Over the next few pages we will build up a more-and-more complex system and try to explain things as we go. Ok, for the most basic systems we need two things: 1. Solar Panel(s), and 2. Battery(s). All your purchasing choices revolve around the battery so that is the first thing we need to work out.
All- in- all it depends on the battery so that is the first thing we need to work out. What battery you choose depends on what you need to power. Work out what appliances you will need to run. Write down how long in a 24 hour period they will be on, how many Watts per hour they require and add them all up. Then we need a battery that will supply the same voltage and all the Amp hours we will require.
For our example we will be running a 12 Volt, 6 Watt pond pump for 2 hours each day. A deep cycle 12 volt lead/acid battery has the right voltage. What size one though? The different sizes are all described in Amp Hours. We work this out by, Watts = Volts x Amps, 6 = 12 x Amps. The pump needs 0.5 Amps for 2 hours, or in other words 1 Amp Hour ( 0.5 Amps x 2 hours = 1 Amp Hour). So we need, at least a 12 Volt 1 Amp Hour battery as a minimum.
After reading a bit about batteries you quickly discover if you want your batteries to last a long time, it’s best not to drain the below about 50%. So double whatever estimate you have worked out so we hopefully won’t need to go below half way. So now we buy a 12 volt 2 amp hour battery which is pretty small.
Of course the basic idea is to replace what you use but there are some things to note. Firstly your panel needs a higher voltage than the battery. Think of voltage like pressure. If the pressure the battery is pushing out is higher than the pressure you are pushing in, then you are not going to get any power in. So for our example to fill our 12 volt battery we will need more than 12 volts.
As you read about batteries you will also quickly realize that a 12 volt battery is pretty much dead at 12 volts and full at about 12.7 volts. So for our 12 Volt battery example we want to not only charge above 12 volts but at about 13.5 volts. So we should look at about a 14 volt panel.
Now we need to work out what wattage panel you need. You need to supply all the amp hours you will use in 24 hours in the period that you have full sun, lets say 6 hours to be safe. For example, we need to use 1 amp hours every 24 hours and we need to generate that in a 6 hour period. So over 6 hours we need to generate 1/6 of an amp every hour. Since Watts = Volts x Amps, 14 Volts * 1/6 Amp = 2.33 Watts and, as always to play it safe (and to make it a round number), lets go for 3 Watts. So now we can go out and get our 14 Volt 3 Watt Panel or several panels that add up to 14 Volt 3 Watts. See our electricity basics page for more information on adding voltages together.
That’s the basics. A battery and a solar panel to match it that covers what we take out. Unfortunately though the system needs some management to make sure it is running efficiently and not breaking our equipment. On the next page we will go through this.
As we mentioned on the last paragraph, unfortunately solar systems need some management. We need to make sure it is running efficiently and not ruining out equipment. Lets look through some of these (Note: the first two are the most important ones).
Batteries can be overcharged. This can be dangerous at worst and at least will damage your batteries. When you have bought some batteries you will realise how expensive they are and when you have had a system for a while you will realise that you system is only as good as it’s batteries so you really want to protect these. You can do it manually or automatically, manually is cheap but a really big pain and visa versa.
Basically, your batteries will have a voltage at which they will be full and this is the point at which you want to switch off charging. Manually, you can check a voltmeter and when it reaches the max, flip a switch that disconnects a battery. This is of course fairly labor intensive, as you have to be there. Automatic systems use a circuit that detects the voltage of the battery and disconnects the battery if it hits the maximum and reconnects when it dips below the maximum. You can built your own, there are a few circuits out there on the web, or you can buy them without too much trouble. If you are looking about these are normally called charging regulators. I picked mine up of ebay pretty cheaply, and they had a few cool functions built in.
In our example we had a 12 volt battery and we learnt at 12 volts it’s pretty much empty and at 12.7 it is just about full. You can check you battery’s state but looking at this voltage.
Note: These are resting voltages (not power being drawn from the battery, none at all!)
If you leave you batteries connected to the panels when the are not producing any power, they will drain a bit of power from your batterys. To prevent this it is a good idea to introduce a diode (diodes only let current flow in one direction) into your system. That way current easily flows from the panel to battery but can not flow from the battery to the panel, thanks to our little one-way device.
Most of your equipment will operate perfectly within ranges of Volts/Watts/Amps. But just incase there are spike in power it is best to lose a $1 fuse than a $100 charge regulator or panel. So between your equipment install some fuse that match your needs. You may never need them if you planned correctly but if anything goes wrong you’ll love them. And they are very cheap insurance.
Temperature has an effect on charging, some more complicated systems take this into account and charge differently depending on the temperature. A hot battery will overcharge before the voltages tell you it’s full, so if you have your battery with your panel it might get exposed to the sun and become hot and your charging regulator needs to keep this in mind. And if you are in an area that drops below 0 degrees Celsius, battery capacity can drop to about 1/4 of normal. batteries tend to like 18 to 25 degrees Celsius(68 to 77 degrees Fahrenheit). So if they are out in the sun keep the shaded and ventilated, or if they are in the cold keep them insulated and perhaps used a bit of your battery power to keep them warm.
Because batteries are so expensive people often try to get the most of them and make them last as long as possible. Batteries often last alot longer if you only every deplete them to at most 50%. Some people even only ever try 20% leaving 80% in the battery at all time. To do this you need to monitor under charge (as well as overcharge) and cut off usage. The same methods and circuits for overcharge can be used. Manually, if you are using some of you power keep an eye on a voltmeter, or get out a calculator and work out how long you can keep devices on for. Or you can build or buy automatic circuits to do this for you.
Every 6 months or so you should check you batteries clean the off, especially if dust builds up between the poles. And if corrosion builds up on your contacts whip out the baking soda and give them a clean too (Don’t get any in the cells though).
Cables and connections
It’s safer to use larger cables and good connections that can make good contacts and support whatever current loads you need. If a large current tries to go through a small wire it might burnt it out (and burn other things along the way) bringing your system to a hold and maybe even destroying it.
From the last paragraph we now have Direct Current (DC) power stored in our batteries but quite a few devices use Alternating Current (AC). To convert our stored DC to AC we need to use an Inverter. While inverters are fairly simple, modern ones have many more functions rather than just changing the current flow. Most of them also up the voltage to a industry standard for the country. When choosing an inverter there are a couple of things we must look at, size and quality.
You will need an inverter that will support the loads you wish to draw through it (current and future loads). Add up what you will be using to work out how many watts at most you will want to use. And the easy bit, you need an inverter than will handle that load. Even if your inverter is not that expensive its a good idea to buy a fuse to protect it. Past that you need to choose one that produces the right voltage and frequency. Most AC devices either use 240V, 50 Hertz or 120V, 60 Hertz, and it’s pretty standard across any country, so check one of your devices and the rest should be the same.
It’s also worth a not to check if the power output is continuous or surge. When some items like motors are turn on they can require up to 7 times as much power as when they are running. The inverter you get should be rated to handle those those surge loads as well as the continuous loads. And it’s always safer to get a larger one than you need just incase.
The quality of the electricity is determined by what is called it’s waveform. If you are running sensitive devices like TV’s and computers then you will need a better quality of power. Lets go through some basics of waveforms.
You can see above roughly what AC looks like over time. One moment current is flowing in one direction (12 VDC) and the next minute in the opposite direction (-12 VDC), ie over time current changes directions – aka it alternates. The frequency or cycles over time give your hertz (50 Hertz means 50 cycles per second) and the amplitude the voltage. The waveform, the important thing in determining the quality, is the shape of this wave. The best quality wave is the sine wave (given by the formula Y=sin(x) ) and approximately like the picture above. And the poorest quality wave is a square wave, like below.
A Square waveform is very cheap to produce and the sine wave is the most expensive. Square waveforms are pretty hard to find as they are such low quality but as you can imagine there are many shapes of waveforms between these, one of the most popular is called the quasi sine or modified wave.
Inverters can be driven mechanically (by motors) or electronically. Mechanical inverts deliver a pure sine wave and handle changes in demand easily but the are not great with surges and are not that efficient. There are two types of electronic inverters, high frequency switching units (cheap and light, but have a short life and don’t handle surges well) and transformer based (more expensive but last longer). If you have access to grid power there is also a third option of synchronous inverter, that gives and takes power from the grid depending on demand (very expensive).
Positioning Solar panels
The direction and angle that your panel faces can have a big impact on it’s performance by affecting the amount of light that hits the panel each day through the year. Some solar panels move continuously to track the sun but most will not go to the expense and difficulty of implementing that. If your one of the majority, that are going to fix your panels in one place, then you have to get it right first go.
To get it right we have to make sure that the panels get hit by the maximum amount of light. This happens when the sun is directly above the panel.
As you can see from above, the angle that the sun hits a panel changes the ammount of exposure. At 30 degrees from the panel, the panel is only exposed to 50% of the light of the sun, at 60 degrees, 87% and at 90 degrees, 100%. This happens because the sun emits the same number of photons in a square cm, but once we put our panels on an angle, those photons are spread across a larger area.
As we all know, at different times of the day the sun moves through the sky and so any stationary panels get exposed to different angles, so what is directly above at one time of the day will not be at the next. What you might not know through is when the sun it at it’s highest it is not necessarily straight up, but may be off by an angle. And that angle is different at different times of the year and different at different latitudes. This angle is to the south in the northern hemisphere and the north in the southern hemisphere.
So we need to take all of this into account. Luckily what is good for your neighbour (aka your rough latitude), is good for you too. So below is a table that will show you what angles to hav your panels on, at different latitudes, at different times of the year.
|0°||70°S||79°S||90°||78°N||70°N||67°N||70°N||78°N||90°||78°S||70°S||67°S||Kuala Lumpur, Malaysia|
|40°S||70°N||61°N||50°N||38°N||30°N||27°N||30°N||38°N||50°N||62°N||70°N||73°N||Wellington, New Zealand|
The angles expressed in the table above are represented by A in the diagram, and the direction to the South or North, eg 90 degrees is perpendicular to the ground. So for example, in Sydney at 40 degrees latitue, we have the angle to put the panel on each month. If we wanted to set it in place for the whole year we might want to average these figures to put your pannel facing north at an angle of 60 degrees.
There are also a few different easy things we can do to improve the exposure of our panels. While constant tracking is complicated, it is easy to move your panels once or twice a year. We can adjust the angle of our panels to have two settings and in summer or winter adjust the angle to get the best of the availible light. So, if we look at our sydney example from before, we can break the year into two and take two averages. That way, in Sydney, we can have our panels at 75 degrees, North facing in the warmer months and 45 degrees in the colder months. And, that should be all you need to know!
If your not sure what you latitude is or you would like to work it out yourself, here is a good method for you.
You will need two pieces of paper, some sticky tape and a pen. First cut out a strip 10cm long and 2 cm thick and fold it in half. Sticky tape this to one side of the paper in roughly the spot of the black stick on the above picture. Now all you have to do is put this out in the sun and every hour or two place a mark where the shadow reaches. Connect these dots into a smooth curved line. Fine the shortest place from the line to the center stick and measure the distance, lets call this distance X. Now the best angle for that time of the year is given by inverse sin of 10/X. Pop it into your calculator and set your panel at that angle. and it should be good for that time of the year every year.
Solar Panel Types
Choosing the right panel can depend on a few different things, quite often its price first. But quickly after that comes, how long do you want them to last, how much space have you got and what environment will it be in.
|Life Span||20 Years||20 Years||5 Years|
|Cost||Most Expensive||Bit cheaper than Mono||Pretty Cheap|
|Made From||Large carefully grown crystals||Multiple small crystals||No Crystalline Structure.|
|Advantages||Best efficiency||Good efficiency with a cheaper cost.||Cheapest|
Can be made into flexible shapes
High temperatures do not effect them
Handle Partial Shading better
|Disadvantages||Expensive||Multiple small crystals||Low efficiency.|
What can you expect to pay?
There are basically two price ranges for panels under 40 watts and over 40 watts. For under 40 watts you can expect to pay $15AUD a Watt for one or two panels, $10AUD a Watt for 10 to 50 panels and above 50 panels, probably around the $7AUD a Watt.
Over 40 Watts gets a bit cheaper, around $8-10AUD per Watt getting cheaper in bulk down to about $6AUD per Watt
Most solar panels last a long time, so don’t be afraid to buy a second hand one. Try ebay or other online auctions, boat yards, caravan sales or your local second hand newspaper.