Guide to Sizing Victron MPPT Solar Controllers
There’s a lot to learn when you’re first diving into solar. To help simplify things, we’ll walk you through the basics of sizing Victron MPPT solar controllers so you can make informed decisions for your off-grid solar system.
What Is a Solar Controller and Why Do You Need It?
A solar controller is a critical component in an off-grid solar system. Its job is to manage the flow of energy from the solar panels to your batteries. Without a solar controller, your batteries could be damaged by overvoltage or receive inefficient charging, reducing their lifespan.
Victron Energy produces high-quality MPPT (Maximum Power Point Tracking) solar charge controllers. These devices adjust to changing light conditions and maximize the energy harvest from your solar panels, making them far more efficient than PWM (Pulse Width Modulation) controllers. In fact, MPPT chargers can provide faster charging in ideal conditions.
Why Victron MPPT Controllers?
At LUX Solar, we highly recommend Victron Energy for their commitment to quality, reliability, and performance. Their MPPT controllers come in two main product lines:
SmartSolar: Includes built-in Bluetooth for easy monitoring and configuration via the Victron app.
BlueSolar: A cost-effective option without Bluetooth connectivity but still offers great performance.
As solar technology advances, Victron’s pricing has become increasingly competitive, making it easier to invest in their world-class equipment.
Sizing Tools
Below we will dive deep into the calculations and considerations you need to take in choosing the right solar controller - but first, here are two handy tools that do the calculations for you!
Shortcut - Victron's MPPT Sizing Calculator
To help you find the right MPPT controller for your system, Victron offers an online MPPT Sizing Calculator:
https://www.victronenergy.com/mppt-calculator
Simply input details about your solar panels—such as the Voc, current, and configuration (series or parallel)—as well as your battery type and voltage. The calculator will recommend the best-suited Victron MPPT model for your setup.
Shortcut - LUX Anykit - Automatically Designs Complete Solar Kit
LUX Solar’s AnyKit is an end-to-end solar design tool that performs all the required calculations to automatically create effective, safe, and compliant solar kits. It ensures that all components in the kit are not only compatible with one another but also optimized for maximum energy generation.
https://www.luxsolar.co.nz/pages/lux-solar-anykit
You can use the tool to quickly figure out which controllers will work with LUX Solar panels. Optionally, you can add pre-wired connection kits to connect the controller to your solar panels and battery.
Understanding Victron MPPT Model Numbers
When choosing a Victron MPPT solar controller, you’ll notice the product names include two key numbers, such as MPPT 100/50 or MPPT 75/15. These numbers represent:
Maximum Input Voltage
This is the maximum open circuit voltage (Voc) that the controller can safely handle. For example, an MPPT 100/50 can handle up to volts.
It’s crucial to ensure that your solar panel Voc does not exceed this number, as exceeding it can permanently damage the controller.
Tip: If you’re connecting solar panels in series, add up the Voc of all panels in the string to get the total input voltage. Make sure the total is below the controller’s maximum voltage rating.
Important: Remember to account for temperature! The colder your panels get, the higher the voltage. The Voc in the datasheet is typically specified at 25°C. Be sure to adjust for temperature variations.
Maximum Output Current
This is the maximum amperage (charge current) the controller can output to your batteries. For example, the MPPT 100/50 can output up to amps.
Why It Matters: The controller must be capable of handling your system’s voltage and be sized correctly to run your loads and extend the lifespan of both the controller and your batteries.
Practical Example: MPPT Model Selection
Let’s break it down with a simplified example:
Scenario:
A solar panel has a Voc of V and a short circuit current (Isc) of A. You plan to connect panels in series.
Step 1: Calculate Total Voltage
Voc of each panel = V. Total Voc in series = V + V = V.
Choose a controller with a maximum voltage rating higher than V. For example, an MPPT 100/50 (which supports up to V) would be a good choice.
Step 2: Calculate Maximum Current
If the panels are in series, the current remains the same as one panel: A. Now look at the output amperage of the controller. The MPPT 100/50 can handle up to A, making it suitable for this system.
Note: An MPPT acts like a DC-to-DC converter where power in ≈ power out.
So:Vmp x Imp ≈ Battery charge voltage x Charge current
Or:Charge current ≈ Panel wattage / Battery charge voltage
(For a 12V system, the battery charge voltage is typically around V.)
Other Numbers That Matter
Getting the Battery Voltage Right
When sizing a Victron MPPT solar controller, it’s not just the solar panel specifications you need to consider—your battery voltage is also very important.
What You Need to Know About Battery Voltage
Victron MPPT solar charge controllers are compatible with multiple battery voltages, such as 12V, 24V, 36V, 48V, or even higher in certain models. Matching your battery voltage to your solar setup ensures optimal system efficiency and prevents issues down the road.
Key Factors to Consider:
Match the Controller to Your Battery Voltage
MPPT controllers must be set to the correct battery voltage, either automatically (in auto-detect models) or manually. For example, if you’re using a 24V battery bank, ensure the controller is configured to output the appropriate charging voltage (e.g., V during bulk charging for lead-acid batteries).
Solar Panel Voltage Must Support the Battery Voltage
Ensure your solar panel voltage (Vmp) is sufficiently higher than the battery voltage. For example, a 24V battery typically requires at least a V solar panel Vmp for efficient charging.
Consider Temperature Compensation
Temperature affects battery charging parameters significantly—especially with lead-acid batteries. Victron MPPT controllers can account for temperature by adjusting charging voltages. If you’re operating in extreme climates, enable this feature or use an external temperature sensor.
Battery Capacity and Current Matching
Lead-acid batteries typically recommend a charge current of 10–20% of their capacity (e.g., a battery can safely handle A of charge current). Lithium batteries can handle faster charging rates, so higher charge currents may be acceptable based on the manufacturer’s guidelines (C is a good approximation).
Key Takeaways for Sizing Your MPPT Controller
Always check the Voc and current rating (Isc) of your solar panels. Voc determines the voltage range your controller must handle. Isc helps you understand how much current your controller needs to process.
Use Victron’s MPPT Calculator or LUX Solar's Anykit tool to simplify the sizing process.
Prioritize quality: A well-sized and high-quality controller like a Victron MPPT will ensure your system runs efficiently and reliably for years.
With this guide, you’re well-equipped to choose the right MPPT controller for your off-grid solar system. Getting things right can make a big difference in the performance and longevity of your setup, and save you time, money, and headaches. Stay powered, stay off-grid!