Types of Solar Panels - Solar panels are the heart of your system and for grid tied applications are either multi or single crystalline panels.  Amorphous panels are more expensive and used mostly on metal roofs.  Their low relative output means much more roof space is needed. 

    A.  Crystalline -  Either single crystal or multi-crystalline.  Both are very efficient and come standard with 20 to 25 year warranties.  You can identify single crystal panels by their solid blue, deep blue, gray colored cells.  Multi-crystalline are sparkling pieces that are very beautiful to look at.  Most off the grid tied homes use crystalline panels.  

    B.  Amorphous is made by blowing silicon gas onto an aluminum substrate.  6-8% efficient. 

2.  Voltage.  For grid tied systems we series (see education section) all the solar panels.   This means we are working with high and very dangerous DC voltage.  Experienced installers only please.

3.  Understanding Solar Panel Labels - All of the numbers needed to understand what solar panel you have can be found on a label on the back of the solar panel.   Generally, we want to know the wattage, voltage and amps of a solar panel.  Unlike voltage sources such as batteries which produce current at a relatively constant voltage, solar panels may produce current (amps) over a wide range of voltages or electrical pressure. 

The maximum output is where the maximum voltage meets maximum current (amps).  We can get the wattage by multiplying the maximum output voltage (Vmp) times the maximum current (Imp).   For example, a 16.9 volt panel at 7.1 amps is equal to about 120 watts output.

Output Characteristics of a Solar Module
Isc Short Circuit Current
Voc Voltage Open Circuit
Vmp Voltage Maximum Point
Imp Current Maximum Point
MPT Maximum Power Point.  Where Vmp and Imp meet

 Voc - When we troubleshoot a panel that a customer says is not putting out any energy we do an Voltage Open Circuit test.  Basically we take the solar panel, without being hooked up to the controller or batteries.  This is called open circuit because no current (amps) are flowing.  They cannot flow because they are not hooked up to anything so there is not a complete circuit.  It is like a switch that is turned off or without the contacts on the switch touching, OPEN.  We then put our multi-meter on the DC volts setting and red multi-meter lead on the positive and the negative or black lead on the panels negative and face the solar panel into the sun.  We should get an Open Circuit voltage reading of about 20+ volts.  

4.  Factors that Affect Panel Performance - The panel's output is directly proportional by the intensity of the sunlight falling on it.  More intense sunlight the greater the output.  A cool crisp clear day is best for solar output.  In fact, being at higher altitudes with less atmosphere can cause the solar panel to exceed its nameplate ... and space is heaven for a solar panel.  Bring in cloudy weather with half as much sunshine and the panel will put out half of its nameplate.  

  1.        Insolation is the term used to describe the amount of solar radiation.  When solar panels are rated they are "flash tested" in a chamber and the flash is set to be 1,000 watts per square meter (Watts/m2) of brightness.  This is another way to say a clear bright crisp sunshiny day.  Output ratings for the panels label, such as watts,  are then collected based on this flash of light. We have a light meter, called the "The Daystar Meter" which has a solar cell built into the top of it and a needle that points to a scale that goes from 0 to 1,200 Watts/m2.  Today, January 2, 2001 at 12:16 p.m. is an overcast day and when pointed at the sun it reads 200 Watts/m2, this tells us that a solar panel will put out 20% of its rated power.  So a 100 watt panel will put out about 20 watts per hour in this cloudy condition.   During a day the sun will be at various intensities so we need get an average to help us figure out how much energy a panel will put out.  While a panel might be putting out only 20% of its rated power it has so many hours per day to do this.  We call the energy received during TOTAL daylight hours the PEAK SUN HOURS.  Peak sun hours is also defined at the number of hours per day when the solar insolation equals 1,000 Watts/m2.   Luckily we have Solar Data from around the U.S. and world.   The data is compiled for the National Solar Radiation Data Base, our tax dollars at work, which is hourly measured solar radiation data collected by the National Weather Service from 1961 to 1990.  There are 239 sites.  The solar radiation data is displayed as monthly and yearly averages, expressed as kWh/m2/day, which is convenient for us so we can get a monthly per day average.  For example, Redmond, OR (16 miles north of Bend) says that in August (solar panel tilted at is optimum angle) we can count on an average of 7.1 hours of pure (read 1000 Watts/m2) sunshine a day to hit our solar panel.  This would mean that a 100 watt solar panel can be counted on to put out 710 watts of power per day (100 watts x 7.1 peak sun hours).  Interestingly if we look at December around the high desert of Oregon we can count on only 2.9 peak sun hours per day.

  2.     Shading:  Even partial shading of crystalline solar panels will result in dramatic reduction of solar panel output.  One completely shaded cell can reduce a solar panel's output by as much as 75%.  Three cells shaded can decrease 93% of the panel's output.  Call for information on amorphous panels.

  3.     Orientation:  A compass is off from TRUE south, where your solar panels should be aimed.  This is because a compass aligns with the earth's magnetic field, which is not aligned with the earth's rotational axis.  We are basically tilted from the sun.  The deviation from magnetic south to TRUE south must be compensated for when setting your solar panels or your house towards the south.  For example, here in Bend, Oregon we must compensate 15 degrees east of south.  We just move the compass 15 degrees (there are markings on all compasses) to the east to find our TRUE south mark.

  4.     Tilt Angle:   If all we have is a fixed solar panel rack, for example flat and bolted to a roof,  then to optimize year round solar performance, we tilt at the LATITUDE of the area.  Latitude is the distance north or south of the earth's equator.  The earth is round so we want to get our solar panel perpendicular or directly into the sun.  In Bend, Oregon we are 44 degrees so we tilt our panels to 44 degrees.  We can do even better if we use seasonally adjustable racks.  With tilt racks, in the summer we use 44 degrees minus 15 degrees in Bend.  We subtract or make the panels flatter to the sky because the sun is higher in the sky.  Remember our goal is to get the solar panels facing the sun.  In the winter we must get the panels at a steeper angle, again to face the sun so we use 44 degrees plus 15 degrees in Bend.   

  5.     Temperature:  What we mean by this is cell temperature.  This, of course, is made up of air temperature, the color of the cell, air movement, etc.  The bottom line is that as the cell heats up the voltage or pressure becomes sluggish or falls off.  Unlike solar water heating where the hotter the temperature the hotter the water with solar electricity higher temperatures are detrimental to solar output.  Heat for solar electricity may be thought of an an electrical resistance to the flow of the electrons or electricity.  

  6.     Load Resistance:  The output of a solar panel is affected by the voltage or the pressure it is working at.  Run the panel at too high a voltage and its output decreases.  Efficiency also decreases at voltages less than the modules ideal.  

5. Real World Considerations - Every manufacturer tests their modules under something called Standard Test Conditions (STC).  These are a set of rules they all follow.  While these rules allow consumers and solar designers to compare panels apples to apples they are not what the solar panel sees in the real world.

To look at how a panel is actually affected in the real world we must look at the following:

Environmental Condition Losses
Module Production Tolerance 5%
Temperature 11%
Dirt and Dust 2%
Wiring losses 2%
Inverter losses 10%
Total De-Rating Factor 30%

6. PTC Rated Solar Panels - PTC stands for  "PV USA Test Conditions", I have also read they mean Practical Test Conditions.  Either way they are way to rate panels relative to a more real life (out of laboratory) condition.

STC Versus PTC Ratings
Module STC PTC Watt Decrease % Change
BP 160, 3160 160 142.1 17.9 11.2
Kyocera 158, 158G 158 139.7 18.3 11.6
Sharp 165, NE-165U1 165 144.8 20.2 12.2
Sharp 185, NT-185U1 185 163.3 21.7 11.7
Shell 150, SP-150PC 150 134.9 15.1 10.1


7.  Solar Array - An array is a group of modules wired together to produce the desired voltage and current.  

8.  Our Most Common Solar Panels

Nominal Volts

Size Weight Type Manufacturer
Product Photo 160 24 volts 31.1" x 62.7"  call Poly Crystal BP Solar
KC-167G 158G  24  50.8" x 39" 35.3 Poly Cristal Kyocera


Manufacturer/Model Watt Width Length Sq. in Effic. Cells Vmp Voc
  BP 160/BP3160 160 31.1 62.7 1950 8.21% Multi 35.4 44.2
  Sharp 165, NE-Q5E2U 165 32.52 62.01 2017 8.18% Multi 34.6 43.0
  Sharp 185, NT-S5E1U 185 32.52 62.01 2017 9.17% Mono 36.2 44.9
  Kyocera, KY167G 167 39 50.8 1981 8.43% Multi 23.2 28.9
  Kyocera, KY158G 158 39 50.8 1981 7.97% Multi 23.2 28.9
Shell/Siemens SP-150 150 32.05 63.76 2044 7.34% Mono 34.0 43.4