AC / DC

It's not just a cheesy band from England who played weird fast-paced music using weird voices.

The photoelectric effect is only able to produce DC power.  Your home works with AC electricity.  In order to change that power over to something your home can use, it requires the use of a device known as an inverter.

This page will explain a bit more about the differences between AC and DC electricity.

Alternating Current

AC is "Alternating Current"

AC electricity is manipulated using coils to induce a current that fluctuates from + to - in voltage at a certain number of times per second.  Meaning that 120v AC actually fluctuates from 120v+ to 120v- at a rate of 60 times per second (60 hertz).

Conductors

With AC current, you can complete a circuit with one conductor and a neutral, which does not conduct electricity.  The two-prong plug that goes into the wall everywhere you go?  One of those is the neutral, the other is a "line."

In a home, most devices are 120v AC, meaning that it is using one line.  Some devices are 240v, thus using two different lines to combine their voltages.  Inverters are typically 240v.

Benefits of AC Electricity


The main reason AC power is so popular is because it travels better over long distances since it is able to be "stepped up" and "stepped down" using transformers.  That's why the grid is AC power.

However you'll notice that many of your devices at home contain power adapters  which convert electrical energy back into DC again, because many electronic devices work using DC power.


Direct Current


DC power is direct current.  It does not fluctuate in polarity like AC power, nor can it operate with a single conductor.  It requires a positive and a negative wire in order to operate.

Without a way to induce AC current, electrical components like batteries and solar panels will only generate DC power.

Additional Reading:Grounding

External Links:
PBS Explains This Same Concept

Kilowatt

A Kilowatt is a measurement of electrical energy.
It is similar a mile-per-hour rating.

When a system is described as a 5 kilowatt system, that means that it is estimated that it operates at 5 kilowatts per hour when it is at full power.  That's not an indication of how much power it will generate every hour, but what it's rated at its very best performance.  

Determining the kilowatt rating of a system usually involves multiplying the STC wattage of the panels by the number of panels included in the system.  

Coming Soon:  exercises

(illustration)

Equipment Grounding

In a solar electrical system, everything needs to be grounded.  That means that there needs to be electrical continuity with every part of the system, including all of the rails on the roof, and every panel frame.

Code Rules About Grounding
The NEC requires any unsheathed ground wire to be #6 AWG or the size of the largest conductor.  It also insists that ground wire outside of conduit must be unsheathed, therefore all ground wire on the roof is #6 AWG.  Many installers will continue the #6 size ground wire all the way to the inverter.

After the ground wire has effectively connected every part of a system, it makes its way to the grounding electrode conductor terminal in the inverter.  Then, it goes out to a ground rod.

The color of ground wire is green.

Ground wire from the roof, connecting to the combiner box
on a bus bar, then continuing to the inverter 

Split Bolts and Acorn Clamps, used for grounding

Standard Test Conditions

STC
In order to rate solar panels fairly, the Department of Energy, in conjunction with Underwriter's Laboratory, have developed a system for rating panels so that the consumer is getting unbiased and accurate information about panel performance.

All panel power ratings are determined by their performance in laboratories under Standard Test Conditions, in which a light with the strength of 1000 w/m2 flashes on a panel.  The temperature of the room is 25 degrees C (about 75 degrees F).

It also assumes an air mass of 1.5.

Air Mass

Air mass is the thickness of atmosphere that the sunlight must penetrate in order to reach the panels.  The more direct the sunlight, the less air mass.  In the illustration to the left, you can see that the sunlight is more direct at (b).

In STC, air mass is accounted for as a factor of 1.5.

Amps And Volts

Difference Between Amps and Volts
Understanding the basics of electricity

In order to understand series/parallel wiring, it becomes necessary to understand the concepts behind electricity.

Electricity has two components:  amperage and voltage.  As a comparison, you can consider voltage to be the pressure of the flow of electrons, while amperage is the size of the field, as demonstrated by this visual description.

You can adjust the voltage and amperage of your array in the way that it's wired.

Further Reading:
Series/Parallel Wiring




Exercise:

Angle and Orientation: Efficiency Table

You can expect a different percent of production from your array, depending on its position.

The "Orientation" of a solar array defines whether it is facing south, southeast, and so forth.  Essentially, what part of the horizon the panels are facing.  It's favorable in the northern hemisphere to have panels facing south.  The closer to South you are, the better the system performance.

The "Angle" is whether the panels are flat or vertical, or anywhere in between.  It benefits the system to have the panels roughly 15 degrees less than your latitude.  In other words, in CT where latitude is about 38, it's beneficial to have the panels at about a 25 degree angle.

As demonstrated in the table, you can see what the difference between panels that are 30 and 15 degrees in angle will produce, comparatively.

Magnetic Declination

Depending on where you are, a magnetic compass may be incorrect because of differentiations in the Earth's magnetic field.

Why This Matters

This is important to consider, when using a compass, because your estimation can be off by the amount of degrees that should be accounted for in magnetic declination.

In America, the declination never reaches beyond 15 degrees. Check out the graphic to the left, and it will give you a better idea of some of the declination factors around the world.

Affects: Panel Orientation

Question:  What's your area's degree of magnetic declination?  How much does it affect system production by accounting for it?