Dear Solar Guy,

        When I look in catalogs for solar cells, I see that no matter how
big the cells are the power is always about a half volt per cell.  This
can't be true.   Are they right or are they misrepresenting their products
to confuse people like me.     

R.H. Volta, Electra, TX.

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Dear R.H.,

        Well this is really a problem with words.  Speaking of power in
terms of volts is the problem.   There are three characteristics (measures)
of electric energy:  voltage (potential), amperage (current) and wattage
(power).  The power (watts) is the product of voltage and amperage.  A solar
cell is really very little different from a battery, in that each silicon
solar cell puts out about 1/2 volt, irrespective of size, just as each cell
of a lead acid battery puts out about 1.5 volts, irrespective of size.  It’s
the current (amperage) that increases with PV cell size or battery cell
size.  And as the current increases with cell size, the power (wattage) does
also.  In fact, the current and power increase proportionally (linear) with
cell size (area).

        Different applications require different voltages and wattages to
operate, so how do we get the required volts and watts for a particular
application ?  Well to begin with good silicon solar cells are typically
about 15% efficient and good solar conditions provide about 1000 Watts/m2. A
full sized solar cell is about 10 cm by 10 cm, or 0.01m2. Therefore the
solar energy falling on such a cell would be about 10 Watts and at 15%
efficiency the output would be about 1.5 Watts.   Now let’s say the
application we have requires about 14 volts and 200 Watts.  To get the 14
volts one needs to wire about 28 cells in series (28x0.5), and this would
result in a power output of about 1.5x28 = 42 Watts.   Thus, to get the
required 200 Watts one would need to wire 5 of these series strings in
parallel, i.e. 5x42 Watts = 210 Watts.

        So back to your real question.  Yes, the quoted 1/2 volt for all the
different cell areas is correct.  But one needs to look also at the amperage
and wattage specified for the cells.  The quoted current and power values
should be proportional to cell area.      

The Solar Guy.