208.530.2393

Point zero energy logo

Solar panels

(page 4 of 7)

Now that we have a basic understanding of electricity we will go over the components of a solar generator.  Basically a solar Generator is a portable mini off-grid system.  The first component is the solar panel.

Solar Panels:

The solar panels convert light (not heat) into power (voltage and current).  This is the heart of the solar generator.  So we are going to cover them in detail.

Types of Solar Panels:

1- Rooftop Solar Panels
These are what are usually used in grid tie or off grid systems.   They are inexpensive, durable, and usually very efficient.  However for portable systems they are very bulky, heavy, and not very portable.
solar panels

2- Foldable Solar Panels
These are basically a smaller, portable version of roof top panels . They are durable and efficient and can be folded for storage and stand on their own with a built-in metal stand. They are still heavy, and take can take a lot of space to store (if you have more than a 200 watts or so).  They are also more expensive per-watt than the roof top panels.  

Many times these panels are setup with a cheap pwm charge controller and aligator clips to charge a battery.  These types of panels are designed to charge a 12V battery directly, and won’t work with a solar generator unless you remove the charge controller and use appropriate connectors on the panel. It is best to find a foldable panel that is setup with just mc4 connectors, and no charge controller.

foldable solar panel

3- Light flexible solar panels
These are by far the most portable solar panel. They are also very thin, making them great for storing in small spaces. However, they have been reported to be less durable, due to being flexed over and over again (usually when mounted on the roof of an rv, or such), or due to cheap panels that are not UV resistant.  If these panels are used, make sure they use a good UV rated surface (ETFE).

flexible solar panels

Solar Panel Specs:

It is important to understand what the specs on the solar panels mean.   This is especially important if you are putting together a system with your own array of solar panels.

Pmax:  This is the maximum power the solar panel is rated for.  If it is a 100 watt solar panel this will be 100 watts.  It is important to note that the panel will only produce this power under ideal conditions with direct sun, and a low temperature (solar panels actually produce less power when the are hot).  It is also important to know that some manufacturers don’t get this number very accurate.  This is usually with brands that are not very well known.  Sometimes they actually produce more than they say, but most of the time they don’t produce the rated Pmax power.

Efficiency:   Many people think a higher efficincy solar panel will charge your batteries faster than a lower efficiency solar panel with the same pmax power rating.  The efficiency has nothing to do with how fast it will charge.  However as mentioned in the Pmax section sometimes panels don’t output there rated power, and if they don’t, that will effect the charge time. 
The only ting efficiency effects is the size of your panel.  A low efficiency solar panel will be much larger than a high efficiency solar panel of the same watt rating.   A good monocrystaline solar panel will be in the 18-23% rating, which is about as high as it gets (as of writing this).  Now if you get a 9% efficient solar panel it is going to take you at least twice the space in solar to produce the same output.  This can hurt the portablitliy of your system.

Ipmax:  This the current the solar panel will put out in amps at the maximum power output of the solar panel (when it is putting out the Pmax power).  This is a good number to know, and usefull when testing solar panels, but not as critical as the ISC.

ISC:  This the the current when the solar panel is short circuited.  Esentially this is the maximum amps the panel will put out.  This number is important because you want to make sure your wires and connectors can handle this amount of amps.  For example if you parallel 5 panels with a ISC of 7A (when you parralell panels the amps goes up, but the voltage stays the same) your maximum amps would be 35A.

Max System Voltage:  When you parallel solar panels together the system voltage will go up.  You want to make sure the voltage is not higher than the Max system Voltage.  Usually this voltage is high (600-1000v), however I have seen some panels with a system voltage as low as 45v.  

Voc:  This is voltage open curcuit, which is the voltage of the solar panel when there is no load on it, or the maximum voltage (at the rated temperature, this voltage will go up when it is colder, and down when it is hot).  If you are running solar panels in series, the voltage will increase while the amps stay the same.  You want to make sure the Voc voltages don’t add up to go over the charge controllers maximum voltage, or the solar panels max system voltage.  For example, if the Voc is 22V, and your charge controllers maximum voltage is 100v, you would not want to run more than 4 panels in series (4×22=88V,  5×22=110).  It is usally a good idea to leave some buffer in here, so if you figure your voltage at 95Voc, and your charge controller can only handle 100v, I would do 1 less panel.

Vmp:  This is the voltage of the solar panel at its maximum power.  At open circuit (no load) the voltage is at its maximum, but since the current is 0, it is also putting out 0 watts of power (power = volts X amps).  As you put a load on the panel allowing current to flow, the voltage will start to go down as the current goes up.  At short circuit, you will get your maximum amps with 0 volts.  This also produces 0 watts of power.  The voltage where it produces the most power is the Vmp.  This voltage is under ideal conditions, and will change depending on how bright the sun is.

Solar Panel Wiring: Series/Parallel

When connecting solar panels in a series, you simply connect the positive of one panel to the negative of the other.  They are designed with mc4 connectors that only connect that way.  When you wire solar panels in parallel, you connect all the positive wires together, and all the negative wires together.  You can also wire in both series and parallel at the same time. 

Each panel:
Pmax = 100 watts
Voc=22v
Isc=7A

Combined in series:
Pmax = 300 watts
Voc=66v
Isc=7A

Each panel:
Pmax = 100 watts
Voc=22v
Isc=7A

Combined in parallel:
Pmax = 300 watts
Voc=22v
Isc=21A

You can also wire your panels in series and parallel at the same time.  For example if you were to series 2 panels, then series another 3 sets exactly the same, and then parallel them all together, you would have 4 series of two panels each.

Here's what you need to know:

Series:  You add the voltage of each panel without changing the amps
Parallel:  You add the amps, without changing the voltage
 
In the above example, since we series two panels together the voltage is 19V +19V = 38V
And since we paralleled 4 strings (a string is a set of panels run in a series) The amps add to: 8A+8A+8A+8A = 32A
 
The watts will be the same no matter how they are wired.  You simply add the watts of the panels.  8 panels X 152 watts = 1216 watts