Using Solar Panels

Solar panels or more correctly photovoltaic panels are devices that convert photons from sunlight into electricity. This is called the photovoltaic effect. The photovoltaic effect was first discovered in 1839 and the first solar cell was produced around 1880. initially electrical output was very small. The first solar cells were used in light measuring equipment, mostly for up and coming photographic market. A truism then that is still just as relevant today is that the output of a solar panel is directly proportional to the amount of light falling on it. Efficiencies have of course increase enormously but a photon is still a photon and the more that reach the panel, the higher the output will be.

The output of a solar panel is in direct proportion to the amount of light falling on it! Forget all marketing hype about shade tolerant panels, panels that work in overcast conditions and panels that work in moonlight! The first lesson regarding solar panels is that output is directly related to light!

Some useful information to know:

• A solar panel is supplied ready to mount. Solar panels are usually supplied with an aluminium frame ready to be mounted to any solid surface. You simply use tags of metal, mounting bar, hinges or whatever to attach your panel via its aluminium frame to the surface you wish to mount it on.

• A solar panel should have a tilt angle wherever possible to face it square on to the sun at midday. The correct tilt angle is the latitude of the installation location in degrees. If you live 20 degrees south or north of the equator then this is the ideal year round angle. Solar panels powering homes with battery banks are usually tilted a little further to increase winter output and solar arrays for grid feed non battery systems are usually tilted a little less for maximum summer performance. In reality the tilt angle is not that critical and panels work fine in most latitudes at angles a lot less than the angle of the latitude. The minimum recommended tilt angle is 10 degrees but this is related to the ability of the panel to "self clean" in a rain shower.

• No matter what a solar panel salesman tells you there is really no such thing as a “shade tolerant”, "partial shade tolerant" or “cloud tolerant” panel! A small loss of sunshine equals a large loss of output. A slight overcast typically wipes out 90% of a panels output. Partial shading from that tempting tree in the desert will mean little or nil battery charging. Do not believe otherwise!

• A solar panel is not magic! If you use a 60 watt light globe for one hour at night you will need full sun on a 60 watt solar panel for one hour + to generate what you have used.

• Your solar panel will typically be 12 volt rated although there are now some 24 volt panels on the market. You will need pairs of panels to generate 24 volts if you use 12 volt panels. You will need your panels in groups of four for a 48 volt system. Voltages different from 12 or 24 volts are also now becoming common. These are usually on larger panels designed predominantly for the grid feed market.

• A 12 volt panel will have a typical output of 20 volts! You need an excess of voltage for power to flow from your panel to your battery.

• A solar panel will either be supplied with a junction box containing a positive and negative terminal or with “flying leads” which are your positive and negative connections

• Power output from a solar panel is calculated in sun hours. This is not the amount of sun you expect per day! A sun hour is the equivalent of one hour of strong midday sunshine with the panel "square on" to the sun. It may take 3 hours to get a single "sun hour" in the morning or afternoon when the sun is at an oblique angle to the panel. On an overcast day it may take all day to get a single "sun hour". With a heavy overcast you may not get a single "sun hour" at all! A much used average is four "sun hours" per day. Say using a 60 watt panel correctly mounted in a shade free area you could expect 4 x 60 = 240 watts per day. Add a bit of partial shading and you could expect half this or less!

Calculating the Size of Your Solar Array

Determining the size of a solar system that will power your electrical needs requires some simple calculations and a chart. The chart is called a load chart, and is your next solar lesson.

Below is a sample load chart and the steps required to fill one out. Move through this then move along and get started on your own load chart in the next section, (link above left and below).

• Prepare a simple chart to list and calculate total electrical load.

• Itemise all electrical appliances, the power they use and the length of time they are on per day. (For appliances used occasionally a weekly power use can be divided by seven). You will get the approximate power consumption of all modern electrical appliances from a placard on the appliance. This placard will most likely state watts however sometimes it may state current ... Remember back to Ohm's Law (on previous page) and use this current x the appliance voltage to get the watts needed for your load chart.

• Total these electrical loads to arrive at a watt/hour per day electrical load. Divide this power requirement by 0.7 to achieve a factored power requirement. Factoring the power requirement compensates for losses and inefficiencies in the batteries, wiring, inverter etc.

• Calculate the sun/hours per day average for your area. Information on this may be available from the meteorology office, library etc. If a wind turbine is going to be used as well, wind figures could be obtained at the same time. It is useful to have the sun hours for each month if possible to determine if any particular month or period in the year is lower than average. A back-up generator could be considered for months with below average sun/hours.

• Divide your total factored load by the average sun hours per day to arrive at the size of the solar array.

• Decimal your minutes, this makes for easy calculations. 0.1 of an hour is 6 minutes, 0.2 is 12 minutes and so on.

• Break the chart into rooms if you like. This is what I have done below:

"Grab yourself a sample chart here!"

Let's say we live in a place where we could expect around 4 "sun Hours" per day on average. 5000 / 4 sun hours gives us our solar array size. In this case we need a solar array of around 1250 watts.

A popular high quality solar panel is the Kyocera 130 watt unit. 1250 / 130 = (9.6) 10 panels. These panels are 12 volt panels and this system would really need to be 24 or 48 volts to retain efficiency. We need panels in pairs for 24 volts or in groups of four for 48 volts. Personally I would be inclined to make the system 24 volts and use 12 panels.

Efficiency revisited

An industry standard has always been around 0.7 For over a decade in the solar industry I have pondered this and looked at what I have installed and what people get from these systems. Given that a modern inverter is at least 90% efficient and a modern new battery absorbs charge current at around 94% efficiency I think that the 0.7 efficiency factor is ultra conservative! while 0.7 is conservative, 0.9 would be considered best possible and a little optimistic.

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