Using Solar Panels

A solar panel is a device that consists of a few bits of silicon, usually glued under glass, that you put in the sun to generate electricity. There are no moving parts and very little to wear out. A solar panel will go on producing electricity year after year with only one proviso ... it must be in full sun to produce full power. Forget about shade tolerant panels and other marketing hype, sunlight = power from your solar modules, shade, overcast, rain and all the inclement stuff means nil or very little output.

Some useful information to know:

• A solar panel is supplied ready to mount. It will have an aluminium frame that you use to attach the panel to any surface. Typically this will be with tags, strips of metal, wood or anything else that suits the particular location. Because aluminium reacts with some dissimilar metals you should either use aluminium for tags or separate different materials with a non-conductor like plastic.

• 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 location in degrees. At 42 south, (here in Tasmania) the tilt angle should be 42 degrees … but this is an average year round angle. If you want maximum output on winter days the tilt angle should be latitude plus 10 degrees. Here in Tasmania I mount (fixed installation) panels at around 52 degrees. A few degrees here and there is not critical. If you are mounting your panels flat, such as on the roof of a caravan they will still work fine, you will however get slightly less power per day compared with a correctly tilted array.

• No matter what a solar panel salesman tells you there is really no such thing as a “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 no 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.

• 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.

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, this is covered extensively next.

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

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

• 2. 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).

• 3. 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.

• 4. 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.

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

This cabin is in an area that receives an average 4 hours of sun per day over 1 year. Dividing the factored daily power requirement by the average sun hours will give you the size of the solar array required. 1721/4 = 430. In this instance a 430 watt array should be sufficient. Given that solar panels are commonly sold in 60, 80 and 120 watt sizes the array would in reality end up being 480 watts. The voltage could be 12, 24 or 48 volts. The most common size would be 24 volts, 12 would be OK, 48 would be the most efficient however it would require 4 x 120 watt panels or 8 x 60 watt panels.

You should now be ready to fill out your own load chart.