Buying solar panels

Physical size

Compare the physical size of the panel with its wattage to get an idea if what the seller is claiming is realistic. For example, if someone is trying to sell a 200 watt panel but the size is a lot smaller than other 200 watt panels, be wary. There are variations on how much power you can get out of a certain area of panel, but those variations are not big.

Consider using smaller panels (we use a lot of 60W panels). It takes only a small amount of shade from a power line or even a bird poo to drastically cut the amount of power you'll harvest, if any at all from that panel. By splitting your solar up into smaller panels, you'll lose less from any one interference.

Monocrystalline or polycrystalline?

Polycrystalline solar panels are older technology, but tend to be a little less expensive. They are more heat tolerant than monocrystalline panels.

They also tend to be a little larger than monocrystalline panels for the same power output, so where space on an RV roof is at a premium, it's probably best to buy monocrystalline. Polycrystalline panels are usually used on large solar installations such as homes and solar farms.

Historically, monocrystalline panel technology has undergone a lot of R&D, resulting in the improved power per area. However, monocrystalline panels have been developed for the European market where temperatures are much lower than in Australia.

Polycrystalline panels are now undergoing a resurgence in R&D, particularly in relation to their superior heat tolerance and as large solar installations are becoming increasingly popular as a power source.

Semi-flexible or rigid?

In terms of output, semi-flexible panels work very well - better than rigid panels in fact.

The weight advantage of semi-flexible solar panels is also significant - around 3kg for a 100W panel compared to around 10 to 13kg for an equivalent rigid panel. But semi-flexible panels come with their own challenges, not the least of which is that they're quite expensive for quality panels.

The lower weight can also be a problem. You need to be very careful that they can resist the force of wind (consider that you may travel at around 100kph to 120kph on the open road), potentially getting under the panel and lifting them off your roof. The wind problem is even more severe when a truck is passing you from the opposite direction, particularly for panels at the front of your RV. The obvious answer would be to glue them directly to the roof of your RV, yes? No. Read on...

The importance of cooling

The output of a solar panel is measured at 25°C. As the temperature of the panel rises - and in Australia the panel can get to 70°C - the claimed output of a panel drops dramatically.

Rigid panels also have issues with air circulation, particularly if the frame of the panel is glued directly to the roof. Heat from the panel gets trapped in that pocket of air under the panel and there is no air flow, due to the frame of the panel, to allow that heat to escape.

Consider also that by gluing the panel directly to the roof, you are also providing ideal conditions for water to collect in that space and cause damage to your RV roof.

Semi-flexible panels don't have a frame, so they require some method of keeping them off your RV roof and as cool as possible. Some 4WD vehicles come equipped with a roof-top cargo area, which is essentially a rectangular frame with cargo mesh in the middle. We've seen instances of semi-flexible panels attached to the cargo frame and supported by the mesh, and the cooling provided with this setup works very well indeed.

Semi-flexible panels are also prone to expansion and contraction with fluctuations in temperature. So is the RV that you attach them to. However, the panel will not expand and contract at the same rate as your RV. You should therefore consider that whatever method you use to attach them to your RV will allow for this difference in expansion and contraction.

Blocking diode

Diodes are used to ensure electricity can flow in only one direction.

A blocking diode is used in a series connection to stop electricity from flowing backwards into a solar panel, thereby burning out the connections between the photovoltaic cells that make up a solar panel, even getting hot enough for a poorly soldered joint to separate.

This backward flow of electricity can occur when there is no longer a load using the electricity generated by the panel, such as when your battery is full.

Most rigid solar panels on sale these days come fitted with a bypass diode, but rarely with a blocking diode. Flexible panels also often have bypass diodes, but they tend to fail when they get hot. We have learnt from bitter experience that a blocking diode is very important in an RV because you're not feeding into an ever-present mains grid, you're feeding into a battery which in good solar conditions can become full.

Blocking diodes can still be installed at the end of each solar cable, before its connection to the neutral link in the junction box on your RV roof where the cabling from all your panels meet.

Bypass diode

A bypass diode is connected in series and it runs across the negative to the positive, either inside or outside the panel. You will most often find these in the box behind the panel where the positive and negative cables come out.

A bypass diode allows current to flow from the previous section of a series-connected solar array (eg, panels 1 and 2) and flow through the bypass diode of the shaded panel (eg, panel 3), rather than through the panel itself, to the next panel in the array. This is important, because if current flows into a solar panel that is not producing current itself, that current will cause the panel to overheat and it will be become damaged. Remember, a solar panel doesn't need much shade to stop working, especially if the shade is across the panel.

A bypass diode is more likely to be needed with panels that are connected in series

Deciphering the back of a solar panel

Maximum rated power, short circuit voltage? What does it all mean, what's important and what is just a number to lead you up the garden path? How to interpret the information on the rear of a solar panel and apply it to your setup.

Series or parallel connection?

Serial connection and parallel connection each have their advantages and disadvantages from the size and amount of cable required, to what type of solar regulator works best with each.

Solar cabling

Our recommended cable sizes to connect multiple panels; extending that cabling to a junction box and the size of cabling from the junction box to your solar controller. We also look at the vagaries of MC4 connections and the need for circuit breakers so you can isolate your solar input.

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MANNUM  SA  5238

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