Understanding Electricity

To design a good solar energy system you need to become familiar with the fundamental principles of electricity that are at play. In this article we will look voltage and current and why they are important. We will also look at how to calculate the correct size of cable for a given application.

WARNING: The equipment that is used in solar power generation generates high voltages and currents (see below) that can kill. The information provided here is for your education and should you decide to put together your own system, please used a qualified electrician to ensure that the system is safe on commissioning. Take extra care when working with your assembly to avoid electric shock. You accept that RevelationSky Harmony Foundation will not be held responsible for injury or death resulting from this knowledge.

Voltage

Voltage is measured in Volts (V) and is defined as the electrical potential difference between two points. The technical definition of a volt is as the amount of energy required to move a unit of charge from the first point to the second.

Current

Current is measured in Amperes or Amps (A) and is a measure of the flow of electric charge. In a electrical wire when a voltage is applied these electrons immediately flow through the conductor from one end to the other creating an electric current.

It is common to see current not only expressed as Amps (A) but also as Milliamps (mA) which is a thousandth of one Amp, e.g. 1000mA = 1A or 5000mA = 5A or 200mA = 0.2A.

The Water Pipe Analogy

Voltage and current tend to be abstract for most people to get a clear picture of as they relate to solar energy design, so in every-day language it helps to think of the combination of voltage and current as a water pipe. In this analogy voltage is the equivalent of the water pressure. In the pipe the water pressure moves the water from the high pressure end to the low pressure end of the pipe and the more water pressure applied the more water that flows. If you think of the rate of flow as current, then taking this back to the electrical terms the higher the voltage the higher the electrical current experienced in the wire.

Calculating Current

From the above you may be thinking that you could increase voltage and increase current and these would be be equitable. In a superconductor, this happens. But since we do not have the millions of dollars needed to play around with superconductor technology, in the real world the pipe (i.e. the electrical cable) that the current flows through has resistance. Different materials have different levels of resistance and all resistance is measured in a unit of resistance called an Ohm (W).

An equation known as Ohm’s law relates voltage, current and resistance. Ohm’s law states that when a voltage (V) is applied across a resistance (R) then a current (I) shall flow across that resistance. This is expressed as the equation V = IR, or Voltage is equal to Current multiplied by Resistance. This equation is rearranged to show that Current is equal to I = V / R or Current is equal to Voltage divided by Resistance.

For example if we had an electrical cable with a resistance of 12W and a voltage of 48V then by Ohm’s law we would have a current flowing through that cable I = 48 / 12 = 4A.

When you purchase electrical cable (the “conductor”) the amount of current it can carry is not infinite due to the resistance of that cable. As you increase voltage, the more current that tries to flow and the hotter the cables gets due to that resistance. A toaster element is an excellent example of a high-resistance conductor and what happens as current is induced. Eventually the cable will melt and / or catch fire which is not what you are designing for with a solar energy plant.

In general the resistance of a cable is determined by:

  • The material used (e.g. copper, gold, etc);
  • The cross sectional area of the cable;
  • The surface area of the cable;
  • The cables maximum acceptable temperature (which is usually the maxiumum temperature acceptable for the insulation used around the cable);
  • The environment the cable is in (on it’s own, installed under glassfibre insulation, installed in a big bundle with other power cables, i a hot environement, cold environment, ventilation, etc).

The most common conductor is copper as this is the most likely cable you will purchase for your system. There are some very interesting equations that you can use to calculate the current carrying capacity of a copper cable which you can research yourself if interested (start here). Far easier is to use one of the many online calculators that will work this out for you such as this one from http://www.axon-cable.com. There are also standard charts that show capacity such as this one from http://www.openelectrical.org.

When you purchase cable, the thickness of the cabled will normally be given as a cross-sectional area of the conductor, e.g. 3mm2. The area of the cable combined with the insulator used will determine how much current a cable can safely permit before it becomes a fire risk.

Power (Watts)

Power is energy per unit time and meaured in Watts (W). In the world of electricity power is defined as the voltage times the current and is expressed in the equation P = IV. This can rearranged to work out how many Amps a given appliance requires for its operation by using I = P / V. This is very important for your system design.

For example a kettle is rated at 1400W on a 240V power outlet, the kettle would be drawing I = 1400 / 240 = 5.83A of current.

Insulators

As insulator is a material that has a extemely high resistance and so current does not like to flow through it. Insulators are placed around copper cable to protect us from the dangerous effects of electricity flowing through conductors, however any sufficently large application of voltage will allow an insulator to conduct (normally resulting in fire). The most common insulator used in electrical wiring for solar power is PVC. Using the calculator from Axon Cable, you can calculate the maximum current capacity for a given cable size (which is measured in mm2). For example below is a snapshot of the calculated rating for a very common size of copper cable of 2.5mm2 with a PVC insulator and in a room at 25oC.

In Australia we tend to get some very summer hot days, so it is advisable that when doing your calculations you work on the assumption of a 40oC day occuring every now and then. Using the same cable and insulation but putting in an ambient temperature of 40oC the allowed current falls to 30.5A.

Sourcing Cable

I have found that the best place for getting cable to suit solar energy systems is your local electrical warehouse, although call ahead as not all of them stock solar supplies.

For most solar installations you need to purchase standard 4.0mm XLPE Solar 2-core cable in 100m rolls for around $150 a roll. This cable is UV protected and rated for outdoor use.

AC vs DC Power

AC stands for alternating current, which means the electrical current frequently reverses direction. AC electricity is measured according to its cycles, with one complete cycle being counted each time a given current travels in one direction and then doubles back on itself. An electrical current is able to complete many cycles per second, and is then given its frequency rating based on that number.

The unit of measurement for an electric cycle is “Hertz” (Hz). The typical frequency in Australia is 50 hertz (Hz), which indicates that the current is performing 50 cycles per second. In other countries they use 60 hertz just to be different. The wall sockets in your home provide AC power as its voltage can easily be changed through a transformer to suit a variety of needs.

DC is actually short for direct current, which is a type of electrical current that travels through a circuit in only one direction instead of backwards and forwards. This is the type of electrical power that is produced by batteries and solar panels.

So you might now catch onto something here. Your household appliances operate on AC but your solar panels and batteries operate on DC. Later we will look at a specific and expensive device called an inverter that has the job of fixing this situation so that you can use your household appliances as designed.

As a final note, when differentiating beween AC and DC voltages the voltage is written, for example, as 240VAC (volts AC) or 240VDC (volts DC).

The Watt-Hour (Wh) and Kilo-Watt-Hour (kWh)

A Watt-hour (Wh) is generic meaure of energy consumption. It is an expression of the relationship between power (W) being used and time (t) in hours to give energy (watt-hours). It is expressed as E = Wt or Energy (Wh) is equal to Power (W) multipled by Time (t) in hours.

The power company charges in what is called Kilo-Watt-Hours which is a unit of 1000 Watt-Hours. In order to get kilo-watt-hours from watt-hours you simply divide the Watt-Hours number by 1000.

Using the equations above we can calculate the amount of energy that a given appliance will use per month. As an example, we shall calculate the amount of energy a television will consume for one month. Lets assume the television uses 100W of power when it is on and is typically used for 4 hours a day. Assuming 30 days in a given month, we first multiply 4 hours by 30 days to get total of 120 hours the TV is used each month. Now we multiply 100W by 120 Hours to get 12,000 watt-hours (Wh) of energy. We then divide that number by 1000 to get 12 kilo-watt-hours (kWh) of energy.

Transformers

We need to know some basic transformer theory as the inverter that changes our battery output or solar panel output is basically transformer that takes the DC from our batteries, changes it to AC and then multiplies the voltage up to mains power voltage and frequency.

A transformer uses handy effect of AC power in that it produces an oscillating magnetic field to transfer power between two coils of wire, which are electrically isolated from each other. Neglecting losses in the transformer, the power going into coil one must equal the power being delivered out of coil two. This is useful as depending on the number of windings in the coils, you can increase or decrease the voltage being supplied.

For example if you have a primary coil with 10 windings and a secondary coil with 100 windings, then there is a 1:10 ratio meaning that the input voltage to the primary coil is multiplied by 10 on the output of the secondary coil. E.g. 24V in becomes 240V out.

This increase or decrease in voltage is not without cost though. The effect of increasing voltage output is to decrease the available current, where as decreasing voltage output increases the available current in the same ratio as the voltage is increased or decreased. E.g. in our 1:10 ratio example above if we had 1A of power being drawn from our 240V side, then we would need to supply 10A on our 24V side to provide that 1A on the 240V voltage side.