When you are planning to select solar panels to collect free power from the sun, there is an almost endless list of technical terms rumours and myths to wade through.

Terms like polycrystalline, amorphous and temperature co-efficient to name just a few.

Sales pitch aside, how is the average person supposed to decide which solar panel is right for their situation?

Aside from brand specific issues like warrantee and quality of manufacturing, there are some clear differences in the technologies used in creating the solar panels commonly available today.

The three panel technologies are :Polycrystalline, monocrystalline and amorphous. Below is a feature by feature comparison of each type.

Polycrystalline

These are the most common and the cheapest (in terms of dollars per watt).

They have a conversion efficiency of between 12% and 12.5% – this means that 1 square meter of panel exposed to the sun will on average produce between 120 and 125 watts. All solar panels are rated at a cell temperature of 25 degrees.

Unless you live in sub zero temperatures, your panels will almost always have a cell temperature far in excess of 25 degrees.

Polycrystalline panels reduce their output as the cell temperature increases. Thus you should expect your new 125 watt panel to produce about 14% – 23% less than the rated value – this is the temperature co-efficient of the panel.

Polycrystalline panels have a hardened glass surface that is polished and thus self cleaning (to some degree).

Monocrystalline

These panels were among the first panels to appear on the market.

They have a higher efficiency at around 15% . They are typically a little more expensive but perform slightly better than their polychrystaline cousins in hot conditions with a typical reduction in output of between 12% – 15%.

Well known brands include BP and Siemens.

Monocrystalline panels also have a hardened glass surface that is polished and thus to some degree self cleaning.

Amorphous

These panels are quite different from the other two technologies already discussed. They have the photovoltaic material bonded onto a stainless-steel material then costed with a rippled polycarbonate.

This has a number of advantages (and one major disadvantage) – the panels are light and very easy to handle, they are almost indestructible (I have seen a panel that has been ripped off a caravan by the wind only to be run over by a truck – testing showed it was unaffected by this treatment. Don’t try this with either of the glass panels!  These panels also reduce their output less than the other two types when partly shaded (by a twig or bird dropping for example).

The two major downside of this construction is that the surface is not smooth and the panels have a very low efficiency rating.

The small ripples in the polycarbonate have a bad habit of holding dust and dirt.

If the panels are mounted flat on the roof of a motorhome, be prepared to get up there and clean them every few weeks.

Amorphous panels have an efficiency of just 6.3%. This of course means that an 64 watt amorphous panel will be more than twice the size of a glass panel with the same rating. This is a major factor if you have limited room on the roof of your motorhome or caravan.

Amorphous panels do not reduce their output when the cell temperature rises above the rated 25 deg – this means that you are far more likely to get all 64 of the watts that you paid for when you buy one of these panels.

Therefore, there are many factors to be considered when selecting solar panels. In our situation, we have all three technologies on the roof of our motorhome (not a good idea – it is best to install just one type of panel) . We have no more room left to install additional panels – this is largely due to the four huge 64watt UniSolar panels that take up more than their share of the roof space. With the benefit of hindsight – I would not have purchased these panels, I would install only glass panels. Your situation may of course be different.

So how Many Solar Panels Do I need? We often get asked “how many solar panels do I need?”. The person asking the question normally expects me to look at the size of their caravan/motorhome then immediately offer a valid technical opinion.

To offer any kind of meaningful answer a number of factors must be known:

1. Average daily power consumption (normally measured in amp-hours).

2. The size and voltage of the battery bank.

3. The average number of sun-hours for the location and time of year.

A few years ago I developed a very comprehensive Excel spreadsheet to calculate all these and other variables. By spending about an hour talking to the occupants of a motorhome or caravan and making a few simple measurements, it is possible to produce a fairly accurate estimate of the amount of solar required to keep the battery system charged.

In developing the spreadsheet, one of my biggest challenges was to find data showing how many amp-hours should reasonably be expected from a given solar panel during the course of a day. After many hours of searching and a few emails, I finally found a table that showed the average sun-hours per day for xxx Australian locations for each month of the year.

It needs to be said that in this context, sun-hours is NOT the same as hours of sun. One sun-hour could arise from one hour of full sun OR two hours of sun at half intensity (or three hours of sun at one third intensity – you get the picture). This data is perfect for calculating the average energy production from a given solar panel or group of panels.