Electricity “consists” of two parts – voltage (V) and current (I). A common metaphor is water – the voltage is the water pressure, how much “push” it has, and the current is the volume of water flowing. These are related by resistance (R) – in the water analogy, you can think of the diameter of the pipe the water is flowing through.
Voltage, Current, Resistance, Power, and Energy
Voltage is measured in volts (V) and current is measured in amps (A). Resistance is measured in ohms (Ω).
The relationship between voltage, current, and resistance, is described by Ohm’s Law, which states that voltage equals current multiplied by resistance (V=IR). For example:
6V = 3A x 2Ω.
If you want more current, but the same voltage, you must reduce the resistance:
6V = 4A x 1.5Ω
In our water analogy, this is the same water pressure, but more water flow. You reduce resistance by increasing the diameter of the pipe.
If you keep the same resistance, but increase the voltage, current will also increase:
8V = 4A x 2Ω
In our water analogy, this is increasing the water pressure, but keeping the same pipe. Water flow increases.
I’ll also mention watts (as in, a 6.6 kilowatt (kW) solar system). The watt is a unit of power, and is simple voltage x current. So 6V x 3A = 18W.
There is a relationship between voltage, current, and power (P, measured in Watts W). This is P=IV. For example: 18W = 3A x 6A. Usually you’ll see solar power systems given by their size, e.g. 6.6kW (6600W).
Finally, energy. Energy is the total amount of power over a given time – as an equation, E = PT. For example, if your solar system generates 5kW for one hour, it generates 5kWh.
Parallel and Series
Solar panels are either connected in series, parallel, or both.
Example panel: V=35V, I=10A, P=350W
Series is when the negative of one solar panel is connected to the positive of another. This is the way most solar panels are connected, and is called a string.
By doing this, the voltage of the string is the sum of each individual panel, and the current is the same as that of one. Using the water analogy, the push is higher, but the total volume of water flowing is the same. The total current cannot exceed the current of the lowest-current panel. Power is also the sum of the power of each panel.
10 panels in series:
V = 10 x 35V = 350V
I = 10A
P = 10 x 350W = 3500W = 3.5kW
Parallel is when the positive of one solar panel is connected to the positive of another, and the negatives are also connected.
By doing this, the current of the string is the sum of each individual panel, and the voltage is the same as that of one. Using the water analogy, the push is the same, but the total volume of water flowing is the higher (imagine multiple pipes). Power is still the sum of the power of each panel.
10 panels in parallel:
V = 35V
I = 10 x 10A = 100A
P = 10 x 350W = 3500W = 3.5kW
You can also do both:
2 parallel strings of 10 panels in series:
V = 10 x 35V = 350V
I = 2 x 10A = 20A
P = 2 x 10 x 350W = 7000W = 7kW
(also P = V x I = 350V x 20A =7000W = 7kW)
Why are they stringed the way they are? Well, most inverters require their inputs to be in a certain range. For a 5kW inverter, 200~800V, and a maximum of 10~20A is common. This is why you have 2 strings (parallel) of 8 panels (series), not 8 strings of 2 panels. The total power is the same, but the voltage would be too low and the current too high for the inverter. This is one of the many reasons it’s important to have a qualified engineer design your system.
AC vs DC Power
Solar panels generate DC power. DC – direct current – is a simple flow of electrons from negative to positive (while current is defined as flowing from positive to negative, the actual electrons flow in the opposite direction). Most electronics – computers, TVs, phones, etc. – also use DC power. Batteries store DC power.
Your house and the grid use AC power. AC – alternating current – is harder to explain. The electrons don’t move as such; they simply oscillate back and forth. AC is beneficial as it can be transformed (i.e. have the voltage or current increased or decreased), whereas DC cannot.
To convert between the two (i.e. from a plug in your home (AC) to power your (DC) computer), a special device must be used. An inverter (like a solar inverter) converts DC to AC. A rectifier (like in the plug of your computer) converts AC to DC.
