solar charger design

Assemble your solar charging system in four simple steps:





If this is intimidating, or more than you want to read, just let us design your system or give your plan a sanity check.  We’ve been doing this for a very long time and we’re happy to help.

Determine how much solar wattage you may eventually want.

Contemplate Constraints

Solar charging systems are typically limited by physical space, budget, and energy requirements.  Since a system can be expanded over time, the most important constraint is the physical space where you can mount solar panels.  If you’re pretty sure maxing out your available roof space is going to be cost-prohibitive, factor in a budget constraint.  If you are only going to power some small loads, and don’t see that need changing in the foreseeable future, then you’ll likely want to do some production calculations and establish an energy production goal.

Estimate Solar Production
In Eugene, Oregon (near our headquarters) a tilted 100W solar panel will produce on average, throughout the entire year, 295Wh per day.  Since we are in a somewhat overcast climate, at a latitude further north of most of our customers, we feel confident using a 3x multiplier for keeping the math simple and approximating daily solar productions in terms of watt-hours based on a known solar array wattage.

This means that if you have 400W of solar panel, you’ll get about 1200Wh of energy production per day.  Of course, you get more than this in the summer, less than this in the winter, and none if you park in the shade.

Measure your roof

Some simple ways to see what solar panels fit on your roof would be to make cardboard rectangles the size of the solar panels and place them on your actual roof.  You can also use painter’s tape to make a scaled version of your roof on a floor.  Don’t forget to take into account space for mounting hardware, a path to walk on or access the roof, and any obstructions like vans, vents, antennas, etc.  Panel dimensions are clearly listed below.

Plan for portable panels
Keep in mind that you aren’t always limited to what can fit on your roof.  AM Solar sells a wide variety of portable panels that can work along with a roof-mounted solar array.  When we work with portable panels, we prefer to remove the factory solar charger, and route the portable panel to the same charge controller that the roof array is using.  This gives you several advantages:  A better quality MPPT charge controller, higher line voltage means less line losses, monitoring of all solar on one system.

Sum the wattage

After you determine what solar panels fit on your roof, and what portable panels you might want to add, sum the wattage, and use that number to select the appropriate charge controller kit.

Select the appropriate Charge Controller Kit.

Quick Current Approximation:

When you are dealing with small, parallel-connected solar panels that have a Vmp of around 20V, you rarely bump into the limitations of maximum open circuit voltage (Voc) or maximum short circuit current (Isc), and you can get away with a simple calculation:

Array Wattage / Battery Voltage x Efficiency = Charging Current

Wattage – Sum the wattage of all the solar panels in your array.  It doesn’t matter if you plan to connect panels in series or parallel, the wattage stays the same.  With an MPPT charge controller, the only the voltage x current of the solar array is proportional voltage x current of the MPPT charge controller’s output.  In other words, just because your solar array current is 10A, doesn’t mean your charge controller ouput will be 10A.

Voltage – This will either be 12V or 24V in an AM Solar system.  Even though you may have a 12.8V nominal lithium battery bank, just round to 12V.

Efficiency Factory – We use 90% and this accounts for line losses, less than perfect weather conditions, and a charging voltage slightly higher than the nominal voltage

With this, you will get a good approximation of the peak charging current that your array is capable of producing.  For example, if you have room for two 200W solar panels, and a 12V lithium battery bank, the math works out like this:

2x 200W / 12V x 0.9 = 30A

This formula has wiggle room, and they are summarized below with a charge controller vs. solar wattage recommendations.

15A < 230W

20A < 300W

30A < 450W

50A < 700W

85A < 1200W

100A < 1400W

This shows that you would use a 30A charge controller for two 200W panels.

As mentioned above, this Quick Current Approximation works most of the time.  You can run into problems with Isc on large arrays, this is especially common with the Victron 150/70, and 150/100 charge controllers.  For example, 7x 200W panels would give a 105A with the formula above, and this is fine for a 100A charge controller except that the 200W panels have an Isc of 10.2A and the controller can only handle 70A.  You would exceed the Isc limit by 1.4A, and possibly damage your charge controller.

For more details, and specifics on making an efficient solar array keep reading.


Critical Considerations:

A solar array delivers voltage and current to a charge controller.  In order for your system to work, you need to make sure that the current and voltage are within the tolerances of the solar charge controller.  The highest current a charge controller will see is the Isc of the array, or the Short Circuit Current.  The highest voltage a charge controller will see is the Voc of the array, or the Open Circuit Voltage.

You can determine the Voc of your array by looking up the Voc of the panels in your array and summing them for series connections.  For example, if you have four panels in series with a Voc of 25V, your array Voc is 100W.  If you have four panels connected in 2×2, your array Voc is 50V.  If you have four panels in parallel, your array Voc is 25V.

This Voc number is so important that it is the first number you see in the part number of a SmartSolar MPPT charge controller.  For example, the 100/30 charge controller can handle an open circuit array voltage of 100V.  The 150/100 charge controller can handle 100V.

You can determine the Isc of your array by looking up the Isc of the panels in your array and summing them for parallel connections.  For example, if you have four panels in series with an Isc of 5A, your array Isc is 5A.  If you have four panels connected in 2×2, your array Isc is 10A.  If you have four panels in parallel, your array Isc is 20A.

Lastly, and this comes up very infrequently with 12V systems, is that you’ll want to make sure your Vmp  (Voltage at maximum power) exceeds your nominal battery bank voltage by a healthy margin.  A voltage that is too low won’t damage anything, but it also won’t result in any charging current.

For example, we have had success with 17.7Vmpp panels on a 12V battery bank, but I prefer to keep the array over 18V.  That means if you have a 24V system, you need an array with a Vmp greater than 36V.  With standard, 36-cell solar panels, this means you’ll need some series connections when charging a 24V battery bank.  If you have 18Vmp panels, you won’t want to connect them in parallel for a 24V battery bank.

If you can meet these current and voltage requirements, you will have a system that works and won’t damage equipment.

Output Clipping and Under Sizing:

If you graph the wattage output of a solar array over time from morning to night, you will see a bell curve distribution, with a peak at mid-day.  If you have a solar array that doesn’t exceed the Voc or Isc limit of the charge controller, but is capable of producing more power than the charge controller can pass, you will get output clipping.  This doesn’t harm the charge controller, it just gives you a bell curve with a flat peak.

An example of this would be four 200W panels on a 100/50 charge controller.  On a very bright day, an 800W array might be able to feed 60A onto a 12V battery bank, but if you use the 100/50 charge controller, it will be limited to 50A.  The 10A that could be, with a larger charge controller, isn’t dissipated as heat, it just doesn’t exist, because the charge controller draws power at a less efficient point along the IV curve in order to keep from overloading.  If you don’t know what I’m talking about in terms of the “IV curve” I have a video on that.

Along similar lines, if you have an undersized solar array, or oversized charge controller, this is perfectly reasonable.  We design many of our systems this way, to give our clients room to expand if they want to enlarge their solar array.

Factoring In Temperature:

A question that came up a few times in the YouTube comments on my last video was “What about low temperatures raising the voltage beyond what the charge controller can handle?”

For us, we recommend parallel connections, or maybe two panels in series at the most, so we are typically nowhere near the input voltage limit of the charge controller.  But, I can go over a scenario showing how this would work.

Let’s say you have four 200W panels that you want to put in series on a Victron 100/50 charge controller, and the Voc of the panels is 24.35V with a temperature coefficient of Voc of -0.30% per degree C.

At 25 degrees C, the Voc of the array is 97.4V and the charge controller is rated to handle 100V.  If you go down 1 degree C, the Voc will go up 0.30%.  That means, at 24 degrees C, you will have a Voc of 100.3V.  I don’t think Victron is going to warranty an operator error like that.  If you are close to the Voc, it might be worth running some calculations to see how close you get when it is really cold.

Add Solar Panels and build your array.

Series vs Parallel:

If you are using a 12V battery bank, try to keep all your solar panels parallel connected.  If you are bumping into an Isc constraint or using a 24V battery bank, connect your solar panels in parallel groups of two in series.  Try to avoid putting more than two panels in series, because the higher voltage that this creates may require you to use different cables and disconnect switches.


Mixing different panels:

All of our panels can be mixed in the same system, but some configurations are more efficient than others.  Using one type of solar panel, or parallel connections of solar panels that have the same Vmp is the most efficient.  If you parallel connect two panels with a different Vmp, or series connect two panels with a different Imp, the combined panels will be forced to operate at a weighted average Vmp (parallel) or Imp (series).  This new operating point won’t be optimal, but it will be close.  The loses from the sub-optimal configuration will be much smaller than the wattage contribution of the odd panel.

Panel Kit Variants:

Each panel we sell on our website has a drop-down menu with several kit variants.  Make sure you get the variant that matches what you want to connect your panel to.  For example, kits that use the new Zamp combiner box (15A and 20A kits) work with panels that kits that end in Cable or ATP connectors.  Our larger kits use the AM Solar combiner box which is compatible with panel kits ending in Cable.  If you want to add a series panel to a panel going to a combiner box, select panel kits ending in “S”.  If you want to connect to a standard Zamp combiner box, get the panel kits ending in SAE.  If you already have mounts and a wire harness, and just want a panel by itself, get a Raw panel.

Add Sealant.

Why you need sealant:

Sealant is used to protect your VHB tape from the elements.  Completely bury the mount foot in sealant to ensure that the tape is sealed.

What sealant to use:

If you have an airstream, van, skoolie, or metal roof, use SikaFlex.  If you have a fiberglass, or TPO roof, use Dicor.  You can use Dicor on metal, but don’t use SikaFlex on TPO. Dicor sealant is used to seal the inside of the combiner box.

How much you need:
Your first tube of sealant will come with your charge controller kit. This will be sufficient for the combiner box and your first two panels.  Beyond that, you will want one tube for every three panels.