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'Free' power from the sun is getting better all the time.

These days no-one goes bush without a fridge and lights, so powering them when you’re not running your engine is vital for happy camping.


Before the availability of affordable solar panels you had to run your engine for a few hours each day, or power up a generator and risk the ire of other campers.

Now, it’s possible to have ‘free’ fridge and lighting power from the sun, by way of solar panels, a regulator and an auxiliary battery.

When you’re shopping for solar battery charging components you’ll notice that there are differently shaped and coloured silicon cells in the panels you’re inspecting. This is not a styling exercise by the makers, but is indicative of the type of cells used in the panels. All solar cells are not the same.

Some cells are uniformly dark and have silver-coloured, square cut-outs at their junctions to adjacent cells. These are mono-crystalline Grade A silicon cells, formed by cutting wafer-thin slices from a cylindrical ingot of single-crystal silicon.

A variation of this type is a panel with half-sized mono-crystalline Grade B cells, with triangular cut-outs where they join adjacent cells.

Other cells are paler in colour, have a highly crystalline look about them and are rectangular, with straight silver-coloured joining strips and no corner cut-outs. These cells are multi-crystalline or poly-crystalline cells and are sliced from blocks that are formed by carefully cooling molten silicon.

Mono and poly crystalline solar cells are nearly always frame-mounted behind toughened glass, but there are some flexibly mounted examples.

The third common type of solar cell is amorphous silicon, which is a dull-looking, thin-film layer most often seen on flexible plastic solar panels.

There are other types emerging in the market place, including some polymer and nano-technology cells, but the above three are by far the most common used in portable solar panels.


Watt Cell for You

Monocrystaline solar panel


Mono-crystalline cells are the most efficient, delivering the highest current flow for a given cell area, followed by poly-crystalline and then amorphous. This means that poly panels are larger than monos and amorphous panels are larger again.

The up-side is that polys and amorphous panels are usually cheaper than those made of Grade A mono-crystalline cells. Another quirk of amorphous cells is the Staebler-Wronski effect that causes an irreversible drop in output when first exposed to sunlight.

Amorphous panels are manufactured with a higher rating than they have after stabilising, so it’s important that they’re initially oversized.

The decision about which panel type you need is not as simple as opting for the greater efficiency of a mono-crystalline one, because offsetting the mono cell’s theoretical efficiency is its greater sun-heat absorption, caused by its darker colour. A hot solar panel loses efficiency compared with cool one.

The poly-crystalline panel is lighter in colour and reflects more of the sun’s heat, so its ability to remain cooler than a mono panel closes the theoretical efficiency gap.

Amorphous panels are even less efficient, but it’s possible to roll up a flexible panel and that may well be more important than panel area.


Why Solar Power

Solar kits The starting point for solar camping isn’t with the purchase of a panel, but with an assessment of your camping electrical power needs.

It’s important to remember that a solar panel is designed to charge a deep cycle battery that runs your camping appliances. Having the right sized battery is vital, if the panel is to operate effectively, so that’s where your calculations should start.

Assuming you have a 12-volt compressor type fridge that is being run in the shade and used to maintain around +4°C (not in freezer mode), it will need 4-6 amps for up to five hours each day. Converting that to battery drain (6A x 12V x 5hr = 360 Watt-hours per day) and adding four hours of two-LED light operation each day (4hr x 0.6A x 12V = 30Wh)  you have a daily auxiliary battery drain of nearly 400Wh, or 34 Amp-hours.

The typical 4WD dual-battery system or deep-cycle system in a small caravan or camper trailer uses a 100-120Ah auxiliary deep-cycle battery, which may seem like overkill, but battery makers urge that a battery shouldn’t be discharged below a 70-percent level.

Hence, a 100-120Ah deep-cycle battery should be relied upon to provide only one day’s camping electrical power without recharging.

Lithium ferro-phosphate batteries are different, in that they can accept much lower discharge than lead-acid types. However, even lithium gets to the point where it needs recharging.

You can run your vehicle engine – overkill – or a generator – noisy – or more sensibly, use a solar regulator and panel.



It may seem like putting the cart before the horse, but it’s important to choose the type of regulator or battery management system (BMS) before you select the panel type and size.

For example, if you want to integrate the solar panel(s) with generator power and 240V mains power, it pays to buy a purpose-designed battery manager. This type of BMS can integrate different power inputs, including vehicle alternator amps, and handle battery types from lead-acid to lithium.

Also, you should choose a controller that can accept system upgrades, should you find that the solar system needs more panel and/or battery capacity.

Small ‘maintenance’ type solar panels, of the type intended merely to trickle charge batteries of vehicles that are idle for long periods don’t require regulation, because the panel voltage and amperage rates are low.

Solar regulator However, panels that are rated above 40W need regulation to avoid over-charging deep cycle batteries, because larger-panel voltages exceed recommended battery charge voltages. A regulator also stops current leakage from the battery when the panel loses sunlight.

The regulator needs to have sufficient capacity to handle not only the panel’s rated output, but also the ‘spikes’ that can occur in circumstances such as a cold panel suddenly being exposed to full sunlight.

Regulators vary in size and price, from a few dollars up to a few thousand. The cheapest ones are simple on-off devices that monitor battery voltage and switch off solar panel current when a pre-set voltage is reached. These controllers can waste some panel power, as the following example shows.

Consider the case of a solar panel, rated at 80W, with a current capacity of 16V/5A. If your battery is partly discharged, at 12V – down from 12.5V – and the panel is connected, your battery won’t get 80W and 5A of charging value, because the battery voltage pulls the panel voltage back around 12.5V. Your 80W panel is now effectively only 12.5V x 5A = 62.5W.

The next step is a two-stage switching regulator that reacts to a full battery charge by selecting a lower voltage switching level, to extend battery life.

A pulse width modulation (PWM) regulator is more efficient than a switching type, because the PWM unit controls panel current while maintaining a constant voltage at the battery terminals. After a pre-set period the PWM regulator reduces charge voltage to a ‘float’ setting.

The latest development in solar regulators is ‘maximum power point tracking’ (MPPT), which has nothing to do with moving the panels around!

It’s electronic tracking of the panel output and battery voltages, followed by calculation of the best voltage setting that will get maximum amps into the battery.

If we take the same panel and battery example as before, but replace the simple switch controller with an MPPT one we can have a situation where the battery doesn’t get the former 5A, but a boosted 6.4A.

MPPT Chart Another helpful feature of MPPT trackers is that they can use a solar panel which has a nominal rating of 24V or more to charge a 12V battery.

However, many MPPT chargers aren’t dust and rain tolerant and need to be positioned carefully.

Also, there’s a slight efficiency trade-off in small panel installations compared with PWM regulators.

Solar regulators are rated in amps and it’s easy to work out how much capacity you need, by dividing panel wattage by charge voltage – typically 15V maximum. MPPT controllers are rated for maximum panel size in watts and maximum input voltage.

Electrical connections are critical components in any solar panel kit, guaranteeing that all the hard earned solar power is going where it needs to go. The ideal way to connect the regulator to the battery is with heavy duty cable that’s directly connected to the battery posts.

The next best is via a heavy plug such as an Anderson unit. If you must have a small-plug connection between regulator and battery, merit plugs are better than most cigarette lighter plugs.


Beware the cheap solar controller

There are many different solar panels available these days and many on-line kits come with a solar charge controller as part of a package that’s usually around $120 or even less. The chargecontrollers in these kits have a retail value around twenty bucks and should be treated with great suspicion. At OTA we bought one and fitted it to our LandCruiser 75 Series, in conjunction with a 130-watt solar panel.

The panel was connected to our under-bonnet second battery via the low-cost controller and we measured the input voltage at 14.4 volts. So far, so good.

However, even with the second battery fully charged the voltage remained at 14.4V, meaning the controller wasn’t dropping to recommended ‘float’ level, around 13.6V.

We checked the controller’s ‘diode’ capability by running the engine, so that the alternator charged the second battery at the same time as the solar panel was charging it. The controller failed almost immediately and never worked again.

We replaced this cheap controller with a Redarc SRP0120 solar regulator – RRP around $85 – and it’s been functioning perfectly ever since. The regulator is programmable to suit standard lead-acid, AGM, Gel and Calcium lead-acid batteries. It also has a diode function that prevents alternator charge harming the regulator or the solar panel.



Panel Type 

Fold out panel You need to decide whether you’ll use vehicle-top or portable solar power.

The big issue with roof-top units is that at the very time you need solar power – when you’re camped and the vehicle engine isn’t running – the panels are most likely to be in the shade.

Some panels are marketed as ‘shade tolerant’, but that’s misleading, because all solar panels need sunlight.

Roof-top panels in deep shade will deliver very little current.

Another misconception is that roof-top panels are needed to help charge your deep cycle battery when you’re driving along, but panel output is far less than alternator output, so you’re much better off using the vehicle charging system when mobile.

Of course, the best on-board chargers can combine alternator and solar inputs, so you get the best of both worlds.

By far the biggest restriction on roof top panel output is dirt build-up. What’s on the roof tends to be forgotten and it doesn’t take more than a few days camping for roof panels to be contaminated by dust, tree sap and bird poo. Anything that interrupts sunlight cuts power.

Movable panels can be positioned in direct sunlight, if you’re at a campsite, several times during the day. Otherwise, movable panels can be left in a compromise tilted or flat position all day.

Although roof-top panels may be restricted in power delivery they’re much more secure against theft than portable panels.


Panel tilt

The ratings on solar panels are laboratory-measured, using sunlight striking the panel at 90 degrees. In the real world, with a roof-fixed solar panel, that situation happens only the middle of the day, in Australia’s tropical latitudes.

For the rest of the country and at other hours of the day, a roof-mounted solar panel never receives optimum sunlight. A portable panel(s) kit can be positioned every few hours for more optimum sunlight angle and can deliver power closer to its rating.

Some RV and caravan owners have roof panels that can tilt to optimise sunlight angle, but it’s tricky to have an optimum tilt angle that can suit varying campsite positions. If you always orient your vehicle in one direction it can be done.

However, a simple cost calculation – putting the cost and fitting expense of tilting brackets against the cost of an additional flat panel – often sees many owners opting for the latter.

Campervans and some slide-ons and trailers have lifting roofs that automatically tilt their solar panels towards the sun – provided the vehicle is oriented in a generally northerly direction.


 How Efficient

Solar panels convert sunlight directly into electricity with an efficiency between seven percent and 22 percent, which isn’t very good. Of the bright sun’s 1000 watts per square metre of energy, a one-square-metre solar panel can convert this into 70-180 watts of electricity, depending on the type of panel and the angle at which it’s disposed to the sun’s rays.

Although a solar panel isn’t very efficient it provides ‘free’ power (once you’ve paid for the panel and controller) and should have a service life of at least 20 years, barring accidental damage.

Early in the morning and late in the afternoon solar panels have a much reduced output, with maximum output near noon. You can expect to average 40 percent of maximum output per 12 hours of sunlight, on clear days.

A complication in this regard is that some companies are claiming wattages for panels that are simply untrue. We’ve seen test results on some panels that were sold as 120-watt units proving they were actually 80-watters! If a deal seems too good to be true; it probably is.


 Light Not Heat is the Key Power Factor

There’s some confusion in the public mind about how the power from a solar panel is generated. House roofs can be fitted with solar cells or solar collectors. A solar panel is flat, where a solar collector has a flat panel, surmounted by a horizontal tank.

Solar cells generate ‘free’ electricity by converting photons of light energy into electrical energy. Solar collectors produce free hot water by absorbing the heating power of the sun.

A solar cell panel needs sunlight, but not sun-heat, because heat reduces panel efficiency. A solar collector runs on heat input alone.

In solar power terms size does matter: an undersized panel may keep your battery alive for a day or two, but it will gradually go downhill and may never fully recover. A panel that replenishes battery power guarantees good fridge performance and long battery life.


Adding an additional solar panel

Many campers find that their needs change, once they’re accustomed to the convenience of modern batteries – particularly lithium – and solar charging. A soar system that seemed adequate a couple of years ago can be found wanting!

Adding additional solar power is easy, provided you have the correct BMS installed. However, it’s best to seek professional advice about different systems, before adding a panel.

For example, in our case we’ve modified the original system on our Tray Tek slide-on camper that came with a 100 Ah AGM deep-cycle AGM battery and a 200-watt solar panel.

The initial change was to swap the AGM battery for a 100Ah Revolution lithium ferro-phosphate battery and Redarc lithium charger. When the Manager30 came out a few months later, we installed that.

However, it suffered from an overcharging issue, so we swapped it for a Projecta Intelli-RV charger April 2023. We also doubled battery capacity by adding a Projecta 100Ah lithium battery.



We also have a 100-watt panel on the roof of the ‘Cruiser that charges the under bonnet, marine-type, lead-acid second battery and, when that’s full, it adds charge to the slide-on camper battery.

In this form the system works perfectly, provided we get around four hours of sunlight every day. On desert trips in winter that’s no problem, but in cloudy or bushy summer campsites it’s marginal, if we want to camp in semi-shade.

Our solution is a Projecta 120-watt folding panel that we use to supplement the roof-mounted panel. Being portable, it can be placed in full sun, while the roof panel is in semi-shade and we still have ample power to keep the batteries charged.

Connecting the portable panel kit was easy. We simply wired red and black 8mm leads in parallel with the existing panel connections and connected them to a top-quality, weatherproof cigarette lighter socket on the camper.

We located the housing vertically, with the socket facing downwards and covered by its integrated rubber cover.

We cut out the controller that was wired into the portable panel kit and connected  its leads directly to a heavy-duty cigarette lighter plug.

A word of caution: not all existing battery and charger installations allow such simple retro-fitting, so be sure to contact the supplier of your original equipment to ensure that’s the correct procedure.













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