PWM

ALMM and inverter compliance: MNRE’s current ALMM page publishes solar PV module and cell lists, not an inverter list. Verify the inverter’s exact model documents and applicable scheme or DISCOM requirements separately.

What is PWM

PWM stands for Pulse Width Modulation. It is a switching technique where a transistor or switch is rapidly turned on and off, and the relative on-time is varied. The average voltage or current delivered over many cycles approximates the desired output.

PWM appears in two distinct roles in solar systems. As a charge controller topology, PWM refers to a controller that connects the PV array directly to the battery through a switch, with the switch pulsing on and off to regulate charging current. As an inverter modulation technique, PWM refers to the high-frequency switching of IGBT or MOSFET devices to chop a DC bus into a synthesised AC waveform.

This article focuses primarily on PWM as a charge controller topology, with a section on PWM as the inversion technique inside modern solar inverters. Both meanings appear in solar engineering documents, and confusion between them is one of the most common mix-ups in installer training.

PWM explained simply

For charge controllers: imagine a wire between your solar panel and your battery, with a switch in the middle. PWM flips the switch on and off many times a second. By varying how long the switch is on, the controller regulates how much current reaches the battery. The panel is forced to work at the battery voltage, which is usually lower than where the panel would naturally produce the most power.

For inverters: imagine the same switch, but operating at 20,000 times a second. By varying the on-time pattern, the inverter chops a constant DC voltage into a sequence of pulses whose average traces out a sine wave. A filter smooths the pulses into the clean AC that runs your home.

Same trick, different scale. The first is a coarse on-off pattern at hundreds of hertz, the second a high-frequency pattern at tens of kilohertz.

Why PWM matters

Charge controller cost. PWM controllers are 3 to 5 times cheaper than MPPT controllers at the same current rating. For solar lanterns, small home lighting and entry-level rural systems, this cost difference matters more than the yield gain.

Inverter modulation. PWM is the fundamental modulation technique in every modern solar inverter. Without PWM there is no chopped DC bus and no synthesised sine wave.

Motor drive control. PWM at the heart of every VFD allows variable-frequency, variable-voltage output for AC motor speed control, including in solar pumping.

Power converters. DC-DC converters of all kinds, including the boost converter inside MPPT itself, use PWM to control duty cycle.

LED dimming. PWM dimming is the standard for solar streetlights and LED drivers. Other techniques like analogue dimming are noisier and less efficient.

How PWM works

As a charge controller

1. The PV array is connected to the battery through a power switch, typically a MOSFET.
2. A voltage sensor measures battery voltage.
3. A controller compares the measured voltage, used as a proxy for battery SOC, to a target setpoint for the current charge stage (bulk, absorption or float).
4. If battery voltage is below the target, the switch stays on, current flows freely.
5. As battery voltage approaches the target, the controller starts pulsing the switch on and off.
6. The pulse width shrinks as the battery approaches full charge, tapering current.
7. In float stage, the pulse width is just enough to maintain battery voltage against self-discharge.

There is no DC-DC conversion. The panel voltage is pulled down to the battery voltage during the on-time of each pulse, well below the panel’s open-circuit voltage. Power transfer happens at battery voltage, current times battery voltage, not at panel Vmp.

As an inverter switching technique

1. The DC bus inside the inverter is held at a fixed voltage, typically 380 to 800 V for grid-tie inverters.
2. An H-bridge of four IGBT or SiC switches connects the DC bus to the AC output.
3. A controller modulates the switching pattern at 16 to 50 kHz.
4. The duty cycle varies sinusoidally across the AC cycle, producing a chopped output whose average traces a sine wave.
5. An LC or LCL filter smooths the chopped output into a clean sine wave.
6. The fundamental frequency of the AC output is set by the modulation envelope, typically 50 Hz for India or 60 Hz for the US.

The carrier frequency of the PWM, the switching frequency, sets the inverter’s losses, EMI signature and filter size.

Technical deep dive

Duty cycle and average output

Here D is the duty cycle from 0 to 1.

For inverters, D varies sinusoidally with time. The peak of the AC waveform corresponds to D near 1, and the zero crossing corresponds to D near 0.5 for bipolar modulation.

Charge stages in PWM controllers

LFP batteries do not need absorption-float cycling like lead acid batteries. PWM controllers in LFP mode hold a constant voltage near 14.0 V for a 12 V LFP pack and rely on the BMS for cell balancing.

PWM efficiency in charge controllers

The switching efficiency is high, 95 to 99 percent. The overall yield efficiency from panel to battery is lower:

The MPPT alternative at 98 percent conversion efficiency captures the panel’s full power and delivers it at battery voltage, yielding closer to 96 percent. The gap is the energy lost to operating below Vmp. For selection criteria across MPPT inverter options, see Inverter MPPT: 8 Critical Selection Factors (2026).

Inverter PWM techniques

Real-world applications

Solar lanterns and home lighting kits. PWM controllers in the 1 to 10 A range, often integrated with the LED driver.

Small DC home systems. Rural off-grid solar home systems in the 50 to 200 W range with one or two LED bulbs, a mobile charger and a DC fan.

Street lighting. Solar streetlights with integrated PWM controllers manage battery charging and LED dimming.

Solar pumps. VFDs in solar pump controllers use PWM to vary motor frequency. The pump runs faster in strong sun, slower in weak sun, without any battery.

UPS inverters. Home UPS inverters use PWM modulation to chop battery DC into AC at 50 Hz with low THD, producing a true sine wave inverter output rather than the older modified sine wave design.

Variable speed drives. Industrial motor drives use PWM at 4 to 16 kHz to control three-phase motor speed.

Advantages of PWM charge controllers

Low cost. The dominant reason PWM persists in low-power solar.

Simple electronics. Easier to repair in the field, fewer failure modes.

High switching efficiency. The losses inside the switch itself are minimal.

Wide tolerance to panel variation. Different panel sub-types in a small array still work, since voltage is set by the battery.

Limitations of PWM charge controllers

Yield. 15 to 30 percent lower than MPPT for the same array.

Panel-battery voltage matching. A 12 V battery requires a 36-cell, 17 V panel. Modern 60 and 72-cell panels at 30 to 40 V cannot be paired with PWM 12 V controllers without wasteful series-parallel reconfiguration.

Battery wear. Without absorption-float-equalisation discipline, PWM controllers can undercharge or overcharge batteries. Quality matters.

No multi-MPPT. A PWM controller cannot manage two strings of different sizes or orientations; see Dual MPPT vs Single MPPT, When You Actually Need It for when that limitation actually matters.

Common problems

Battery undercharged. A 12 V battery on a hot afternoon with a panel running near short-circuit current never reaches absorption voltage. The user thinks the panel is dead.

Battery overcharged. A controller with the wrong setpoints can boil lead acid electrolyte and destroy a battery in weeks.

Panel mismatched to controller voltage class. A 24 V panel on a 12 V controller leaves half the power on the table. A 12 V panel on a 24 V controller cannot charge at all.

Heat damage. PWM controllers without heat sinks fail in summer attics. Most low-end controllers do not derate gracefully.

EMI. Switching noise from PWM can interfere with radios and other electronics. Filtering on power and ground rails mitigates this.

Best practices for PWM applications

Match panel Voc and Vmp to the battery voltage class. For 12 V batteries, use 36-cell panels. For 24 V, use 72-cell panels.

Check the controller charge profile matches the battery chemistry. Most modern PWM controllers have a switch for lead acid, AGM, gel and LFP.

Use temperature compensation if the battery is in a hot environment.

Mount the controller in a ventilated location. Heat is the leading failure mode.

Use Class II SPDs upstream on PV side for installations in Indian thunderstorm zones.

Do not parallel two PWM controllers on the same battery. They will fight over voltage setpoints.

Reserve PWM for systems where the cost difference against MPPT is significant relative to the system value, typically below 200 W panel ratings.

Comparison: PWM vs MPPT charge controllers

For a full side-by-side comparison of yield, cost and use cases, see MPPT vs PWM Charge Controller, Why MPPT Wins Every Time.

Indian market context

PWM controllers dominate the Indian solar lantern and small home lighting market, which has seen 80 million units shipped under various MNRE and state programmes since 2010. Companies like SELCO, Greenlight Planet, d.light and OMC Power use PWM controllers integrated inside their products.

MNRE specifications for solar lanterns and small home lighting systems under IEC 62257 reference PWM as an acceptable charging technique for panels below 50 W.

For grid-connected rooftop solar and any system using MPPT-capable inverters, PWM is not used.

Standards and certifications

Common mistakes

Pairing a 60-cell panel with a PWM 12 V controller. The controller wastes 40 percent of available power.

Setting the controller to lead acid profile for an LFP battery. Undercharging within months.

Skipping temperature compensation in hot conditions. Lead acid batteries cook in summer.

Treating the PWM modulation in inverters as the same thing as a PWM charge controller. They share a technique but solve different problems.

Using cheap unbranded PWM controllers in lightning-prone areas without SPD upstream. Failure on first storm.

Using PWM where MPPT cost is justified. A ₹2,000 controller saving means ₹40,000 less energy over five years.

Mixing PWM and MPPT controllers on the same battery bank. Voltage setpoints conflict.

Key takeaways

PWM is two things in solar engineering: a low-cost charge controller topology suitable for solar lanterns and small home kits, and the high-frequency switching technique that powers every modern solar inverter. The first is fading as MPPT prices fall. The second is fundamental to power electronics. Knowing which PWM is being discussed is half the battle in design reviews.

Further reading

For how PWM plays out in real projects, these guides go deeper:

• MPPT vs PWM Charge Controller Comparison
• MPPT Kya Hota Hai: Solar Inverter Explained (2026)

Sources

• IEC 62509:2010, Battery charge controllers for photovoltaic systems: Performance and functioning.
• IS 16270, Indian Standard for solar PV charge controllers.
• IEC 62257 series, Recommendations for renewable energy and hybrid systems for rural electrification.
• IEC 62109-1, -2, Safety of power converters for use in photovoltaic power systems.
• Mohan N., Undeland T. M., Robbins W. P., Power Electronics: Converters, Applications, and Design, Wiley.
• Rashid M. H., Power Electronics Handbook, Butterworth-Heinemann.
• MNRE Specifications for Standalone Solar Applications and Solar Lanterns.
• Holmes D. G., Lipo T. A., Pulse Width Modulation for Power Converters: Principles and Practice, Wiley-IEEE Press.
• Sandia National Laboratories Report SAND98-1948, Charge Controller Performance Comparison.