A solar panel cannot charge a battery on its own. Between the panel and the battery there must be a solar charge controller, which converts the panel's output into the voltage and current required by the battery. The charge controller is therefore the true heart and control centre of the entire solar system: it determines how much of the available solar energy actually reaches the battery and how much is simply wasted as heat.
This article explains the difference between PWM and MPPT technologies using a practical numerical example, shows how to correctly size a controller according to the array voltage and current, and finally describes how the controller fits into the broader Victron ecosystem through SmartSolar and Cerbo GX.
Technology Comparison: PWM vs. MPPT
PWM (Pulse Width Modulation)
A PWM controller is essentially a fast electronic switch. It connects the solar panel directly to the battery and rapidly switches it on and off so that the output voltage matches the battery voltage. The problem is that this forces the panel to operate at battery voltage rather than at the voltage where it can actually produce its maximum power.
The current remains the same (5.55 A), but most of the voltage is effectively discarded. Out of the panel's rated 200 W output, less than 74 W actually reaches the battery, while the remaining 63 % of the available power is simply unused. Today, PWM controllers only make sense for very small and inexpensive installations — for example, a single 50–100 W maintenance panel. They are not suitable for a serious solar installation on a motorhome.
MPPT (Maximum Power Point Tracking)
An MPPT controller is actually a sophisticated DC-to-DC converter with a microprocessor. Several times per second it searches for the panel's maximum power point — the voltage and current combination that produces the highest power — and then converts that energy into the voltage and current required to charge the battery. It works similarly to a transformer, but for direct current.
The same panel under the same sunlight can deliver nearly three times more charging current than with a PWM controller. In real-world conditions — changing weather, different sun angles, and varying temperatures — MPPT controllers typically generate 30–70 % more usable energy per day than PWM units. In a motorhome, that often determines whether your battery reaches 100 % by evening or remains at only 70 %.
Controller Sizing
Victron SmartSolar controllers are identified by two numbers separated by a slash — such as 100/30 or 150/35. Both numbers must be calculated independently. Looking only at the total wattage of your solar array is not sufficient.
First Number: Maximum PV Voltage (Voc)
The first number specifies the maximum open-circuit voltage (Voc) that the controller can safely accept. This voltage must remain safely below the controller's limit under all conditions, including the worst-case scenario: a cold winter morning, when the panels are producing their highest open-circuit voltage before charging begins.
A solar panel's open-circuit voltage increases as temperature decreases (typically about +0.3 %/°C). Two panels connected in series with a Voc of 45 V each at 25 °C will produce approximately 100.8 V at −15 °C — 40 °C colder. That exceeds the limit of a 100 V controller and could permanently damage it. For this configuration, the correct choice is a 150 V controller, not a 100 V model.
For this reason, controller sizing should always include a safety margin based on the lowest realistically expected winter temperature rather than average summer conditions.
Second Number: Maximum Charging Current
The second number indicates the maximum battery charging current the controller can deliver — for example 30 A for a 100/30, or 35 A for a 150/35. If the solar array is capable of producing more current than the controller can deliver under ideal conditions (cold weather and bright sunshine), the controller simply limits its output. This phenomenon is known as clipping.
Clipping is not a fault and does not damage the controller. Instead of operating at the absolute maximum power point, the controller slightly shifts its operating point to maintain its rated maximum current. In practice, this only sacrifices a small amount of energy during the brightest midday hours. For this reason, modest current undersizing — roughly 10–20 % — is often more economical than purchasing a significantly larger controller that would only operate at full capacity a few hours per year.
The Phoenix runs 780 Wp of PERC monocrystalline panels (2×220 Wp + 2×170 Wp) in a series-parallel configuration into a Victron BlueSolar MPPT 150/60-Tr. The "150" gives a comfortable safety margin on the series Voc even in hard frost, and the "60 A" output handles the full 780 Wp array without clipping in any real-world scenario.
Integration into the Victron Ecosystem
MPPT conversion is only the foundation. The real value of a charge controller comes from how well it communicates with the rest of the electrical system.
Victron SmartSolar
The SmartSolar series includes built-in Bluetooth and pairs directly with the VictronConnect mobile app. Without any additional wiring, you can monitor:
- current solar power output,
- production history for the past 30 days,
- charging status,
- error diagnostics.
For LiFePO₄ batteries, SmartSolar includes optimised charging profiles with the correct absorption and float voltages, without unnecessary boost charging that lithium batteries do not require. If your vehicle has multiple MPPT controllers — for example, because solar panels face different directions — the VE.Smart Networking feature wirelessly synchronises them so all controllers share the same battery temperature and voltage data instead of competing with one another.
Cerbo GX — The Central Brain
Cerbo GX connects all Victron components into a single integrated system with a shared communication bus.
DVCC (Distributed Voltage and Current Control) — through the CAN bus, Cerbo GX reads live information directly from the battery's BMS: state of charge, cell temperature, and maximum permitted charging current. This information is then shared with both the MPPT controller and the battery charger. Instead of charging solely according to measured battery terminal voltage, the controller charges according to the battery's actual condition — significantly more accurate and kinder to long-term battery health.
Automatic energy management — when the battery is already full but the solar array continues producing excess power, Cerbo GX can automatically switch on selected loads according to predefined rules: compressor air conditioning, or electric water heating via a 230 V heating element (Alde or Truma). Instead of wasting available solar energy, the excess power is put to productive use.
VRM Portal and remote monitoring — when connected to the internet (typically via Starlink while travelling off-grid), Cerbo GX streams system data to Victron's cloud VRM Portal. Battery status, solar production, and energy consumption are visible from anywhere in the world. Many settings, including charging limits, can also be adjusted remotely — particularly useful when your motorhome is parked somewhere unattended and you want to verify that everything is running correctly.
The Phoenix has the full Victron stack: BlueSolar MPPT 150/60-Tr on VE.Direct, two LiFePO₄ 200 Ah batteries with VE.Bus BMS NG on CAN, and a MultiPlus-II 3000 VA on VE.Bus — all tied together by a Cerbo GX running DVCC. In practice this means the MPPT never overcharges the batteries regardless of what the terminal voltage appears to say, and on sunny days in summer I can run the Sinclair roof A/C unit directly from solar surplus rather than drawing down the battery.
Frequently Asked Questions
I only have a small solar system (one 100 W panel). Is MPPT still worthwhile?
Almost always yes, particularly if the panel operates at a higher voltage than the battery — a 12 V system with a panel at 18–20 V Vmp is the typical case. The price difference between a small PWM and a small MPPT controller is minimal today, while the efficiency gain is permanent. The only scenario where a simple PWM might make sense is a very low-cost setup where the panel and battery are already very close in voltage, which rarely applies to modern motorhome configurations.
What happens if my solar array exceeds the controller's current limit?
Nothing destructive — the controller safely limits the current (clipping) and the excess panel power simply goes unused. The risk of damage applies almost exclusively to exceeding the voltage limit (Voc), not the current limit. A controller operating in clipping mode is working exactly as designed: it protects itself while delivering the maximum current it can safely handle. Regularly exceeding the current rating by a small margin is perfectly normal for an array slightly oversized relative to the controller.
Do I really need a Cerbo GX, or is a SmartSolar controller sufficient?
A SmartSolar controller works fully standalone and charges correctly without any Cerbo GX. Cerbo GX adds value once you want to connect multiple components — a controller, a battery with a BMS, an inverter/charger — into one system with more accurate charge management (DVCC), load automation, and remote monitoring via VRM. For a simple single-component solar setup, a standalone SmartSolar with the VictronConnect app over Bluetooth gives you everything you need.
Related articles
- Solar Panels on a Motorhome — what actually reaches the controller's input: panel technologies, temperature losses, and wiring strategies.
- LiFePO₄ Batteries Explained — where the solar energy ultimately goes, and what the battery needs from the charging profile.
- Inverters and Power Converters — how stored energy becomes 230 V AC for appliances on board.
- Electrical Basics for Motorhomes — voltage systems, wiring, fusing, and protection fundamentals.