We receive questions every week from customers who want to confirm which size Victron MPPT is right for their system. There are a few factors to consider, and it helps to understand how you arrived at your choice, especially if you plan to add a solar panel or double your battery bank in the next couple of years. Here’s everything (or nearly everything) you need to know. Still not sure after reading this? We understand, it’s not always a straightforward choice. Send us a message describing your use case, and we’ll confirm the right MPPT solar charge controller for the job.
MPPT Specifications
Victron MPPT Model Numbers And What They Mean
When you start to explore Victron’s extensive product line, you’ll notice that many products have similar names and numbers. While this can be overwhelming, it's important to understand that Victron has specific reasons for this approach. In true Victron fashion, the goal is to ensure that there's a suitable solution for every scenario. Since we're focusing on solar charge controllers, particularly the MPPT (Maximum Power Point Tracker) product line, let's look at an example:
This is a SmartSolar MPPT 150/100 - TR VE.Can. Let’s break it down piece by piece:
SmartSolar = The Victron Bluetooth-capable charge controllers (as opposed to the non-Bluetooth BlueSolar option)
MPPT = Maximum Power Point Tracker (vs a PWM charge controller)
150 = Maximum PV input voltage. In this case 150 Volts
100 = Maximum battery charger current. In this case 100 Amps
TR = terminal style connection. There’s also an MC4 in most larger Victron charge controllers.
VE.Can = capable of connecting via the VE.Can (Victron Energy Can) network, via an RJ45 plug. There is also a selection of non-VE.Can capable MPPTs, which simply lack the VE.Can designation in the product name. All Victron MPPTs are able to be connected via the 4-pin VE.Direct ports.
This unit, one of the larger MPPTs, also has the optional “SmartSolar Pluggable Display” installed, offering some visual feedback on the unit directly. Any unit with the slot on the front can have this optional device installed - a great way to get some extra info directly on the MPPT with a relatively low-cost install.
PV Input vs. Battery Output
That’s a lot of info in a product name, but most of what we are interested in, when it comes to choosing the right size MPPT for your solar array, are the numbers in the middle. From our example this is the 150/100. It’s important to take note that these are on two different sides of the PV > MPPT > Battery sequence.
The 150 means that this unit can handle a maximum of 150V on the PV input. EG: the collection of panels in your array that are in series, adding up their voltage. We’ll discuss how to calculate that below.
The second number, 100, is the maximum amperage that this unit will be able to deliver to your battery bank. You can get a very basic idea of how much power your array might deliver by taking your total array wattage and dividing it by the battery bank's voltage. Let’s say for example, a 1000W array: 1000W / 12V = ±83A. Now this is a really rough estimate, not taking into account a lot of the more fine-grain details that are related to how you design your series/parallel setup in your array, and the exact specifications of your panels, but it’s a great place to start.
Panel Specifications
Now that we understand how the MPPTs are defined, your solar modules are the next, and somewhat more complicated, section to consider before making a final selection. Let’s again look at an example from a real-world product. This example is from a Philadelphia Solar Nexus module, but you’ll see similar information on nearly every solar panel datasheet out there. We’ll be discussing the numbers from the 450W module.
Voc vs Vmpp
Voc and Vmpp are similar numbers, and both refer to the voltage of the modules. Voc = Voltage Open Circuit. This is the maximum voltage that a panel will reach while under no load, essentially unconnected but in full sunlight. In many ways, it’s the “worst case” voltage, and is the primary number we’ll consider when sizing for an MPPT, as this voltage can be seen by the MPPT before it begins its charging cycle. This number is the one that you’ll use when determining the maximum voltage of your PV module series in an array. In our example Voc = 38.84V.
The Vmpp = Voltage Maximum Power Point. You’ll notice the “mpp” is the same as our MPPT solar charge controllers. The job of the MPPT is to hit this target on the panels, which it does through a complex series of rapid tests that it performs each second. A good MPPT will constantly be adjusting the load it places on the panels to deliver the maximum amount of power, and a well performing MPPT will likely result in the panels' voltage being around the Vmpp while under load. In most cases, this isn’t a critical number to track for sizing but in a smaller array, one with lower voltage, it is helpful to make sure that this Vmpp number at least exceeds your battery bank's charge voltage for whatever series of panels you have. In our example Vmpp = 33.51V.
Isc vs Impp
Isc and Impp are similar to Voc/Vmpp, but are referencing the panel's current rather than voltage. Isc, like Voc, means Current Short Circuit. If you were to connect the positive and negative of your array, you’d be creating a circuit with essentially unrestricted flow, and in full sun, it would theoretically result in this Isc current. This number is used to determine the maximum potential current your panels might transmit over your systems wiring, through fuses, etc. It’s the number you’ll use to then determine how large your fuses will need to be, and it’s involved in calculating the sizes of your fuses and wire. In our example Isc = 14.47A
Impp, like Vmpp, is the amperage the panels will produce when under an ideal load in full sunlight. EG: how much power they’ll be passing while charging your battery. In our example Impp = 13.48A.
Wattage: What does it mean?
Wattage is by far the most advertised number when it comes to solar panels, and it’s a very versatile number. At its core it’s simply Voltage x Amperage (V • A = W). In the case of our Philadelphia Nexus 450W module you’ll see that multiplying the Vmpp by the Impp will equal the wattage of the panel. EG: 33.51V • 13.44A = 450.37W. Wattage, while very handy for shorthand calculations, is often best used as a broad reference point.
Voltage coefficients and why they matter
Voltage coefficients, and to a lesser extent current coefficients, are probably one of the more misunderstood (or even entirely ignored) elements when calculating a good fit for your array + MPPT combo.
The key reason these numbers are included on any quality panel is that, as temperature changes, so does voltage and amperage. An increase in temperature will result in higher resistance in conductive materials like the copper that connects all the individual cells in your PV panel, and even in the wire between the array and the rest of your system. As the temperature drops, that resistance also drops. What this means, practically, is that the warmer it gets, the lower the voltage of your system. Perhaps more critically, as it gets colder, that voltage will rise.
Both directions are important to keep track of, though the critical one is most often how temperature drops affect your system's voltage. On a PV module's datasheet, these numbers are often found in a separate section from the voltage/amperage numbers. From our Philadelphia panels:
Let’s discuss the “Open Voltage Temperature Coefficient” specifically (the current coefficient also comes into play, but it’s often so much less impactful that we can mostly set it aside - if you need to calculate it you’ll use a similar method to voltage). This number, in this case “-0.25,” is the amount of change in voltage that your panels will produce for each degree Celsius above “nominal”, which is pretty universally 25°C (±77°F).
When calculating the voltage of a series of panels, it is pretty critical to include this calculation, unique to every solar module, and to account for the climate that your array will be used in. For more temperate climates, this difference will be quite a bit smaller - in Hawaii, where it essentially never freezes at sea level, you would see a much smaller range, but at the peak of Mauna Kea, the 13,803’ peak on Hawaii, this range will be much higher. The right MPPT at sea level might not be right at that peak where temps drop down into the 10-15*F range. In many northern states, Canada, and Alaska, temperatures might reach as low as -40°F (also -40°C).
Let’s calculate this based on some basic numbers: -40°C (-40°F) and +70°C (±158°F). To include the coefficient, which is a % change per degree C, we’ll need to take the Voc, the difference in temperature between “nominal” and the low temp, multiplied by the Voltage temp coefficient, and finally multiplying all of that by the baseline Voc. In math speak, and assuming we use a low temp of -40°C, which is 65 degrees below the nominal 25°C:
Simple…right? Of course, that’s a little complicated to just whip out your calculator for. In the case of our example panels, this means a roughly 4.5V increase per panel when the outside temperature is -40°F. Not a huge difference, but when you start using panels in series, it can add up - 5 panels in series means almost 25V more during that one critical time of the year when the temps are intensely cold. Exactly the moment you don’t want to burn out a power system if you depend on it.
Designing an Array
How do series and parallel circuits work?
As we work toward the ultimate goal of designing your array and selecting your charge controller, we’ve covered a lot of technical details. After dealing with some of the more technical aspects, we often come to a closely related question: “Should I wire in series or parallel?”
“Series” means connecting multiple solar panels in the same circuit loop (positive > negative > positive > etc.). This adds the voltage of each panel while keeping the amperage the same. Let’s use an oversimplified 20V / 5A panel as an example:
Conversely, “parallel” means combining the positives of a set of panels together and the negatives together. This keeps the voltage the same while adding the amperages to produce more current:
Advantages and disadvantages
Series vs. parallel is often not simply a matter of one being better than the other. Series has the advantage of increasing the voltage of the system. This means that your array, which must reach about 5V above the charge voltage of your battery bank to begin charging, can do so under lower light conditions. It won’t gather more power when the sun is out and the light is good, but it might give your array the chance to start charging earlier in the day or under cloudy conditions. Enabling a charge when the sun isn’t full can be incredibly helpful.
The increased voltage of your panels maintains the same wattage while keeping amperage down. This can allow you to use a smaller and lower-cost-per-foot wire between your PV array and the MPPT you choose. This is particularly helpful when you have to run a long stretch of cable between your panels and the MPPT, such as often happens in off-grid systems or particularly large RV installations.
The downside to a series of panels is that something that impacts one panel will have an equal effect on every panel in that series. Say a shadow covers part of a single panel in a string of five panels? Seems like a small thing, right? Unfortunately, that partially covered panel could reduce its output by 50% or more. This reduction in power is mirrored by every other panel in the string, reducing the whole set of panels by that much.
Parallel, on the other hand, adds the amperage of the parallel strings together, while maintaining a static voltage across the array. This makes it much more resistant to shadows; the shading of one panel will have a minimal impact on the rest of the system, but the reduced voltage means you’ll need more light to start charging, and it increases the amperage of the power moving through the main cabling between the array and your MPPT - likely requiring you to increase the size of your cable to accommodate that.
Series, Parallel, or a Mix of Both?
Series and parallel wiring each have benefits and drawbacks. Often, the best approach is a combination of series and parallel in your array design.
Sometimes it comes down to placement—shading, panel angle, and other site constraints. It’s almost always best to keep panels in a series string in the same light and at the same angle. If either of those must vary due to placement or shadow, a parallel setup can help.
Sometimes it’s about ensuring you have enough voltage to charge consistently. If you’re using a set of panels—like our Philadelphia 450W module from above—to charge a 48V battery bank, a single panel won’t even begin charging; the voltage simply isn’t high enough. But if you use 2 panels (or more) in series, you reduce the amount of light needed to start pushing power into your battery bank.
This approach also gives you more flexibility when choosing an MPPT. In the Victron lineup, you’ll find multiple MPPTs with the same (or very similar) charge currents. Take the SmartSolar 150/100, SmartSolar 250/100, and the SmartSolar MPPT RS 450/100: all three are well-sized to charge a 48V battery bank with a 4000-5000W PV array. The 250/100 is a bit more expensive, and you have concerns about shading throughout the day, so you might try 3 strings of 3 panels (3S3P) using a 150/100, since your voltage will stay under 150V. Or perhaps you really need that tenth panel, shading won’t be a problem, and you see advantages to a higher system voltage—so you decide on 5 panels in series with 2 strings in parallel (5S2P), choosing the 250/100 to allow for your increased voltage. The 450/100 allows an even higher voltage limit for longer strings, and it has two independent trackers—often a great choice for off-grid homes, where you may have room for a long string of panels and want to keep voltages as high as possible to reduce wire gauge over a long PV run.
A datasheet doesn’t really explain this, mostly because every PV panel is different and every array is unique. Don’t assume that if your MPPT’s datasheet says it can handle up to 2,000W in a 12V system, it will necessarily be okay to use four 500W panels in series. It can likely work with four 500W panels, but you may need to build a 2S2P array to keep your voltage lower and your system safe.
Parallel strings and when to use multiple MPPTs
While a parallel set of panels will be more resistant to shading or other light-related impacts, this doesn’t make it immune. There will be some voltage drop from a shaded set of panels, which, while it won’t be as dramatic, might still reduce the voltage of the other parallel strings. So, when does it make sense to use a second MPPT instead of a larger single MPPT?
Most of this comes down to consistency and how you’re using the setup. If shading is a passing concern, something you see but which isn’t an “always” kind of problem, parallel can be a useful tool. If instead, the shading or light conditions are always going to be a problem, then a second MPPT can be a much more effective way to get more out of your panels.
Some examples of when you might want a second (or third) MPPT instead of setting your array in parallel strings on the same MPPT:
Using both installed rooftop panels on an RV and a set of deployable panels on the ground.
Mixing panels: any time the panel specs (or ages) are different those differences will be shared throughout the system - the least effective or least powerful panel pretty much always being the point the rest of the panels go to.
When your parallel strings cannot be the same length: one string of 5 panels in parallel with a string of 4 panels? Use a second MPPT.
Dramatically different angle between strings.
Amperage is getting too high: every MPPT has a maximum amperage it can handle on the input, and at a certain point the amperage your wire/cable must carry will make that cable size hard to find and/or much more expensive. Most PV fuses/breakers and wire is at most rated to around the 32A range, though there are higher amperage cables and fuses/breakers out there. Pay attention to your gear selection.
While not a hard-n-fast rule, we often find that 2-3 strings in parallel is the most you really want to do. Four or more strings in parallel will often increase the amperage in the system so much that it loses the advantages it might have had - a small 12V system with ±100W modules is often the exception to this, though then you’re usually going to benefit from increasing the voltage by having more panels in series.
Choosing an MPPT
Options for Victron MPPTs
Victron, with its many options, provides quite a few ways to combine panels with an MPPT to produce the best results for your individual needs. The SmartSolar MPPTs are broken up into general sizes based on the voltage of the PV and the voltage of the battery bank they can charge:
SmartSolar MPPT 75/10 and 75/15: The smallest charge controllers available, mostly for small 12V and 24V systems. These units are not designed to charge a 48V bank, and are going to be the best option when you have ±100-200W of PV with a 12V bank, or perhaps 400W when charging a 24V bank.
SmartSolar MPPT 100/15, 100/30, and 100/50: These MPPTs take the voltage up to 100V and allow for larger arrays to charge 12V or 24V banks. No 48V charging.
SmartSolar MPPT 100/20: This is the smallest MPPT that will charge 12V/24V/48V, making it a remarkably flexible small MPPT. It’s one of our most popular options, suitable for 200-300W when connected to a 12V bank, or even up to ±1200W when charging a 48V bank.
SmartSolar 150/35 and 150/45: Similar to the 100/30 and 100/50, but with an extended voltage range to allow for longer strings. It only charges 12V/24V.
SmartSolar 150/60, 150/70, 150/85, and 150/100. The first larger chargers that will handle 48V battery banks. They are also a good fit for larger arrays charging 12V or 24V banks. The 150V max PV input allows for longer strings. The large units also have an option for the VE.Can model that allows use of a daisy-chained RJ45 cable for comms with a GX device.
SmartSolar 250/60, 250/70, 250/85, and 250/100: the largest single tracker MPPTs most often used for larger arrays, especially when charging 48V. These units also have a VE.Can capable model.
SmartSolar MPPT RS 450/100 and 450/200: these are the largest MPPTs that Victron currently makes. The 450V PV maximum is significant, allowing for very long strings of high-wattage panels, and each unit has multiple trackers (the 450/100 has two trackers, while the 450/200 is equipped with four independent trackers). These potent 48V-only chargers are well-suited to off-grid systems and capable of powering a home almost single-handedly.
Considerations when connecting to a battery bank
The amount of charge provided by an MPPT, or several MPPTs, is important to compare against the amount of power your battery, or battery bank, can handle. Don’t overdo the PV with a small battery. Lithium will handle much more charge; lead acid/AGM will need to be charged more slowly in terms of Amps/Wh.
To begin charging your battery bank, the voltage from your PV array must be 5V higher than your current charge voltage. If your battery bank is sitting at 24.5V after a night carrying its loads, the PV voltage will need to reach at least 29.5V the next morning before it will start to charge.
Multiple MPPTs in a single system need to be set to the same charge settings. If you have multiple Victron MPPTs, combining them with a VE.Smart Network, or via the use of a GX device like the Cerbo GX, is highly recommended to coordinate and synchronize their charging curves. When the battery bank is nearing full, you will also likely see one or more of your MPPTs slow and/or stop charging as the battery nears 100%. This is completely normal, and one of the advanced features of a Victron system, where the reduced amperage is drawn from fewer MPPTs rather than reducing all the MPPTs equally.
Final Step-by-Step Summary
That’s a lot of info, let’s summarize it all into a step-by-step for sizing your own MPPT.
Gather your details.
How much PV do you want to use? Will it all be facing the same direction with the same light? Panel specifications or datasheet? Battery bank details? Etc.
Determine your array design. Number of panels in series? Parallel strings?
Calculate the PV array voltage input and battery charging current output.
For a basic estimation, add the Voc of the total PV panel series to determine a rough Maximum PV Input Voltage. Also, divide the total system wattage by the battery bank voltage to determine the likely output amperage your MPPT will need to accommodate.
Account for cold and hot weather impacts on the voltage of your array.
Choose an MPPT.
Victron has provided a helpful calculator for sizing your MPPT. It’s a great tool that can get you a good fit after entering some of the details from step one above.
*Note: Victron's MPPT calculator is calibrated to size you with the smallest and lowest cost MPPT they make that can handle your array details, not necessarily the MPPT that will provide you with the most power for your battery bank as they tend to undersize them, up to 130% utilization if that option is enabled - meaning they might suggest an MPPT that throttles the output in certain conditions.
Conclusion
Yes, this is complicated and it's a good idea to get a second opinion. Send us a message describing your use case and we'll help you find the right Victron MPPT for your project.
