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The intent of this post is to help you understand the difference between PWM (Pulse Width Modulation) and MPPT (Multi-Point Power Tracking) charge controllers and when to use each.
Before I go further, I want to go over a quick reminder of power calculations. Solar panels are rated by their actual output when connected to a load. For example, a generic 300W 24V nominal panel has an actual output of 36.1 volts (Vmp) and 8.3 amperes (Imp). When you multiply them together, you get the rated wattage which is 299.63W (300W after rounding up). You can see the specs on the back of the solar module or in its data sheet.
A charge controller (SCC) is an important component in a battery based system. They are not used in grid type systems as they don’t have batteries to charge. Their primary role is to manage charging the battery bank. It prevents it from overcharging. Many of them control the current and voltage at which it charges. At night, the voltage of the battery bank is higher than that of the array that it is connected to. Without a charge controller, the tendency would be for the voltage to flow out of the battery bank. A charge controller prevents that from happening. It allows the flow to only go one way into the batteries.
Charge controllers are available with two different technologies, PWM and MPPT. You can’t often tell the difference between a PWM and MPPT charge controller just by looking at them. So how can you tell the difference between the two? Generally, this information will be printed on the controller’s label or you will find it by checking the product manual. Beware there are many unscrupulous sellers who simply print MPPT on a standard PWM controller. To prevent being fooled, it is worth noting that MPPT is substantially more expensive to manufacture than PWM. If the price seems too good to be true, it certainly is.
Both PWM and MPPT ensure the batteries are being charged at the right voltage based on their state of charge. How they perform in a system are very different from each other. An MPPT charge controller is more expensive than PWM. Let us go over why it is often worth it to pay the extra money.
PWM charge controllers operate by making a connection directly from the solar array to the battery bank. During bulk charging, when there is a continuous connection from the array to the battery bank, the array output voltage is pulled down to match the battery voltage. As the battery charges, the voltage of the battery rises so that the voltage output of the solar panel rises as well using more of the solar power as it charges. The PWM is in essence a switch that connects a solar array to the battery. The result is that the voltage of the array will be pulled down to near that of the battery.
MPPT charge controllers measure the Vmp voltage of the panel and down convert the PVv voltage to the battery voltage. Because power in equals power out, when the voltage is dropped to match the battery bank, the current is raised. You are using more of the available power from the panel.
The MPPT is more sophisticated (and more expensive): it will adjust its input voltage to harvest the maximum power from the solar array and then transform this power to supply the varying voltage requirement of the battery plus load. Thus, it essentially decouples the array and battery voltages so that there can be, for example, a 12V battery on one side of the MPPT and panels wired in series to produce 36V on the other.
It is generally accepted that MPPT will outperform PWM in a cold to temperate climate, while both controllers will show approximately the same performance in a subtropical to tropical climate.
Best MPPT Charge Controllers:
Let us see how this affects our system with a 100W 12V nominal panel with a 12V nominal battery bank. We will do the math assuming 100% efficiency which is not what you will see in the real world. But it will really help illustrate the difference between PWM and MPPT quite clearly.
With PWM let us assume the battery voltage is low, say 11V. When the charge controller connects the panel’s output to the battery, the solar panels output is pulled from 18V down to 11V. With the maximum power current of 5.56A, the charge controller’s output into the battery is 11V x 5.5 6A which equals 61W. When the battery is fuller at 14V, more of the panel’s available voltage is used and with the same 5.56A, this charge controller is outputting 78W into the battery. 14Vs x 5.5 6A equals 78W.
Now let us do the math with an MPPT charge controller. When the battery is low it drops the voltage from 18V down to 11V and that drop is a ratio of 1.6 (18V/11V). When it drops the volts by 1.6, to keep power constant, it increases the current by 1.6 as well increasing the current from 5.56A to 8.9A. 11V x 8.9A equals 97W. Compare that with a PWM output of only 61W. If we do that same math for a fuller battery at 14V, the in versus out ratio works out to 1.28, so that increases the current from 5.56A to 7.1A. 7.1A x 14V equals 99.4W. Let us compare those two outputs with the same battery and same panel just going from a PWM to an MPPT charge controller (see the picture below):
You have seen how an MPPT charge controller can maximize the output when the solar panel’s nominal voltage matches the battery bank’s nominal voltage. Here is another huge advantage of using MPPT. If you have a solar panel array that has a higher nominal voltage than the battery bank, a PWM charge controller is just going to throw away that extra voltage. Let us use as an example a 12V battery bank with two different 100W panels: one a 12V and one a 24V. Since W = V x I, a 100W 12V nominal panel with a Vmp of 18V has an Imp of 5.56A. 18V x 5.56A equals 100W. Likewise, a 24V nominal 100W panel has twice the voltage but half the current 36V x 2.78A equals 100W. As we saw previously, a PWM charge controller brings the panel voltage down to the battery voltage. So in the 12V panel, the volts get pulled down from 18V to 13 V. 13V x 5.56A equals 72W. So on the 24V panel, it does the same thing. It pulls the voltage from 36V down to 13V but remember that the current was half of that on the 12V panel. So with the current at 2.78A times 13V, that equals 36W. You have lost most of the power from that 24V panel by throwing away those extra volts.
Now let us do that with an MPPT charge controller. The 18V is dropped down to 13V at a ratio of 1.38. 18V divided by 13V equals 1.38. Increasing the current by 1.38 as well raises it to 7.7A. 13V times 7.7A equals 100W. The 24V panel’s 36V is dropped down to 13V with a ratio of 2.7. 36V divided by 13V equals to 2.7. So the current is raised by 2.7 to 7.7A. That is the same voltage and current output from the 12V panel. So we are able to use all 100W of the power of the 24V panel in the 12V battery bank getting all the power available. This is very useful as higher voltage panels are usually also higher wattage.
You can get a panel that is 200W to 300W in a 20V or 24V nominal panel. 12V panels tend to go well under 200W. The higher wattage panels are also generally less expensive per watt than the smaller panels.
When should you use a PWM versus an MPPT charge controller? PWM works great on smaller systems and where the nominal voltage of the panels match the voltage of the battery bank. Remember that wiring panels in series increases the voltage. If you have two 12V panels and a 24V battery system, you can wire the panels in series to make 24V. In most cases, with a small system, the cost of increasing the panel size to get more power is less than the cost of going from PWM to MPPT. Going from a 100W panel to a 130W panel to get 30% more power will cost less than going from a PWM to an MPPT charge controller. An MPPT is worth it if you have a larger system that can benefit from the extra percentage of power from an MPPT. For example, if you have a 2,000W array and you can increase the output by 25%t with MPPT, that is like adding another 500W of solar panels at approximately 1USD/W, that is 500USD just for the panels not to mention the extra wires and racking needed. In that case, it is probably cheaper to buy an MPPT charge controller to get that increase than to add extra panels. An MPPT charge controller is required when the voltage of the array doesn’t match the array bank voltage. Another great reason to purchase an MPPT controller is when the size and space that the solar panels take up is needed to be kept to a minimum but the user wants to extract the absolute maximum amount of charge from their solar array. For example portable solar power generator kits.
PROS and CONS
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Which one is the right Charge Controller for your solar system? PWM or MPPT?
PWM charge controller
When a solar array is connected to the battery through a PWM charge controller, its voltage will be pulled down to near that of the battery. This leads to a suboptimal power output wattage (Watt = Amp x Volt) at low and at very high solar cell temperatures.
In times of rainy or heavily clouded days or during heavy intermittant loads, a situation may occur where the battery voltage becomes lower than is normal. This would further pull down the panel voltage; thus degrading the output even further.
At very high cell temperatures the voltage drop off point may decrease below the voltage needed to fully charge the battery.
As array area increases linearly with power, cabling cross sectional area and cable length therefore both increase with power, resulting in substantial cable costs, in the case of arrays exceeding a few 100 Watts.
The PWM charge controller is therefore a good low cost solution for small systems only, when cell temperature is moderate too high (between 45°C and 75°C).
MPPT charge controller
Besides performing the function of a basic controller, an MPPT controller also includes a DC to DC voltage converter, converting the voltage of the array to that required by the batteries, with very little loss of power.
An MPPT controller attempts to harvest power from the array near its Maximum Power Point, while supplying the varying voltage requirements of the battery plus load. Thus, it essentially decouples the array and battery voltages, so that there can be a 12V battery on one side of the MPPT charge controller and two 12V panels wired in series to produce 24V on the other.
If connected to a PV array with a substantially higher nominal voltage than the battery voltage, an MPPT controller will therefore provide charge current even at very high cell temperatures or in low irradiance conditions when a PWM controller would not help much.
As array size increases, both cabling cross sectional area and cable length will increase. The option to wire more panels in series and thereby decrease current, is a compelling reason to install an MPPT controller as soon as the array power exceeds a few hundred Watts.
MPPT charge controller is therefore the solution of choice:
- If cell temperature will frequently be low (below 45°C) or very high (more than 75°C).
- If cabling cost can be reduced substantially by increasing array voltage.
- If system output at low irradiance is important.
- If partial shading is a concern.