Solar Charge Controller Comparison: MPPT vs PWM

NCTlighting will help you understand the difference between PWM or pulse width modulation, and MPPT or multi-point power tracking charge controllers and when to use each.

Before go further, we want to go over a quick reminder of power calculations.

Solar panels are rated by their actual output when connected to a load. For instance, a generic 300 watt 24 volt nominal panel has an actual output of 36.1 volts VNP, and 8.3 amps.

When you multiply them together, you get the rated Watts, 299.63 watts or round it up to the 300 Watts. You can see the specs on the solar modules back or in its datasheet.

A charge controller is an important component in a battery based system. They’re not used in straight 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 and many of them control the current and voltage at which it charges. More on that in a moment, at night, the voltage of the battery bank is higher than that of the array that it’s 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, allowing 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 these two controllers.

Generally, this information will be printed on the regulator’s label, or you’ll find it by checking the product manual. Beware, there are many unscrupulous sellers particularly on eBay who simply print MPPT on a standard PWM controller.

To prevent being fooled, it’s worth noting that MPPT is substantially more expensive to manufacture than PWM. So if the price seems too good to be true, it certainly is.

Both PWM and MPPT ensure the batteries have been charged at the right voltage based on their state of charge. However, how they perform in a system are very different from each other.

An MPPT charge controller is more expensive than PWM. Let’s go over why it’s 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.

MPPT charge controllers measure the VMP voltage of the panel and down convert the PV 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. So you’re using more of the available power from the panel.

Let’s see how this affects our system with a 100 watt 12 volt nominal panel with a 12 volt nominal battery bank. We’ll do the math assuming 100% efficiency which isn’t what you’ll see in real world, but it will really help illustrate the difference between PWM and MPPT quite clearly.

With PWM when the battery voltage is low, say 11 volts, when the charge controller connects the panel’s output to the battery, the solar panels output is pulled from 18 volts down to 11 volts. With the maximum power current of 5.56 amps. The charge controllers output into the battery is 11V times 5.56A which equals 61 watts (11V*5.56A=61W).

When the battery is fuller at 14 volts, more of the panel’s available voltage is used, and with the same 5.56A. This charge controller is outputting 78W into the battery. 14V times 5.56A equals 78W (14V*5.56A=78W).

Let’s do the math with an MPPT charge controller. When the battery is low, it drops the voltage from 18 volts down to 11 volts. And that drop is a ratio of 1.6, 18 volts divided by 11 volts. So 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 times 8.9A equals 97 watts (11V*8.9A=97W). Compare that with a PWM output of only 61 watts.

If we do that same math for a fuller battery at 14 volts, the in versus out ratio works out to 1.28. So that increases the current from 5.56 amps to 7.1 amps, multiply that times the 14 volts, that equals 99.4 watts (14V*7.1A=99.4W).

So let’s compare those two outputs with the same battery and same panel, just going from a PWM to an MPPT charge controller. You’ve seen how an MPPT charge controller can maximize the output when the solar panels nominal voltage matches the battery banks nominal voltage.

Here is another huge advantage of using in 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’s give an example of a 12-volt battery bank with two different 100 watt panels. One a 12 volt and one a 24 volt. Since what’s equals volts times amps, a 100 watt 12 volt nominal panel with a VMP of 18 volts has an eye MP of 5.56 amps. 18 volts times 5.56 amps equals 100 watts (18V*5.56A=100W).

Likewise a 24 volt nominal 100 watt panel has twice the voltage, but half the current 36 volts times 2.78 amps equals 100 watts (36V*2.78A=100W).

PWM Charge Controller

As we saw previously, a PWM charge controller brings the panel voltage down to the battery voltage, so in the 12 volt panel, the volts get pulled down from 18 volts to 13 volts. 13 volts times 5.56 amps equals 72 watts (13V*5.56A=72W).

So on the 24 volt panel, it does the same thing, it pulls the voltage from 36 volts down to 13 volts, but remember that the current was half of that on the 12 volt panel. With the current at 2.78 amps times 13 volts that equals 36 watts (13V*2.78A=36W). You’ve lost most of the power from that 24 volt panel by throwing away those extra volts.

Now let’s do that with an MPPT charge controller.

The 18 volts is dropped down to 13 volts at a ratio of 1.38. 18 volts divided by 13 equals 1.38 (18V/13V=1.38). Increasing the current by 1.38 as well raises it to 7.7 amps. 13 volts times 7.7 amps equals 100 watts (13V*7.7A=100W).

The 24 volt panels 36 volts is dropped down to 13 volts with a ratio of 2.7. 36 volts divided by 13 volts equals to 2.7 (36V/13V=2.7). So the current is raised by 2.7 to 7.7 amps.

That looks familiar. That’s the same voltage and current output from the 12 volt panel. So we’re able to use all 100 watts of the power of the 24 volt panel in the 12 volt battery bank getting all the power available. This is very useful.

As higher voltage panels are usually also higher wattage. So you can get a panel that is 200 to 300 watts in a 20 or 24 volt nominal panel whereas 12 volt panels tend to go well under 200 watts. The higher wattage panels are also generally less expensive per watt than the smaller panels. So depending on your system, they can often be the less expensive solution to go with.

Conclusion

Compared to the bigger panels and the MPPT charge controller, even though the charge control is more expensive. When should you use a PWM versus an MPPT charge controller. PWM works great on smaller systems, and where the nominal voltage of the panel’s match the voltage of the battery bank

Remember that wiring panels in series increases the voltage. If you have two 12-volt panels and a 24 volt battery system, you can wire the panels in series to make 24 volts, 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 100 watt panel to a 130 watt panel to get 30% more power will cost less than going from a PWM to an MPPT charge controller. And MPPT is worth it if you have a larger system that can benefit from the extra percentage of power from an MPPT.

For instance, if you have a 2000 watt array, and you can increase the output by 25% with MPPT. That’s like adding another 500 watts of solar panels. At approximately $1 per watt, that’s $500 just for the panels. Not to mention the extra wires and racking needed.

In that case, it’s 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 kits for camping.

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