PWM vs. MPPT
PWM vs. MPPT: Which one is better?
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.
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 PO (Watts) = V (Volts) x I (Amps)
Resistance loss PR(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 ( PR = 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 PO = V x I, there is no change of total power outputPO , so the current through cable I should be half.
Finally, the resistance PR(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.
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.