PWM vs. MPPT: Which one is better?

2021/8/16 15:01:53

We have learned about the features of both controllers (PWM & MPPT) in previous chapters. We well noted that PWM does not convert extra voltage into currents, which results in low power conversion rates. In other words, PWM doesn’t transfer all energy that is collected by solar panels to batteries, but MPPT is always tracking the maximum power point from the panels and adjusting its currents and voltage accordingly so that it can transfer all energy collected by solar panel to battery.

A concrete example will explain this clearly:

The basic physical formula:

Power (watts) = V (Volts) x I (Amps)

If we use a nominal 12V, 100W solar panel to charge a 12V battery system, the actual Vmp is 17V, and we can calculate its current output:

I = Power / V

I = 100 / 17 = 5.88 amps

Now we know the panel output is 17V and 5.88A.

Scenario 1: The photovoltaic system is with PWM solar charge controller.

PWM will drag the voltage down to battery charging voltage – approximate 14V. After going through the PWM, the solar energy only remains 14V and 5.88A.

That is:

P = V x I = 14 x 5.88 = 82.32 W

Scenario 2: The photovoltaic system is with the MPPT solar charge controller.

The MPPT not only drags the voltage down to 14V, but also increases the current, so that the power almost equals to power out.

So, if the voltage decreases by 17/14 = 1.21

Then the current to battery increases by 1.21, we get

5.88 x 1.21 = 7.11A

Total power out

P = 14 x 7.11 = 99.54 W

In this example, the power wasted by PWM is

99.54 – 82.32 = 17.22W

Almost 20% energy was not converted to battery chemical energy. If we consider the scenario in a large solar array, the loss could be tremendous.

So, it’s better to use MPPT for large solar array.

6.2 The strengths of MPPT

a) High conversion efficiency

If your photovoltaic system comes with a large solar array, MPPT would be the best choice to boost the solar energy conversion, especially in cold weather, since the panel voltage will rise as the temperature drops. The MPPT conversion rate could rise from 20% to 40%. That’s green and free energy that really saves money on your bill.

b) Lower energy loss in cables or lower cost for purchasing cables.

Please remember Ohm’s law formula

V (Volts) = R (Ohm) x I (Amps)

Output power  P_O (Watts) = V (Volts) x I (Amps)

So

Resistance loss  P_R(Watts) = R (Ohm) x  I² (Amps)

Then, if your PV panels are installed a long distance from your battery bank, the power loss of cable resistance is considerable ( P_R = R x I² ). Here R represents the resistance of cables. R increases as cable length increases:

But if we double the Voltage of the solar array by wiring them in more series, according to P_O = V x I, there is no change of total power outputP_O , so the current through cable I should be half.

Finally, the resistance P_R(Watts) = R (Ohm) x  I² (Amps) will be a quarter than before.

In fact, with MPPT, you could raise the voltage of the solar array even higher to reduce the current flow.

In this case, we increase the panel’s voltage to reduce the resistance loss through cables, and since we are using MPPT, which always tracks to harvest maximum power from panels, we have no voltage waste as PWM may have.

We could review this topic from another aspect. If you cannot raise the panels voltage, then you have to find out some solution to reduce cable resistance, as resistance = resistivity × length / area, it seems the only way is to use cables with large transverse areas, and that will be another huge sum of money to spend.

To recap, when it comes with small systems, PWM is a good solution since it is inexpensive, but for large systems, in order to improve conversion rates and not waste the solar panel capacity of harnessing solar energy, MPPT is preferable. MPPT would always be applied to higher power systems.

6.3 Pros and cons

Learning knowledgeable information from the previous content is necessary before making the decision to purchase a solar charge controller for your PV system. A comparison table, listing the difference between PWM and MPPT, is also suggested. So, we put their pros and cons together to make it more convenient for you to review.

	Pros	Cons
PWM	PWM technology has been available in PV systems for a long time and is a relatively stable and mature technology They are cost-effective and are affordable for most consumers PWM can withstand up to 60 amps load currently Most PWMs have a reasonable heat dissipation structure that allows them to work continuously PWM comes in different sizes to suit a wide range of applications	If PWM is applied to photovoltaic solar systems, the voltage of the solar panel has to match that of the battery bank The current load capacity of a single PWM has not been developed and is still only up to 60 amps Some smaller sized PWM charge controllers cannot be UL listed due to their poor structure design Some smaller sized PWM do not have fittings of conduit PWM has signal interference problems sometimes. The controllers generate noise in TV or radios PWM limits the expansion of photovoltaic solar systems to some extent It cannot be applied to high voltage off-grid solar arrays
MPPT	MPPT maximizes the conversion of solar energy from PV panels, and the rates can be 40% more efficient than PWM MPPT can be used in cases where the solar panel voltage is higher than the battery voltage. MPPT can withstand up to 80 amps load current MPPT features longer warranties than PWM MPPT does not limit the expansion of solar panels in the system MPPT is the only solution for a hybrid solar power system	MPPT are more expensive than PWM. The pricing of some models is double that of a PWM charge controller Since MPPT has more components and functions, its physical size is larger than PWM. MPPT are more complicated, so most time, we need to follow a guide when sizing the solar array MPPT solar controller constantly compels the solar panel array wired in strings

6.4 Does every solar PV system need a charge controller?

The answer is no.

Generally, if your solar panel is less then 5 watts for every 100 amp hours battery, then you do not need a solar charge controller.

Here is a formula we can use:

Quotient = Battery Capacity (Amp Hour) / Imp of solar panel (Amps)

If the quotient is larger than 200, you don’t need a controller; otherwise, you’d better install a controller.

For example, if you have a 200AH battery and 20W panel, the quotient would be 200/1.18=169.5; in this case, you need a controller.

If you have 400AH battery and 10W panel, the quotient would be 400/0.59=677.9; in this case, you do not need a controller.