PWM vs MPPT controllers

The solar charge controller is the "brain" between your panels and your batteries.

It governs how charging happens and affects:

• How much energy you actually get out

• Charging speed

• Efficiency

• How the system performs in cold and poor weather

There are two main types, called PWM and MPPT.

In this guide we cover:

• How they work

• The differences between them

• Why MPPT is almost always the better choice

A solar panel cannot be connected straight to the battery without control.

The controller's job is to:

• Regulate the voltage

• Protect the battery

• Optimise the charging

• Prevent overcharging

The controller sits between the solar panel and the battery bank.

PWM stands for pulse width modulation.

A PWM controller works by:

• Connecting the panel and the battery directly, in pulses

• Pulling the panel's operating voltage down to the battery's level

It is a simple, cheap technology that has been around a long time.

Solar panels work most efficiently at a higher voltage than the battery.

For example:

• Solar panel: around 18–36V operating voltage

• Battery: around 12–14V

A PWM controller cannot turn that surplus voltage into extra charging current. Instead, a large share of the power is simply lost.

That means:

• Lower efficiency

• Slower charging

• Worse performance in cold and cloud

• Greater energy losses

MPPT stands for maximum power point tracking.

An MPPT controller is considerably smarter and more advanced.

It works by:

• Continuously finding the panel's optimal operating point

• Converting surplus voltage into extra charging current

• Maximising the energy sent to the battery

That lets the system make use of far more of the panel's capacity.

An MPPT controller works rather like an intelligent DC-DC converter.

If the panel produces high voltage and low current, the controller can turn that into lower voltage and higher charging current for the battery.

The result is higher efficiency, faster charging and above all a better energy harvest.

In our example we have:

• Solar panel: 200W

• Panel voltage: 20V

• Battery: 12V

With PWM the panel is forced to work close to the battery's voltage.

That means a large share of the power disappears.

With MPPT the controller can instead convert the voltage into extra charging current and use considerably more of the panel's output.

The difference can be 0–30% more energy, and sometimes more still in colder weather.

In cold temperatures a panel's voltage usually rises. That is a good thing in itself — but only if the controller can make use of the extra energy. A PWM controller cannot do that efficiently.

An MPPT controller, on the other hand:

• Optimises the charging continuously

• Works better in changeable weather

• Harvests more energy across more of the year

For Nordic conditions, MPPT is therefore extra important.

The sun is rarely perfect. Cloud, trees, shadows and the time of day shift the panels' operating point all the time.

An MPPT controller adapts dynamically to the conditions and can therefore:

• Find better operating points

• Deliver more energy in poorer conditions

MPPT controllers often handle a higher input voltage (Voc) and panels wired in series.

That brings several advantages:

• Thinner cable

• Lower losses

• Longer cable runs

• Better system efficiency

PWM systems are considerably more limited.

PWM still turns up in:

• Very small systems

• Budget installations

• Simpler leisure setups

For example:

• Small maintenance chargers

• Simple low-power 12V systems

But in a modern off-grid system, MPPT is always the better choice.

At Kraftur we always recommend MPPT controllers because they:

• Harvest more energy

• Charge the battery faster

• Work better in a Nordic climate

• Handle shade better

• Allow more efficient cable runs

• Work better with larger solar arrays

• Give a better total cost over time

A cheap controller can quickly become expensive if the system produces less energy every day for many years.

Yes — but the gap has narrowed a great deal in recent years.

In most modern systems the extra cost is small compared with:

• Higher energy production

• Better battery charging

• Better efficiency

In practice MPPT is often cheaper over time, because the system performs better every single day.

• A controller that is too small
  Can cap the system's output.

• The wrong input voltage
  The controller must handle the panels' maximum voltage.

• A cheap PWM on a large system
  Usually means large energy losses.

• Cables that are too long in a low-voltage system
  Causes voltage drop and worse charging.

For modern systems we recommend:

• An MPPT controller

• Correct sizing

• A margin on input voltage and current

• Quality components

Especially important for:

• Van life / overlanding

• Motorhomes / caravans

• Boats

• Off-grid cabins

• Larger battery banks

When the sun does come out, you want to get as much energy from it as you can.