7 Tips for Sizing your PV Array to Meet Your System Goals

By Brad Berwald, Senior Product Manager, and Douglas Grubbs, Product Applications Engineer

Solar charge controllers offer a variety of input voltage windows.  They can vary from 30V to over 500V.  Sizing your PV isn’t as simple as just “filling up” the controller’s capacity.  There are many considerations to take into account depending on the needs and goals of your off-gird system. PV module choice and the arrangement of the modules in the system is essential.  The number of modules in series and the number of parallel strings impact system efficiency, array optimization, and other benefits you may not take into account at first glance.  Let’s explore several examples of where making the right choices can not only prevent system problems but give you an advantage in your system’s performance and solve a few unique challenges!

  1. Obtaining Site Flexibility For Residential Systems 

Any solar systems need a clear view of the sky for collecting the most available solar power. Still, that location does not always coincide with where the system needs to provide power, especially not in off-grid applications.  You’d love to bring the freedom of a renewable energy system to your Northern cottage, but it’s a little too far into the woods from that clearing to make it practical.  So what options do you have?  Placing the PV Array a distance from the system can be a challenge given wiring needs and the fact that you have to deal with voltage drop over distance.  But there are ways around this.  

Sizing for a very high input voltage using Morningstar’s TS MPPT 600V is just one solution.  With an operating PV input voltage of up to 525V, you can easily make runs of up to 100m or more feasible and still provide 48V charging at the system.

  1. Overcome Difficult Site Conditions

One of the oldest and earliest adopters of remote power is the railroad industry.  The national railway network consists of thousands of miles of remote track, and monitoring and safety are a top priority.   For this reason, the railroad industry often turns to Morningstar products to offer remote power solutions.  A railroad project that Morningstar participated in had a unique need.  A solar system was required to power new signal installation on a railway bridge.   The challenge was in the location.  The signal was on the north side of the bridge, where there was no direct sun access.  The array needed to be placed on the south side to be most effective.  Most railroad systems are sized at low-voltage 12VDC for the trackside equipment, and Morningstar’s Tristar MPPT 150V provided the perfect solution. The solar array was placed on the southern side of the bridge over 20 meters away.  At 80VDC, the array voltage was more than sufficient to provide the power needed to cross this distance to the signal load while maintaining good conversion efficiency and accommodate the low voltage equipment.  Problem solved.

3.  Sizing with Efficiency as the Top Priority

The MPPT charge controller’s efficiency in converting solar voltage to battery voltage is dependent on a few factors. The most significant contributor is the ratio of input and output voltage.  In the above examples, high to low voltage conversion gives real benefits, but it does reduce efficiency by a few percentage points due to the significant voltage difference. When the system’s goal is maximizing solar output to the battery above all else, opting for a nominal solar array that closely aligns with the battery voltage is ideal.  For example, in a 48V battery system, a solar array with a Vmp of !72V is ideal.  When more closely matched to the battery, there is less conversion taking place and this increases efficiency significantly.  When controller loss is reduced, more usable power goes directly into the battery.  This type of array wiring is ideal in hot climates where system cabinet temperatures are a concern since it also minimizes waste heat from the controller.

  1. Use Good System Design to Save Money on Wiring

High voltage doesn’t just have its benefits in long-distance runs.  It is also a source of significant cost. Larger wire gauges, especially when sheathed for burial, can be costly due to the material costs.  Using higher voltage sizing can reduce wire size, which directly translates to cost savings. Try this path when you size your next project, and you can leverage the benefits of an MPPT controller while also being kind to your project budget.

  1. Systems that are “On The Move”

Anyone who has installed a complex roof or ground mount for an array knows that a well-designed PV system’s mounting requirements can pose a challenge.  Positioning for the optimum angle, avoiding shading sources, and securing it for high winds are all required for a system to perform well and last the test of time. But not all systems are stationary.  In many applications, PV systems may face dynamic conditions in a vehicle or mobile system.  They are used to provide temporary site power or be part of trailer-mounted systems providing security and signaling on highways.  Some systems may even accompany you on an extended journey.  RV and Marine systems find themselves with a new location as often as every day, and considerations can be taken when choosing how you configure your array so that it makes the best of a less than ideal location.

An MPPT controller will use the available PV power and optimize its tracking based on the IV curve of the module you have chosen.  That curve may change considerably in different orientations.  The system must be flexible because you can’t always park your vehicle or trailer in an ideal place.  Some systems may even roam freely, leaving their location out of your hands entirely.  When would this occur?  Here is an interesting example of a unique system that floats freely in the ocean!

Navigation signals and lighted waterway markers have employed battery power for decades.  Thankfully, the latest generations of these designs are solar-powered, significantly reducing the amount of primary cell battery waste generated in the past.  But how do you point an array towards the sun when it is free to spin and bob in the ocean’s waves?  By providing multiple PV planes and using an MPPT controller to handle the complex scenario.  Placing three modules on the sides of the buoy, it can now generate power in any orientation.  The IV curve will be the result of the power curve of any of the three modules. Depending on mother nature, the IV-curve may result from the combination of 2 modules providing power in parallel.  Morningstar’s SunSaver MPPT has been a preferred choice in these applications due to its small size, marine rated enclosure, and the fact that it offers Morningstar’s Trakstar MPPT algorithm, the same used on our larger models.  It is easily up to the task of keeping up with such dynamic solar conditions on the array.  Its data connectivity can send solar data along with existing environmental sensor data for many oceanic research efforts.  That’s a lot of value in a small package.

  1. Make the right module choice.  Cell count matters in Battery Systems

With module technology evolving rapidly, more wattage is squeezed into PV module frames each year.  Module manufacturers have been putting an increasing array of cells into their designs, and the count, efficiency, size, and latest semiconductor materials have evolved considerably.  While grid-connected systems are less impacted due to their higher operating voltages, some deliberation should be used when choosing your module type for battery-based systems.  For example, 36-cell and 72-cell modules have always been an excellent nominal match for 12V and 24V systems, respectively.  The intermediate design of 60-cell modules is not a good fit for 24V battery systems with a PWM controller, and 12V systems can only take advantage of them when paired with an MPPT controller.  This is due to their slightly lower cell count, which may not provide sufficient voltage during days of extreme heat and high module temperatures.  

Morningstar has taken much of the guesswork out of these decisions with Morningstar’s String Calculator.  This powerful software tool contains all of Morningstar’s controller specifications and will reveal several ideal PV array configurations when paired with a module choice from its extensive database.  Morningstar regularly updates the String Calculator with the latest modules on the market while providing a historical list of models from years prior (over 110,000 entries, to be exact). These new additions also include the latest modules that incorporate “Perc” cell technology and accounts for their slightly higher voltage characteristics.  Automatic compliance checks are performed for common issues that may arise with a misconfiguration, such as over-voltage or temperature impacts that may limit charge output.  Since all Morningstar MPPT controllers can be oversized to maximize charge controller yield, we provide some extra headroom on sizing to let you know when you can “push” a system a little bit for some extra power.

  1. Oversizing PV When It Makes Sense

It’s common practice in Grid-Connected system design to oversize the array PV Wattage to 120-130% of the inverter’s continuous wattage rating. During full sun, the energy is limited to the inverter’s maximum output rating, but during periods of low insolation, this allows for greater total power delivery over time, increasing the yield of the inverter system.

It’s less commonly known that this sizing practice is also possible on all of Morningstar’s MPPT controllers and it can yield very positive benefits in off-grid systems as well.   Thus, the site can have extra wattage available, over the continuous rating of the controller added at the PV input, and the controller will only use what it is capable of supplying while limiting output power.  Its maximum voltage input rating still does need to be adhered to and you should consult our operation manual for specific detail so maximum power rating, temperature impacts, and input voltage are taken into consideration.

This extra harvest provides better overall system reliability and more consistent power delivery each day despite varying sunlight levels.   It uses more of a less expensive resource (the PV array) and ensures that a more limited resource (the controller and system components) are used to full capacity for a longer period of time. Consult Morningstar’s String Calculator for guidance.  In systems we have consulted on that were used in large telecommunication applications, the array can be anywhere from 3-10kW or more, and this oversizing technique really brings the power level to a more consistent level and helps to offset diesel genset runtime, the fuel for which is costly and difficult to supply in remote locations.