What is a solar charge controller? Do you need one?

If you’re considering installing an off-grid solar project with a battery attached, you’ll want to look into a solar charge controller for your system. Charge controllers act as a gateway to your battery, and ensure that you don’t overcharge and damage your energy storage system. In this piece, we’ll cover what a solar charge controller is, and how the two major types (PWM and MPPT) compare.

Choosing the right solar charge controller: what you need to know

A solar charge controller is a regulator for your solar battery that prevents it from overcharging. Batteries are rated for voltage capacity, and exceeding that voltage can lead to permanent battery damage and loss of functionality over time. Solar charge controllers act as a gate to your battery storage system, making sure damage doesn’t occur from overloading it.

Charge controllers are only necessary in a few specific cases. Most commonly, you will want to look into charge controllers if you are trying to install an off-grid solar system, from rooftop systems to smaller setups on boats or RVs. If you are a homeowner looking to install a solar array with a battery that is connected to the electric grid, there is no need for a charge controller – once your battery is full, excess energy will be directed to the grid automatically instead, helping you avoid overloading your battery.

To help you find the best match, we created a list of the best portable solar products of 2022. Many of these include solar charge controllers for ease of use.

Is a solar charge controller right for you?

For the majority of solar shoppers, there’s no need to worry about charge controllers. Rooftop or ground-mount solar installations with a battery backup are almost always linked to the electric grid, and in the case that your battery completely fills up, your excess solar energy will automatically reroute there.

If you are interested in installing a small off-grid solar energy system with battery backup, you might need to look into a charge controller to ensure that your battery is safely charged. For relatively small batteries paired with low-output 5-10 Watt solar panels, a PWM charge controller should do the job. For more complex DIY solar projects with higher output panels, you may want to consider a MPPT charge controller.

Types of solar charge controllers: PWM and MPPT

If you want to use solar to go completely off-grid, there are two types of charge controllers to consider: Pulse Width Modulation (PWM) controllers and Maximum Power Point Tracking (MPPT) controllers.

Pulse Width Modulation (PWM) solar charge controllers

PWM solar charge controllers are the standard type of charge controller available to solar shoppers. They are simpler than MPPT controllers, and thus generally less expensive. PWM controllers work by slowly reducing the amount of power going into your battery as it approaches capacity. When your battery is full, PWM controllers maintain a state of “trickle”, which means they supply a tiny amount of power constantly to keep the battery topped off.

With a PWM controller, your solar panel system and your home battery need to have matching voltages. In larger solar panel systems designed to power your whole home, panel and battery voltage aren’t typically the same.  As a result, PWM controllers are more suited for small DIY solar systems with a couple of low voltage panels and a small battery.

Maximum Power Point Tracking (MPPT) solar charge controllers

MPPT solar charge controllers are a more expensive and complex charge controller option. They provide the same switch-like protection that a PWM controller does, and will reduce the power flowing to your home battery as it nears capacity.

Unlike PWM controllers, MPPT charge controllers can pair non-matching voltages from panels and batteries. MPPT controllers adjust their input to bring in the maximum power possible from your solar array, and can also vary their output power to match the attached battery. This means that MPPT charge controllers are more efficient than PWM controllers, and more effectively utilize the full power of your solar panels to charge a home battery system.

If efficiency were the only concern in purchasing a controller, an MPPT controller would be the best choice every time. But it’s not always practical. Selecting the right solar charge controller involves several factors beyond just efficiency. To determine what controller is right for you, answer these questions:

1. What type of panels do you have?

Most off-grid solar panels are 36-cell panels designed to charge a 12V battery. These systems work well with PWM controllers. 60-cell and 72-cell panels are typically used with a grid-tie solar panel system and have a higher voltage, thus requiring an MPPT controller.

2. How big is your system?

A PWM controller works with any system size as long as the voltage between the solar system and home battery match—though typically they don’t match in larger systems, making a PWM ideal for smaller setups. MPPT controllers are less efficient unless your array is at least 170W.

3. What temperatures can you expect?

MPPT controllers work better than PWM controllers when it gets colder. As the temperature drops, the voltage increases, and an MPPT controller can capture the excess voltage. In warm climates where the temperature doesn’t typically get very low, there isn’t extra voltage and an MPPT controller isn’t necessary.

4. What is your budget for a controller?

In general, MPPT charge controllers are more expensive than PWM controllers due to their higher charging efficiency. MPPT controllers can reach up to 20% higher efficiency – this is due to its four-stage charging method that is generally healthier for your battery. PWM solar charge controllers are more versatile and more easily installed, but their lower efficiency usually means lower prices.

How much do PWM and MPPT solar charge controllers cost?

Solar charge controllers usually cost a few hundred dollars, with variation depending on features and type.

You don’t have to build your own solar setup to start saving money

On the EnergySage Marketplace, you can register your property to begin receiving quality quotes for solar installations. If you are interested in storage solutions to pair with your panels, you can simply indicate your interest on your profile for installers to see. Connecting your solar project to the grid (even with battery backup) is a smart move, as it provides a second backup for your system, and in the case that your battery storage capacity isn’t enough, you won’t simply run out of power to use.

While going completely off-grid with a DIY solar project may work in some cases, if your main concern is saving money, hiring a qualified installer to help you go solar is still a sound financial decision. What’s more, having a professional installer work on your solar project ensures that you are getting the expertise you need to have a functional and effective solar system. Installers also offer warranties and protections for their products that you can’t always get with a DIY project.