Article at a Glance

MPPT and PWM solar charge controllers both regulate power from a photovoltaic array, but they do not make the PV array operate in the same way.

A PWM controller effectively allows the downstream battery or DC-bus voltage to determine the PV array’s operating point during charging. If that operating point is well below the array’s maximum-power voltage, the array produces less power than it could at its maximum power point.

An MPPT controller uses an active tracking algorithm together with a DC/DC power-conversion stage. The tracking function searches for an operating point near the PV array’s maximum power point, while the DC/DC stage converts that captured power to the voltage required by the battery or DC bus.

For small systems where PV and battery voltages are deliberately well matched, PWM can still be practical.

For a telecom solar overlay using higher-voltage PV strings to supply an existing -48V-class DC system, MPPT/DC-DC is usually the more suitable architecture.

But one specification is frequently misunderstood:

MPPT tracking efficiency, DC/DC conversion efficiency and solar-to-bus efficiency are three different things.

A controller described as “97% efficient” has not necessarily demonstrated 97% MPPT tracking efficiency.

That distinction should be understood before comparing products.

MPPT vs PWM: The Real Difference Is the PV Operating Point

A photovoltaic module does not have one fixed voltage at which it always generates maximum power.

Its current-voltage characteristic changes with irradiance, cell temperature, shading and operating conditions. As those conditions change, the maximum power point moves.

Sandia National Laboratories’ PV Performance Modeling Collaborative describes MPPT as the control process that adjusts PV voltage to maximize power output. Its concept of array utilization reflects how closely a system remains at the maximum power point and how much energy is lost through MPPT error.

That gives us a clearer way to describe the two technologies.

PWM:

PV array → operating point strongly influenced by battery/DC-bus voltage

MPPT:

PV array → MPP tracking → DC/DC conversion → required downstream voltage

PWM does not actively search the PV curve for maximum power.

An MPPT controller does.

That difference matters much more than the labels “basic” and “advanced.”

Why “PWM Wastes the Extra Voltage as Heat” Is an Oversimplification

A common explanation of PWM says that any difference between PV voltage and battery voltage is simply “burned off as heat.”

That is not a good description of what is happening.

With a PWM controller, the PV array is pulled toward the downstream battery or bus operating voltage. The array therefore operates at a different point on its I-V curve instead of remaining at its maximum power point.

At MPP:

PMPP = VMPP × IMPP

At the actual PWM-controlled operating point:

PPV = VOperating × IOperating

The lost harvest is the difference between what the array could produce near its MPP and what it actually produces at the imposed operating point.

You cannot accurately calculate that difference by subtracting two voltage values while assuming current remains unchanged.

This matters because simplistic MPPT-vs-PWM examples sometimes exaggerate the difference between the two technologies.

The real comparison depends on the actual PV curve and the downstream voltage.

Three Efficiency Numbers That Should Never Be Mixed Together

This is the most important distinction in the article.

1. MPPT Tracking Efficiency

MPPT tracking efficiency describes how effectively the controller captures the maximum power that is available from the PV source.

Conceptually:

MPPT Tracking Efficiency = Captured PV energy ÷ Available energy at MPP

It primarily describes the performance of the tracking function.

A controller can lose tracking energy if it:

• Oscillates excessively around MPP
• Responds slowly to irradiance changes
• Remains on an outdated operating point
• Settles on a local rather than global peak under partial shading

2. DC/DC Conversion Efficiency

Once the controller has accepted PV power, that electrical power still has to pass through its DC/DC conversion stage.

Conceptually:

Conversion Efficiency = DC output energy ÷ Controller input energy

Losses here come from the power electronics rather than the MPPT search itself.

HighJoule’s current Stacked-PV MPPT Solar Controller publishes conversion efficiency up to 97%. The product is intended for base-station PV capacity expansion and converts higher PV input voltage into a DC output suitable for the telecom power architecture.

That 97% figure is a conversion-efficiency specification.

It should not be described as:

97% tracking accuracy

or:

97% MPPT tracking efficiency.

3. Solar-to-Bus Efficiency

The usable solar energy that finally reaches a telecom DC bus depends on more than the controller itself.

A simplified relationship is:

Solar-to-Bus Efficiency ≈ Tracking Efficiency × Conversion Efficiency × Other DC Loss Factors

Other factors can include:

• PV cable loss
• Protection-device loss
• Thermal derating
• DC distribution
• Controller operating limits
• Site-level control strategy

The equation is deliberately simplified.

Its purpose is to show that one efficiency percentage cannot describe the whole system.

IEC 62891:2020 makes the same basic distinction in its test framework for grid-connected PV inverters. It treats static MPPT efficiency, dynamic MPPT efficiency and steady-state conversion efficiency as separate quantities.

Tracking Accuracy or Tracking Efficiency?

Both phrases appear in product descriptions and technical discussions.

For engineering comparison, MPPT tracking efficiency is usually the more useful term because it can be defined around captured energy relative to the available maximum-power energy.

“Tracking accuracy” can also describe how close the operating point is to MPP, but it becomes ambiguous if the test method is not stated.

A supplier claiming:

99% MPPT accuracy

should therefore be able to answer:

• Static or dynamic?
• Energy-based or instantaneous?
• What PV characteristic was used?
• What irradiance profile was tested?
• Across which input-voltage range?
• At what output power?
• According to which test procedure?

Without those conditions, the percentage is difficult to compare across products.

Static MPPT Efficiency: Can the Controller Stay Near MPP?

Imagine irradiance and module temperature remain reasonably stable.

The maximum power point is moving very little.

Once the controller has found that point, it should remain close to it without unnecessary oscillation.

This is the basic static tracking problem.

IEC 62891 defines a method for measuring static MPPT efficiency for grid-connected PV inverters under a static PV-generator characteristic.

The standard is specifically written for grid-connected photovoltaic inverters, so it should not automatically be presented as a certification standard for a telecom DC/DC MPPT controller.

Its terminology is still useful.

It reminds buyers that:

Having an MPPT function is not the same as demonstrating how accurately it remains at MPP.

Dynamic MPPT Efficiency: Can It Follow a Moving Maximum?

Real solar conditions change continuously.

Clouds move across the array. Cell temperature rises through the morning. Irradiance can rise or fall quickly around cloud edges.

The maximum power point moves with those changes.

An MPPT controller has to:

• Detect the change
• Move away from the old operating point
• Locate the new maximum
• Settle without excessive oscillation

IEC 62891 treats dynamic MPPT efficiency separately from static efficiency for exactly this reason.

Two controllers may behave similarly during a steady midday test and differently when irradiance changes rapidly.

That can be relevant at telecom sites with:

• Fast-moving cloud cover
• Mountain weather
• Tower shadows
• Different PV orientations
• Temporary shading from surrounding infrastructure

For those sites, a single peak-efficiency number says very little about dynamic tracking quality.

Partial Shading Creates a Different MPPT Problem

Uniform irradiance often produces one dominant maximum on the PV power-voltage curve.

Partial shading can create several local maxima.

A basic MPPT algorithm can therefore settle around a valid local peak while a higher global maximum exists elsewhere.

The controller is technically tracking a maximum.

It is simply not necessarily tracking the global maximum power point.

More sophisticated controllers may use:

• Periodic voltage scanning
• Global-search routines
• Hybrid algorithms
• Multiple independent MPPT channels

But none of these capabilities should be assumed simply because the front label says “MPPT.”

When partial shading matters, ask the supplier:

1. Does the controller have a global-MPP search function?
2. What triggers a full scan?
3. How often does it scan?
4. Does the scan interrupt normal energy harvesting?
5. Is its partial-shading behavior documented with test data?

This is especially relevant to telecom installations where towers, antennas, shelters or nearby structures can shade part of the array.

Why Telecom Solar Overlay Favors MPPT/DC-DC

The electrical architecture of a telecom solar overlay is different from a small 12V battery-maintenance system.

Telecom power systems commonly operate around a -48V DC architecture, while PV strings may operate at substantially higher voltages.

HighJoule’s current Stacked-PV MPPT Solar Controller, for example, is designed for base-station PV expansion. Published versions accept series-connected PV strings and regulate the output toward the communication-site DC bus.

Higher PV string voltage can also reduce array-side current for the same transmitted power:

P = V × I

And resistive cable loss follows approximately:

Ploss = I²R

Lower current can therefore make longer PV cable runs easier to manage.

This is particularly useful when panels are installed:

• On telecom compounds
• On roofs
• Along fences
• On nearby open land
• Some distance from the DC equipment cabinet

MPPT/DC-DC allows the PV array to remain near its useful operating voltage while producing a compatible downstream DC output.

PWM does not provide the same voltage-conversion function.

MPPT Tracking and DC-Bus Integration Are Two Different Control Problems

A telecom solar-overlay system has to solve two related problems.

PV-Side Control

How much of the available PV power can the MPPT controller capture?

DC-Bus Integration

How much of that converted solar power can the telecom site actually use?

HighJoule’s existing telecom solar-overlay architecture uses MPPT/DC-DC modules in conjunction with the site’s existing DC bus, rectifiers, batteries and backup sources. The broader DC-bus prioritization and multi-source operating logic are covered separately in HighJoule’s Solar Overlay Technology guide.

The distinction matters because an excellent MPPT tracker does not guarantee maximum site-level solar utilization.

Solar energy may still be limited by:

• DC-bus voltage constraints
• Battery SOC
• Battery charging limits
• Rectifier settings
• Telecom load
• Controller output limits

Controller performance and site-level solar utilization should therefore be evaluated separately.

When Does MPPT Matter Most Compared With PWM?

There is no technically responsible universal statement such as:

MPPT always produces 20% or 30% more energy.

The difference depends on the electrical architecture and operating conditions.

Operating Condition	MPPT Relevance	Why
PV Vmp well above battery/bus voltage	High	MPPT can hold PV near MPP while converting voltage downstream
Higher-voltage series PV strings	High	Useful DC/DC conversion is required
Long PV cable runs	High	Higher PV voltage can reduce current and cable loss
Rapid irradiance variation	Tracking-dependent	Dynamic MPPT performance matters
Partial shading	Algorithm-dependent	Global-MPP behavior may matter
Small voltage-matched system	Lower	PWM may already operate reasonably close to useful array voltage
Simple battery-maintenance application	Lower	Additional harvested energy may have limited value

The decision should be driven by the actual system.

A fixed rule such as:

PWM below 200W, MPPT above 200W

is not an electrical design rule.

Power level may influence the economics, but it does not determine whether the PV array and battery voltages are compatible.

Cold Weather Changes Both MPP and String-Voltage Risk

PV voltage generally rises as cell temperature falls.

That changes both:

• Vmp
• Voc

An MPPT controller can continue adjusting the PV operating point toward the new MPP as temperature changes, provided the string remains inside the controller’s permitted MPPT window.

A PWM controller remains much more constrained by downstream battery or bus voltage.

But cold weather is not simply an MPPT advantage.

Higher cold-weather Voc can also create an overvoltage risk.

The PV string therefore has to satisfy:

Maximum expected cold-weather string Voc < controller absolute maximum PV input voltage

This should be checked before commissioning.

For professional telecom projects, string design matters at least as much as the statement that “MPPT performs better in cold weather.”

PWM Still Has Valid Applications

PWM remains useful in some solar systems.

It can be appropriate where:

• PV and battery voltages are deliberately well matched
• The system is small
• Cable distances are short
• Loads are simple
• The financial value of additional harvest is limited
• Sophisticated dynamic tracking is unnecessary

A small independent sensor node is one example.

If the module, battery and load were designed together around a simple voltage-matched architecture, PWM may provide perfectly adequate service.

That is different from a telecom overlay using higher-voltage PV strings and an existing DC power system.

In that application, voltage conversion itself is part of the engineering requirement.

How to Test Conversion Efficiency in the Field

Conversion efficiency can be checked relatively simply.

Measure PV-side voltage and current:

Pinput = Vin × Iin

Measure controller output voltage and current:

Poutput = Vout × Iout

Then:

Conversion Efficiency = Poutput ÷ Pinput

Measurements should ideally be synchronized because solar conditions can change between readings.

This test tells you how much of the electrical energy entering the controller appears at its output.

It is useful.

But it does not prove MPPT tracking efficiency.

Why Input and Output Measurements Cannot Prove Tracking Accuracy

Suppose a controller receives 900W from a PV array and delivers 873W to the DC bus.

Its conversion efficiency is:

873 ÷ 900 = 97%

That may be an excellent conversion result.

But what if the PV array could have produced 1,000W at its true maximum power point at the same moment?

The controller may have been converting its captured power efficiently while failing to capture all of the available PV power.

To measure tracking efficiency, you need a reference for the maximum power that was actually available.

That normally requires a controlled method such as:

• A calibrated PV simulator with a defined I-V curve
• Synchronized PV I-V characterization
• Another validated reference capable of determining true MPP

IEC 62891 provides a formal static and dynamic MPPT-efficiency test framework for grid-connected PV inverters.

The key procurement lesson is straightforward:

Input/output power measurement evaluates conversion. It does not, by itself, evaluate tracking.

How to Read HighJoule’s 97% MPPT Controller Specification

HighJoule’s current Stacked-PV MPPT Solar Controller page publishes several relevant specifications.

Depending on the model, the public product information includes:

• HJ-MCJ48H60
• HJ-MCJ48H100
• HJ-MCJ48H120
• 65–250VDC PV operating input range for the listed models
• Up to 97% conversion efficiency
• Adjustable DC output
• RS485 / Modbus communication
• Wall-, pole- or rack-oriented installation options
• Outdoor versions with IP55 protection

These specifications help evaluate:

Application architecture
and
power-conversion performance.

They do not by themselves establish:

• Static MPPT tracking efficiency
• Dynamic MPPT tracking efficiency
• Global-MPP scan recovery time
• A fixed percentage energy gain over PWM
• A universal response time

Those figures should only be published when model-specific test results and corresponding conditions are available.

For a manufacturer, this distinction is important.

A conservative specification that can be traced to test data is more useful than a stronger number that cannot be verified.

A Better MPPT Procurement Checklist

When comparing MPPT controllers, do not ask only for “efficiency.”

Ask what has actually been measured.

Specification	What It Tells You
MPPT operating voltage range	Whether the PV string remains inside the tracking window
Maximum PV input voltage	Whether cold-weather Voc remains safe
DC/DC conversion-efficiency curve	Power-stage losses across operating load
Static MPPT efficiency	Steady-state tracking quality
Dynamic MPPT efficiency	Response to changing irradiance
Number of MPPT channels	Flexibility for different strings and orientations
Global-MPP behavior	Capability under multi-peak partial shading
Start-up voltage	Low-irradiance and morning behavior
Thermal derating curve	Real output capability in outdoor cabinets
DC output range	Compatibility with telecom DC bus
Communication interface	Remote analysis and alarm integration
Electrical protection	System safety and outdoor reliability

Then ask for the conditions behind every important number.

For example:

• Input voltage
• Output voltage
• Power level
• Temperature
• Irradiance profile
• Static or dynamic test
• Test standard or internal method

A percentage without test conditions is difficult to compare.

Evaluating MPPT for a Telecom Solar Overlay Project

For an actual telecom overlay project, a practical engineering sequence is:

1. Confirm the site’s real DC-bus operating range.
2. Determine PV string Voc and Vmp across expected temperatures.
3. Check the controller’s MPPT operating window.
4. Check maximum PV input voltage against cold-weather string Voc.
5. Review DC/DC conversion efficiency over the expected load range.
6. Request static or dynamic MPPT data when tracking performance is contractually important.
7. Review global-MPP behavior if partial shading is expected.
8. Check thermal derating for the actual outdoor cabinet temperature.
9. Verify how the MPPT output coordinates with existing rectifier settings.
10. Measure PV input and DC output energy after commissioning.

This approach gives a much better picture than choosing a controller from one headline efficiency number.

Frequently Asked Questions

What is MPPT tracking efficiency?

MPPT tracking efficiency describes how much of the PV energy theoretically available at the maximum power point is actually captured by the controller’s tracking function.

Is MPPT tracking efficiency the same as conversion efficiency?

No. Tracking efficiency measures the quality of maximum-power-point tracking. Conversion efficiency measures electrical loss through the controller’s DC/DC power stage.

Does a PWM controller have MPPT tracking efficiency?

No in the normal sense. PWM does not perform maximum-power-point tracking, so it should not be assigned a directly comparable MPPT tracking percentage.

Is MPPT always 20–30% better than PWM?

No. The energy difference depends on PV-to-battery voltage mismatch, temperature, irradiance, shading and system architecture. In a well-matched small system, the difference can be much smaller.

Can I measure MPPT tracking efficiency from controller input and output power?

No. Input/output measurements determine conversion efficiency. Tracking efficiency also requires a reliable reference for the maximum PV power available at the same moment.

Why is MPPT suited to telecom solar overlay?

Telecom solar overlay commonly uses higher-voltage PV strings that must supply an existing -48V-class DC architecture. MPPT/DC-DC allows the PV array operating point and downstream DC voltage to be controlled separately within the controller’s operating range.

Core Takeaways

The useful comparison is not:

MPPT 97% vs PWM 75%.

That combines different kinds of measurements and can be misleading.

A better performance chain is:

Available PV Power
↓
MPPT Tracking Performance
↓
DC/DC Conversion Efficiency
↓
DC-Bus Coordination
↓
Usable Solar Energy at the Telecom Site

PWM does not perform maximum-power-point tracking in the same way.

MPPT does, but tracking quality and power-conversion efficiency still need to be evaluated separately.

For small, deliberately voltage-matched solar systems, PWM can remain a practical choice.

For telecom solar overlay using higher-voltage PV strings and an existing -48V-class DC system, MPPT/DC-DC is generally the more suitable architecture because maximum-power tracking and voltage conversion are both part of the integration problem.

The most useful question to ask a controller supplier is therefore not:

“What is your efficiency?”

Ask instead:

“Which efficiency are you quoting, and under what test conditions was it measured?”

That question often reveals more about an MPPT controller than the largest percentage printed on its datasheet.

Engineering Note

IEC 62891:2020 is written for MPPT-efficiency testing of grid-connected photovoltaic inverters. Its static/dynamic terminology is useful for understanding MPPT performance, but compliance with IEC 62891 should not be claimed for a telecom DC/DC controller unless the relevant product has actually been tested according to the applicable procedure.

HighJoule product values quoted in this article reflect currently published product information. Project-specific MPPT performance, PV string design, thermal derating and DC-bus compatibility should be confirmed using the selected controller model and actual site conditions.