What Is Solar Charge Controller? Do You Need One?

Depending on the kind of solar+storage system you have, you may or may not need one.

A solar charge controller

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A solar charge controller benefits a solar+storage system. The solar+storage system allows customers to use solar off-grid, either full-time or as a backup during power outages. The controller controls the amount of energy being sent into the battery backup so that the battery doesn't exceed its voltage capacity—thereby extending the life of the battery and avoiding any damage to it.

Depending on the kind of solar+storage system you have, you may or may not need a solar charge controller. In this article, we'll go through everything you need to know about this tool.

Benefits and Types

The output of a solar panel can vary depending on levels of sunlight, ambient temperature, the quality of the solar cells in the panel, and other factors. All of these factors affect the efficiency of the panels in converting sunlight into electricity.

A solar charge controller smooths out that variability so that batteries receive power at a constant and safe rate. It also sends a “trickle charge” when the battery is nearly full. Since batteries regularly lose a small amount of charge, a trickle charge keeps the battery topped off without overcharging it.

If you have a grid-tied solar+storage system, either ground-mounted or on your roof, you most likely have no need for a solar charge controller. Your excess solar energy will automatically flow into the grid when your battery is full. But if your solar system is operating off-grid, a controller might be a wise investment.

There are two main types of solar charge controllers, Pulse Width Modulated (PWM) and Maximum Power Point Tracking (MPPT). PWM controllers are better suited for small solar+storage systems with low voltage panels and small batteries. MPPT controllers are more expensive, but have more features and advantages; they are recommended for any solar system over 200 watts.

PWM Controllers

The main function of a PWM controller is to protect your battery by making sure solar panels have the same voltage as the battery. The voltage of the battery must match the “nominal voltage” of a solar panel—that is, the voltage the panel is marketed as having, even if the actual voltage can vary and often be slightly higher.

As its name suggests, a PWM controller controls the flow by pulsing the power sent to the battery to slow down or speed up the charging. Some PWM controllers can only handle one voltage level, while others can handle different levels. Either way, the voltage of both battery and panel must still be the same.

PWM controllers are uncomplicated devices, though some can have additional features added to their essentially basic systems.

MPPT Controllers

Unlike PWM systems, where the voltage of battery and panels must be the same, MPPT controllers can charge a lower voltage battery from a higher voltage solar array and, in some cases, a higher voltage battery from a lower voltage solar array.

In electrical systems, voltage and amperage are inversely related: The higher the voltage, the lower the current (amperage), and vice versa. Since an MPPT controller controls the rate and current of the voltage flowing from solar panels to a battery, off-grid solar+storage systems can have panels of different voltage than their batteries.

So while a solar panel by itself with a nominal voltage of 24 volts is insufficient to power a 48-volt battery, an MPPT controller allows it to work by halving the amperage, thereby doubling the voltage flowing into the battery.

Treehugger Tip

For fans of golfing or boondocking (off-grid RV living), an MPPT controller allows you to charge a 36-volt or 48-volt battery with a single flexible 12-volt solar panel attached to the roof of your RV or golf cart.

Solar panels charging an electrified camper van.

Cindy Shebley / Getty Images

MPPT controllers can increase the efficiency of your solar panels by 20% to 30% by tracking the optimal voltage-to-current ratio to maximize output to a battery. It's their increased efficiency that can make their higher cost worth it.

Costs

The simplest PWM controllers can cost $60 or more. Those with additional features can cost up to $200.

MPPT controllers can do more, but cost more, running from $500 to over $1,000, depending on the features. If there is a long distance between your panels and battery, however, you can save money by using lower-gauge electrical wiring between the two, since MPPT controllers modulate current and voltage.

And because MPPT controllers can convert the higher output of a solar array to the lower voltage of a battery, a battery can capture more of the sun's energy. This will increase its efficiency and potentially shorten the payback time of the more expensive system, as well as increase your ability to rely solely on solar energy.

Some optional controller features that come at additional costs include:

  • LEDs to allow users to monitor the voltage and amperage of the system.
  • Internet-enabled controllers to allow for remote monitoring.
  • Multiple outputs to allow for charging two separate batteries.
  • Longer cables for greater distances between panels and battery.
  • Temperature sensors, which will maximize the efficiency of charging since batteries charge at different rates during different temperatures.
  • Low voltage disconnects, which will automatically disconnect an attached DC-operated device (such as a golf cart) when the battery is low.

As always with solar products, anticipate your solar needs and capabilities, and calculate the costs and payback time to see what kind of system works best for you.