Since PV systems are electrical systems, it is a good idea to understand the basics of electricity.
Here are some basics and terms you will see throughout the design process of a PV system.
DC- Direct Current
Direct current, made popular by Thomas Edison, is the one-way flow of electrical charge. Solar modules and batteries produce this type of current. When looking at DC voltage on an oscilloscope, one would see a FLAT LINE.
AC- Alternating Current
Alternating current (Transmission made popular by Nikola Tesla, and used universally for Mains Power Transmission), is the flow of electrical charge that periodically reverses, or alternates direction. AC is used for power transmission, and is the type of electricity that we use in our homes. When you plug in an appliance or device into an electrical outlet, AC power is what you are using. Inverters take the DC power from Batteries or Solar Panels and convert it to usable AC power. When looking at AC voltage on an oscilloscope, one would see a SINE WAVE.
The frequency is the number of cycles or occurrences of a specific event over a unit of time. Almost always in PV systems we are referring to the CYCLES PER SECOND of AC Sine Wave from the inverter. The unit of measurement for the cycles per second is the Hertz, abbreviated Hz. In the U.S., were are on a 60 Cycle frequency. When buying an inverter for use in the U.S., the inverter output will be rated at 60Hz. In many parts of the Caribbean and the majority of Europe, a 50Hz or 50 cycle system is used.
Voltage is what is known as Electromotive Force, or EMF. Voltage is the energy difference of electrical potential energy transmitted between two points. The most common analogy is that of a water hose. The voltage is the equivalent of the pressure pushing the water through the hose. The volume of water in motion (the electrons) is the equivalent of the current. Since power (how much water leaves the hose) is voltage multiplied by current, you may have lots of water (current) and little pressure, or lots of pressure and little water, and would have little water coming out of the end. When you have voltage (pressure) and current (water volume), then the power output will be higher. Voltage is measured in VOLTS, represented by the letter V.
Electrical current is simply the flow of electrical charge. In our water hose analogy, it is simply the volume of water that is in the hose. Without voltage, the flow is non-existent. Current is measured in AMPS, and is represented by the letter I. The higher the current, the higher the number of electrons in the circuit is.
Although not nearly seen as much in PV systems as voltage and current, resistance plays a very important part in electrical circuits. Resistance can be described as the opposition to the flow of electrical current through a given circuit. Resistance is measured in Ohms and is represented by the symbol Omega (Ω). In our water hose analogy, the resistance would be the size of the hose. Even with large amounts of current (water) and voltage (pressure), if the diameter is extremely small (high resistance), we can only get so much water out of the hose (power). Resistance is much lower in copper than in aluminum, which is why copper wire is used for PV/ Electrical system installations. Ohm’s law shows the relationship between voltage, current and resistance with the formula V = I*R.