This article is slightly technical. But I think it’s very important to really understand how different between PWM and MPPT charge controllers.
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.
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 12 volt battery on one side of the MPPT and panels wired in series to produce 36 volts 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.
In this article, the effect of temperature is analyzed in detail, and a quantitative performance comparison of both controller topologies is given.
MPPT vs PWM: The current-voltage curve and the power-voltage curve of a solar panel
To understand the difference between PWM and MPPT charge controller, let’s first look at a typical curve of a PV panel. The current-voltage curve and power-voltage curve is important because it states the expected power generation of the panel based on the combination voltage (“V”) and current (“I”) generated by the panel.
The examples throughout the following pages are based on an average 100 W / 36 cell monocrystalline solar panel.
The current-voltage curve of this panel is shown in below
From this basic curve the power-voltage curve can be derived by plotting P = V x I against V. The result is the blue curve in figure 2 below.
Obviously, the power obtained from the panel is zero when it is short circuited (0 x Isc = 0) or when no current is drawn from the panel (Voc x 0 = 0).
In between those two zero power points the product P = V x I reaches a maximum: the Maximum Power Point (Pm = Vm x Im).
The importance of the Maximum Power Point can be visualized as follows:
The product Vm x Im is proportional to the surface of the rectangle shown in figure 3. Pm is reached when the surface of this rectangle is at its largest. Figure 4 and 5 below show two less optimal results obtained when power is harvested at a voltage which is too low or too high.
The maximum output of a 100W solar panel is, by definition, 100 W at STC (cell temperature: 25°C, irradiance: 1000 W/m², AM: 1,5).
As can be seem from figure 3, in the case of a 100 W / 36 cell crystalline panel the voltage corresponding to the Maximum Power Point is Vm = 18 V and the current is Im = 5,56 A. Therefore 18 V x 5,56 A = 100 W.
In order to get the maximum out of a solar panel, a charge controller should be able to choose the optimum current-voltage point on the current-voltage curve: the Maximum Power Point. An MPPT charge controller does exactly that.
The input voltage of a PWM charge controller is, in principle, equal to the voltage of the battery connected to its output (plus voltage losses in the cabling and controller). The solar panel, therefore, is not used at its Maximum Power Point, in most cases.
MPPT Charge Controller
In this example, Pm = 100 W, Vm = 18 V and Im = 5,56 A.
With its microprocessor and sophisticated software, the MPPT controller will detect the Maximum Power Point Pm and, in our example, set the output voltage of the solar panel at Vm = 18 V and draw Im = 5,56 A from the panel.
The MPPT charge controller is a DC to DC transformer that can transform power from a higher voltage to power at a lower voltage. The amount of power does not change (except for a small loss in the transformation process). Therefore, if the output voltage is lower than the input voltage, the output current will be higher than the input current.
PWM Charge Controller
In this case the charge voltage imposed on the solar panel can be found by drawing a vertical line at the voltage point equal to Vbat plus 0,5 V. The additional 0,5 V represents the voltage loss in the cabling and controller. The intersection of this line with the current-voltage curve gives the current Ipwm = Ibat.
A PWM controller is not a DC to DC transformer. The PWM controller is a switch which connects the solar panel to the battery. When the switch is closed, the panel and the battery will be at nearly the same voltage. Assuming a discharged battery the initial charge voltage will be around 13 V, and assuming a voltage loss of 0,5 V over the cabling plus controller, the panel will be at Vpwm = 13,5 V.
The voltage will slowly increase with increasing state of charge of the battery. When absorption voltage is reached the PWM controller will start to disconnect and reconnect the panel to prevent overcharge (hence the name: Pulse Width Modulated controller).
Pros and Cons of Both Types of Solar Charge Controllers
PWM charge controllers are built on a time tested technology. They have been used for years in Solar systems, and are well established- These controllers are inexpensive, usually selling for less than $350- PWM controllers are available in sizes up to 60 Amps- PWM controllers are durable, most with passive heat sink style cooling- These controllers are available in many sizes for a variety of applications.
MPPT charge controllers offer a potential increase in charging efficiency up to 30%- These controllers also offer the potential ability to have an array with higher input voltage than the battery bank- You can get sizes up to 80 Amps- MPPT controller warranties are typically longer than PWM units – MPPT offer great flexibility for system growth- MPPT is the only way to regulate grid connect modules for battery charging
The Solar input nominal voltage must match the battery bank nominal voltage if you’re going to use PWM- There is no single controller sized over 60 amps DC as of yet – Many smaller PWM controller units are not UL listed- Many smaller PWM controller units come without fittings for conduit – PWM controllers have limited capacity for system growth- Can’t be used on higher voltage grid connect modules
MPPT controllers are more expensive, sometimes costing twice as much as a PWM controller- MPPT units are generally larger in physical size- Sizing an appropriate Solar array can be challenging without MPPT controller manufacturer guides- Using an MPPT controller forces the Solar array to be comprised of like photovoltaic modules in like strings.
Which one is the right Charge Controller for your solar system? MPPT of PWM?
PWM charge controller
Find PWM Charge Controller at Amazon
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
Read more: Best MPPT Solar Charge Controller Reviews
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, whilst 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 12 volt battery on one side of the MPPT charge controller and two 12 V panels wired in series to produce 36 volts 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 (12 V battery), or several 100 Watts (24 V or 48 V battery).
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.
A quick quide to choosing the right charge controller for your PV system: