MPPT vs PWM Charge Controllers: How to Match One to Your System

According to Victron Energy’s wiring guide, solar panels must never be connected directly to a battery. A solar charge controller sits between the array and the battery, converting the higher panel voltage into a voltage suitable for charging. Without a controller, the battery can be damaged. Local regulations may also require a fuse, circuit breaker, or residual-current device between the PV array and the charger.

Why you need a charge controller

Victron Energy’s Wiring Unlimited book explains that a solar charger converts the higher solar panel voltage into a voltage suitable for battery charging. If a panel were connected directly, the battery would receive an unregulated voltage that could overcharge and destroy it. The Hong Kong Electrical and Mechanical Services Department (EMSD) adds that a charge controller regulates the current flowing from the photovoltaic array to the batteries and disconnects the batteries when they are full. In any standalone system, a controller is essential for protecting the battery and managing the charge.

PWM vs MPPT: How they differ

In a 2014 technical article, Victron Energy describes a PWM (pulse width modulation) controller as essentially a switch that connects the solar array to the battery. The result is that the array voltage is pulled down to near the battery voltage. An MPPT (maximum power point tracking) controller is more sophisticated: it adjusts its input voltage to harvest the maximum power from the solar array and then transforms that power to supply the varying voltage requirement of the battery plus load. This decouples the array and battery voltages, so a 12 V battery can be paired with an array wired to produce 36 V, for example.

The EMSD similarly notes that an MPP (maximum power point) charge controller incorporates a DC-to-DC converter so the photovoltaic array can operate at the voltage that delivers maximum output power at the prevailing solar irradiance.

Energy harvest advantage

Victron’s 2014 article states that the MPPT controller’s performance advantage is substantial, 10% to 40%, when the solar cell temperature is low (below 45°C), very high (above 75°C), or when irradiance is very low. At high temperatures or low light, the array’s output voltage drops dramatically, so more cells must be connected in series to keep the voltage comfortably above the battery voltage. An MPPT controller can still extract useful power under those conditions, while a PWM controller would simply pull the array voltage down to battery level and lose the extra potential.

Cable savings with higher array voltage

The same article highlights a second benefit: higher array voltage reduces current for the same power, which lowers cable losses. Ohm’s law shows that cable power loss is proportional to the square of the current (Pc = Rc × I²). Doubling the array voltage can cut the required cable cross-sectional area by a factor of four for the same loss, or reduce losses for a given cable size. Victron notes that this makes MPPT controllers attractive as soon as array power exceeds a few hundred watts on a 12 V battery, or several hundred watts on 24 V or 48 V systems, because the savings in copper can offset the higher controller cost.

When PWM is enough

Victron’s 2014 conclusion is that PWM is a good low-cost solution for small systems only, when solar cell temperature is moderate to high (between 45°C and 75°C). In those conditions, the array voltage is already close to the battery voltage, so the MPPT’s voltage conversion offers little gain. For larger systems or where the array voltage must be substantially higher than the battery voltage, MPPT is the solution of choice.

Matching a controller to your system

Whether you choose PWM or MPPT, the controller must be sized correctly for the solar array and battery. Victron’s Wiring Unlimited book stresses that the array’s open-circuit voltage (Voc) must never exceed the controller’s maximum PV voltage rating. For example, a typical 12 V panel has a Voc around 22 V. A controller rated for 75 V maximum PV input can handle up to three such panels in series. Exceeding the voltage rating can permanently damage the controller.

The same guide notes that the controller’s current rating refers to the current going into the battery. A 15 A controller on a 12 V battery can deliver roughly 180 W (15 A × 12 V), while on a 24 V battery the same controller can deliver about 360 W. Therefore, the array’s total power must be matched to the controller’s output current rating at the system battery voltage. With a PWM controller, Victron notes that the array voltage is pulled down to near battery voltage, so extra array voltage is not converted the way an MPPT controller converts it.

Lithium battery considerations

Lithium batteries add another layer of matching. Victron’s Lithium Smart Battery manual explains that the battery management system (BMS) monitors cell voltages and temperature. If a cell voltage goes too high or the temperature is out of range, the BMS sends a signal to turn off chargers. The charge controller must therefore be compatible with the BMS, either by accepting an on/off signal or by being part of an integrated system that can respond to battery alarms. The manual describes several BMS models that control chargers via “charge disconnect” signals or digital communication. When designing a system with lithium batteries, check that the charge controller’s charge profile matches the battery’s requirements and that the BMS can interrupt charging if needed.

Quick checks for matching a controller

Use the table below as a checklist when comparing controllers to your array and battery. Always refer to the manufacturer’s datasheets and manuals for exact limits.

Frequently asked questions

Can I use a PWM controller with lithium batteries?

Check the battery manufacturer’s requirements: the controller’s charge settings must match the battery, and with Victron’s lithium batteries an external BMS must be able to switch charging off. Some controllers can respond to BMS signals and some cannot.

How do I know if my array voltage is too high for my controller?

Check the panel’s open-circuit voltage (Voc) on its specification sheet. Multiply by the number of panels in series. Ensure this total is below the controller’s maximum PV input voltage, leaving the margin the controller maker recommends. Victron’s Wiring Unlimited book emphasizes that exceeding the voltage rating will damage the controller.

Is MPPT always better than PWM?

Not always. For very small systems in warm climates where the array voltage is already close to the battery voltage, a PWM controller can be sufficient and less expensive. Victron’s 2014 article states that PWM is a good low-cost solution for small systems when cell temperatures are moderate to high. MPPT becomes more beneficial as system size grows or when the array voltage must be substantially higher than the battery voltage.

For more guidance on choosing and installing a charge controller, visit our Charge Controllers section.

Sources

• Victron Energy blog: Which solar charge controller, PWM or MPPT? (2014)
• Victron Energy: Wiring Unlimited
• Hong Kong Electrical and Mechanical Services Department: Balance of system
• Victron Energy: Lithium Smart Battery manual