Solar charge controllers generally come in two varieties, which are abbreviated as PWM and MPPT. These stand for Pulse Width Modulation (PWM) and Maximum Power Point Tracking (MPPT), or the technology each model uses to actually control the flow of charge in a solar electrical system.
We’ll be getting into the purpose of a solar charge controller below, and how to compare PWM vs. MPPT when it comes to the best solar charge controller for your specific system.
What does a solar charge controller do?
A solar charge controller is a core component of any off-grid electrical system. It sits between your solar panels and your battery bank, where it regulates the voltage and flow of electrical current from the panels to the batteries.
This regulation is extremely important for protecting your batteries and your overall system. Too much current all at once (say, on a really sunny day) runs the risk of overcharging your batteries, which can cause damage, overheating, and potential safety issues.
Alternatively, depending on your battery bank’s state of charge (SOC) and its manufacturer, you may need to charge with different amounts of current at different times. A good solar charge controller will be programmable to your battery’s specifications. This is so you can set the appropriate voltages for bulk, absorption, and float charging based on the manufacturer’s recommendations.
This set-up will help keep your batteries in good shape for longer, meaning fewer replacements over time. This programmable, customizable technology will also ensure your batteries charge more efficiently, saving you time and keeping you powered up.
In the most basic terms, think of your solar charge controller as a gatekeeper for the power flowing from your solar panels into your battery bank – and preventative maintenance for your batteries’ long-term health!
How do PWM and MPPT solar charge controllers compare?
You might have heard that MPPT solar charge controllers are more efficient, or that PWM solar charge controllers are less expensive. In truth, the right choice for you will depend on much more than that.
Efficiency
MPPT solar charge controllers tend to be more efficient at charging your battery bank than PWM (some say by up to 30%), simply because of how the technology works.
MPPT In Practice
Maximum Power Point (the MPP in MPPT) refers to the optimal balance between current and voltage. An MPPT solar charge controller works by adjusting the flow of current between your solar array and battery bank to maximize the energy your panels are harvesting, finding that Maximum Power Point on the spectrum of voltage and current and capitalizing on it.
This ensures your batteries and loads are safely able to take advantage of all your potential power. Depending on your batteries’ state of charge (SOC), the weather outside (e.g. how sunny or cloudy it is), and even differing sizes of your solar array and battery bank (e.g. a 90V solar array and 12V battery bank), an MPPT solar charge controller will get you charged up smarter and faster by consistently finding your maximum power point and operating around it.
PWM In Practice
PWM, on the other hand, is less sophisticated. The “Pulse” in Pulse Width Modulation (PWM) can be thought of as an on/off switch between your solar array and battery bank, balancing the solar harvest and batteries’ charging.
A PWM solar charge controller switches the flow of current between your panels and your batteries on and off, in pulses, to reach a full state of charge. This takes place with fewer of the condition adjustments an MPPT charge controller can account for.
While it may still adjust the flow of current based on your state of charge, it will generally take longer for your batteries to charge when relying on a PWM solar charge controller.
Further, a PWM solar charge controller is more limited in the range of solar array voltages it can accommodate. For example, many PWM controllers tap out around 30-50V, whereas MPPT charge controllers can be rated for 400V plus!
Did you know: Many solar panels may be marketed as one voltage (e.g. 12V), but will actually produce a higher voltage (e.g. 20V). Always check the specification sheet to be sure!
System Size
All of that said, efficiency can look very different depending on the actual size of your system.
If you have a smaller system that doesn’t need to power big appliances like an air conditioner or refrigerator, or is just used to supplement your power between electric hook-ups from time to time, you might not notice much of a difference in efficiency.
For example, if an MPPT solar charge controller can get you charged up in an hour, a PWM solar charge controller might be able to do the same in an hour and twenty minutes. That twenty minutes, if it’s only relevant now and then, could be a fine compromise.
However, for larger systems, powering big appliances, and ensuring you have more freedom to get (and stay) off-grid consistently, the efficiency of an MPPT solar charge controller can be a necessity.
An MPPT solar charge controller will save significant, noticeable time when charging a larger battery bank off of solar or shore power, and that time adds up the more you use your system.
Plus, the advanced technology in newer MPPT models will keep your batteries in better shape long-term. This means you won’t need to worry about replacing your batteries as often, which can be a big money-saver on larger battery banks.
Cost
At face value, a PWM solar charge controller will typically cost less up front than an MPPT model. However, paying a little more for an MPPT solar charge controller initially can save you money in the long run.
The overall cost of investing in your solar charge controller will really come down to the size and complexity of your system, and the value of your time spent charging.
If you have a larger system, an MPPT model will greatly improve the life of your batteries – by far the most expensive part of any electrical set-up. If you have a smaller system, the extra time for charging and wear on your battery bank may be less significant if you decide to go with a PWM model.
All of that said, a solar charge controller is one of the least expensive components in your entire electrical system, regardless of which model you choose. We think the immediate cost savings on a PWM model are easily outweighed by the long-term benefits and technical advantages of an MPPT model for most people.
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