been measured at numerous locations and this
data has been summed, usually over each month for several years to provided
average daily values by month.  A typical value may be 5 or 6 Sun-Hours (per
day).  For El Paso the number of sun-hours for December, March and June are
about 3.3, 6.0 and 8.5, respectively, while for Houston for the same months
the levels are 2.3, 4.1, and 6.0, respectively.  These values are for
horizontal surfaces, and solar collectors are normally tilted toward the
south to enhance solar collection.  For a fixed tilt to the south of about
40 degrees (the local latitude plus about 10 degrees) the respective
sun-hours for El Paso are about 5.1, 7.2 and 6.2, while for Houston they are
about 3.3, 4.5, and 4.7.

        The use of sun-hours is quite convenient.  For example, in June a
15% efficient solar (photovoltaic) panel of one square meter tilted at 40
degrees to the south would have a daily output of about 0.93 kW-hrs in El
Paso and about 0.7 kW-hrs in Houston.   

The Solar Guy.