Basic electrical information for solar systems
Basic electrical information for renewable energy users
If you want to fit some sort of renewable power system to your house, camper van, motor home, yacht, satellite or whatever, then a basic understanding of electricity will help. Some knowledge of the basics of electricity is pretty well a prerequisite to owning any form of stand alone power system and anyone contemplating a grid feed solar system should at least understand what electricity is and what happens to power produced from a grid feed solar array.
So what is electricity?
Blowed if I know know! Defining exactly what electricity is, is still a cause of contention. What we are more interested in is how electricity can work for us. A good description of electricity could be: Electricity is a bunch of things called electrons. Electrons are one of the building blocks of the universe. The interesting thing is that when electrons aren't building the universe they can be made to flow through wires and perform work. This is useful.
Some Information About Electricity ...
Electricity is stuff that flows to appliances through wires. Electricity has two characteristics called voltage and current. Electricity as we use it will be in two forms; alternating current (AC) and direct current (DC).
Some Examples of Voltage Are:
- 12 volts DC - A common car electrical voltage and a small solar system voltage
- 24 volts DC - A common small house or large motor home voltage
- 48 volts DC - A more serious voltage for medium to large household solar power systems
- 60 Volts DC and above - Serious voltage from a large battery bank for a larger installation.
- Mains type voltage - This varies depending on what part of the world you come from, commonly 110 in the USA or 240 volts in Australia, with some countries using 220 volts. This voltage will be AC.
Note: All of the above DC systems can have an inverter connected to them which will convert the DC voltage to something more useable around the campervan or house like 240 volts (110 volts in the USA).
Current and it's Relationship to Voltage:
Current is the amount of electricity flowing through a wire and is proportional to the amount of power being used. A definition would be that an item that uses a large amount of electricity, say a heater or an iron will draw more current than a small electrical appliance like a domestic light globe. Current is measured in amps or more correctly amperes. While voltage could be called the potential of electricity, current is the amount of power that comes out of the wire.
The amount of current that flows through a wire is also proportional to the voltage. There is a relationship between current and voltage that determines the amount of work electricity can do for us. This is covered shortly, if you have got this far and are still reading, keep going!
AC and DC Electricity
DC Electricity is; "direct current" and is the electricity generated by your solar panels and stored in your batteries. Direct current has a positive and a negative supply wire.
AC Electricity is; "alternating current" as connected to most houses and buildings in most areas of the world. This is the electricity your inverter will produce to run your appliances like the TV stereo and computer. While DC electricity has a positive and a negative connection, the wires supplying AC current alternate between positive and negative. The speed at which the wires swap between positive and negative is the cycles. This is expressed in hertz. A hertz is simply a number of times something happens in a second. In Australia the mains AC voltage is rated at 50 hertz, that is it alternates between positive and negative 50 times per second. In other parts of the world 60 hertz is not uncommon.
Putting Electricity to Work
Back several hundred years ago a bloke by the name of James Watt was busy changing the future. James invented the steam engine. The steam engine marked the beginning of a new age. To the right is his picture, courtesy the good folk at Wikipedia.
Besides inventing the steam engine what James did for us was define work. When Jim was a lad work was considered in terms of what a horse could do. This was pretty broad as you could imagine. Questions could be asked like: What size is the horse? How old is it? What did it have for breakfast?
Jim fixed all of this by defining a unit of work called "horsepower". Horsepower can be further broken down into things called watts, named of course after Jim who has sadly passed away.
So now instead of needing to know what the horse had for breakfast we can define work in a precise and finite way. The definition of work is a unit called a watt. A watt is a very useful measurement. Just like a litre is a measure of liquid, a watt is a measure of work.
Enter Georg Ohm
Georg Ohm was a brainy German physicist who loved fooling about with electricity. While doing so he discovered a relationship between voltage, electrical current and work performed (watts) This is called Ohm's law.
Ohms law states (in part) that: Amps (current) x Volts = Watts
Later you will find this calculation very useful. Of course you can reverse this for another common calculation; Watts divided by Volts = Current. Useful stuff indeed! At this stage don't fret about remembering all of this stuff though!
Parallel and Series Connections
Later on as you ponder connecting solar modules or batteries together you will hear the term "connecting in parallel" or the term "connecting in series". It is important to gain an understanding of what this means: Basically speaking a device like a solar panel or a battery will have what is termed as a nominal voltage. The nominal voltage of a lead acid battery is 2 volts. The nominal voltage of a solar panel is usually between 12 and 36 volts. "Nominal voltage" by the way, is the generic voltage given to something that actually has a voltage that varies slightly. A 2 volt lead acid cell for example can be 2.4 volts when fully charged and 1.9 volts when flat. A solar panel with a nominal voltage of 12 may have an open circuit voltage of around 20 volts.
A parallel connection between two devices will result in the voltage remaining the same. A parallel connection is connecting the positive and negative terminals of one device to the positive and negative terminals of the other device. The diagram to the left is a parallel connection, it could be two batteries, two solar panels, whatever. Obviously you can take your positive and negative feed or supply wire from either positive and negative terminal, in the diagram we are using the terminals of the right hand side device.
A series connection is somewhat different: Opposite polarities are connected. You will take the positive terminal from one device and connect it to the negative terminal of the other. This will double the voltage. See diagrams below. An example of series connection (left), if this was the same 2 x 12 volt panels then when joined in series the output would be 24 volts. Note how the power is now taken from a terminal on each panel.
The diagrams could equally apply to batteries. Let's sayyou have chosen 2 x 12 volt car batteries. the top diagram would produce 12 volts. The lower diagram would produce 24 volts.
There is generally a limit on the number of recommended items placed in parallel strings. Some panel manufacturers recommend no more than say three panels, usually due to current flow restrictions. Batteries can have charging problems when connected in parallel.
The limit on series strings is more due to voltage requirements. 12 x 2 volt cells in series, for example makes a 24 volt battery bank, you don't use 13 or 11! When connecting solar panels in series, the limitation becomes the maximum array voltage.
Why Watts are Useful
If you are contemplating an energy system for your house, motor home, caravan, satellite or whatever then you really need to know how much work you want it to do and how to calculate the work. Here is a quick example:
A few years after Jim was in his prime another bloke in another country was trying to invent a light bulb. His name was Thomas Edison. As we know, Tom succeeded. On the right is a picky of one of Tom's first successful light bulbs, again courtesy wikipedia.
If we look at a modern light bulb it will be sold with an energy rating in watts. Let's take a common 60 watt light bulb.
The 60 watts bit is what we are interested in ... This light bulb requires 60 watts of work to make it function in the way Tom intended.
When you buy a solar panel for your "whatever it is" project you will discover that solar panels are sold with a wattage rating. I am sure you can now guess that this wattage rating is the amount of work the solar panel can do. So let's rock off to a mythical panel shop and pick a common sized panel, one with a rating similar to the light bulb, say a 60 watt panel (panels come in heaps of sizes, 60 watts is one of them).
Ok, if we get our 60 watt light bulb and our 60 watt panel we have a generator (the 60 watt panel) and a device (the light bulb) and the potential to make power to produce light. If the light bulb and the panel had similar voltage ratings we could connect the bulb to the panel and make the light bulb work from sunlight. This may be novel but the usefulness is somewhat suspect because a light bulb is redundant in sunlight BUT the example shows what work is required to produce a known effect.
So: if we took the panel and put it in the sun and captured the electricity and stored it (capture and storage is the basis for most energy systems) we could calculate how much work we could get by stating something easy to understand like:
If the panel sat in sunlight for an hour then it would store enough power for the light bulb to stay lit for an hour. If the panel sat around in the sun for four hours then the light bulb could be expected to remain lit for four hours.
When you put a solar panel in the sun the amount of power it produces is proportional to the amount of light falling on it. If we got say a 60 watt panel and orientated it so it was perfectly square on to the midday sun it should produce around 60 watts. Assuming it was fixed in this position the question arises: What happens when the sun is not square on, like in the early morning or late afternoon? On a clear blue day, the output would slowly rise from zero at dawn to 60 watts at midday and back to zero at dusk.
A sun hour takes into account changing output and converts it into an hourly output. It may for example take the passage of three hours between 7 am and 10 am for the solar panel to put out what it puts out at midday. Between 7 am and 10 am one sun hour has passed.
Typically when a solar panel is fixed in position facing north and tilted a bit (depending on your latitude) It will put out about 4 sun hours of power per day so the 60 watt panel could be expected to put out around 240 watts of energy (240 watt hours).
If the panel was flat on say a caravan roof then the output could be expected to be a little less, possibly around 3 sun hours. Sadly a panel sitting in the sun all day will not put out its full rated power all day and the sun hour lets us calculate a more realistic daily output.
Of course in winter in say somewhere in the southeastern Australia, on a good day you may only get 2 sun hours. On a rainy day you may not get a single sun hour all day.
For planning a solar system, somewhere between 3 and 4 sun hours per day is a good starting point.
Let's have a quick summary of this page and move on
- You are going to design or own a system that will have a nominal voltage
- There is a relationship between voltage and current
- This is summed up by "Ohm's Law
- If your system includes solar panels the daily output will be governed by a thing called a "Sun Hour"
- If you still don't understand any of this stuff, move on and hopefully the relevance of what I am trying to tell you will sink in later on down the track!