Batteries | Solar | Inverters| T1 Lithium

Technical Stuff


Rigid panels, semi-flexible panels, roof mounted, portable, awnings made of solar panels, a few high watt panels or lots of smaller watt panels, mixed sizes.  There's a lot to write for this section, so stay tuned - more will be added as the blisters on my fingers heal...

Testing your solar panels

The first step is to know how many amps you should be getting from each panel.

So, let's say you have 150 watt panels and your RV is wired for 12 volt, you should get 150/12 = 12.5 amps from each panel.

What you'll need:

  • Old towels or blankets or something to cover each individual solar panel
  • A good solar day, away from shadow
  • It will help to have someone inside to take readings
  • You will need to be not charging from mains, generator or DC to DC.

How to test:

  • Cover all your panels with the towels or blankets
  • When all your panels are covered, remove one covering and take a reading in amps (A) of your solar input from your solar regulator, making a note of which panel you’re testing
  • Cover that panel again, remove the cover from another panel and take a reading again. Repeat until you’ve done them all.

Some things to keep in mind:

  • Each time you want to expose only one panel at a time
  • Make sure there is no shadow over any panel
  • If a cloud comes over, you need to wait for it to go before continuing your readings
  • While you're up there, take a look for dirt, shadow from aerial, vents, etc and damage such as chewed wiring or burns in the photovoltaic cells (they look like cracks):

Burn lines in photovoltaic cells

Burn lines in photovoltaic cells

Do you need more solar panels, or is your cabling not up to scratch?

So you're not getting as much charge out of your panels as you think you should.  You've tested all your panels using the steps above and they're all putting out pretty much what they should.

Logic says put more panels on your roof, but if your cable from panels to solar regulator to battery is not thick enough, you'll lose voltage as charge travels along the cable.

It's a bit like installing a second water tank at your house - if you're piping your water through a garden hose, adding another tank is not going to give you more water.  You need to put in a wider hose so more water can flow from tank to house.

So, how to ascertain if cabling is a problem?

On a good solar day, when you've got good sun coming onto your panels:

  • Use a multimeter to measure the voltage at the back of one of your panels.
  • Now measure the voltage where the solar comes into the regulator.
  • Next, take a reading of the battery voltage.

If there is a voltage drop anywhere between these 3 readings, your wiring is substandard and adding more solar isn't going to help.

Connecting your solar panels in series or parallel - what's the difference?

Whether you connect your solar panels in series or in parallel, the same principle applies as for batteries:  

  • Parallel connection means all the positives are connected together and all the negatives are connected together.
  • Parallel connection gives the sum of the amps of each panel output without increasing the voltage.
  • Panels connected in parallel work best with a PWM (Pulse Width Modulation) controller.
  • Series connection means the negative from one panel is connected to the positive from the next panel and so on until you end up with one negative at one end of the string of panels and one positive at the other end of the string of panels.  That negative and positive then go off to the controller.
  • Series connection gives you the sum of the voltage of each panel output without increasing the amps.
  • Panels connected in series work best with an MPPT (Multiple Power Point Tracking) controller and don't work well at all with a PWM (Pulse Width Modulation) controller.

Advantages and disadvantages of connecting panels in parallel

Advantages of parallel connection:

  • You can use mixed sizes of solar panels.
  • You can buy a very good PWM controller for much less than a good MPPT controller.
  • A PWM controller needs enough power to light up its display.  It only really starts to use power when there is more solar than your battery requires and the controller needs to limit the solar coming in.  In this case, the power consumed is excess to requirements anyway.
  • A PWM controller doesn't produce a lot of heat, so doesn't require special cooling.

Disadvantages of parallel connection:

  • You need more cable
  • You need thicker cable

Advantages and disadvantages of connecting panels in series

Advantages of series connection:

  • You need less cable.
  • Your cable doesn't need to be as thick as that for panels connected in parallel.

Disavantages of series connection:

  • A good MPPT controller is quite a bit more expensive than a PWM controller.  A cheap MPPT controller will not be a good one.
  • MPPT controllers will use some of the energy being harvested from the solar to run the MPPT controller itself.  In the early hours of the morning, later in the day or on a poor solar day, this may mean that the small amount of power being harvested is mostly taken by the controller itself, or that harvest may not even be enough to start the MPPT controller.  So when you most need to harvest all the solar available because it's a lousy day, the MPPT controller will not give you the best return.  MPPT controllers are best suited for large solar installations for this reason.
  • MPPT controllers can generate a large amount of heat, depending on how much it has to alter the voltage coming in to suit the voltage of your RV.  So, if you're harvesting, for example, 150 volts, to feed into a 12 volt RV, the MPPT controller has a lot of work to do and will generate a lot of heat.  Thus, MPPT controllers are manufactured with a large fan and/or heatsink to disperse this heat.

Can you mix different sizes of panels?

Yes, you can, but they must be connected in parallel (all the positive leads joined together and all the negative leads joined together) and your solar regulator must be of the PWM (Pulse Width Modulation) type, not of the MPPT (Multiple Power Point Tracking) type.

Semi flexible panels - do they work?

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

They also have a huge weight advantage over rigid panels. But that 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 can be even more severe when a truck is passing you from the opposite direction. 

Semi-flexible panels also don't have the inbuilt cooling that rigid panels have by design.  The aluminium frame of a rigid panel keeps the panel away from direct contact with your RV roof.  Semi-flexible panels don't have this frame, so they require some method of keeping them off your RV roof and as cool as possible.

What to look for when buying panels

Below are some statistics that should be available for any solar panel you wish to purchase - these figures are available on the back of the panel.  We've also noted the values of those settings that you will get the best performance from and what the acronyms mean:

VMPVoltage Maximum Power16 to 18 volts
OVCOpen Circuit Voltage20 to 22 volts
ISCShort Circuit Current5.97 amps for a 100 watt panel, or as high as possible*
IMPAmps Maximum Power5.6 amps for a 100 watt panel, or as high as possible*

The most important value, and the one to start with, is the VMP. When you find panels you like with the right VMP, then compare the ISC and IMP.

* You'll only get 1 or 2 percent higher ISC or IMP on some panels, but it's worth it, depending on cost of course. There's no point paying heaps more for that 1 or 2%, but it's worth paying a little more. If someone has panels where the ISC or IMP are way higher, say 10% higher, they're taking you for a ride.

Watch out for panels advertised with just a Maximum Working Voltage. A maximum working voltage of 16 is not a figure - it's an advertisement. You need the figures above from the back of the panel.

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, they're trying to take you for a ride. There are variations on how much power you can get out of a certain area of panel, but those variations are not big.


It's important to keep your solar panels clean.  We're probably not telling you anything you don't already know.

We highly recommend that you also polish your panels.  The first time, use a cut and polish compound and thereafter, use a wax polish.  This will make the in between cleans with a hose much more effective at removing the dirt.

Beware the cockatoos!

Yep, they're quite partial to gnawing on solar panel wiring, and the results can vary from annoying and expensive to downright dangerous.  Try to keep your wiring under the panels and/or in conduit.

The use of solar energy has not been opened up because the oil industry does not own the sun. - Ralph Nader, Loose Talk