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Estimated Read Time: 10 minutes

TL;DR: Choose the right-sized solar charge controller by considering your system's energy needs, operating temperature, battery chemistry, and professional advice. Off Grid Direct offers a range of controllers with free shipping, price match guarantees, and a 30-day return policy.

When it comes to solar charge controllers, bigger isn’t always better.

Unless you’re planning to fit more solar panels into your system, an oversized charge controller will be underutilised and a waste of money.

You’re better off to purchase a smaller and more affordable controller that suits your setup and performs all the necessary functions, including:

  • Solar output monitoring to ensure charge effectiveness
  • State of charge monitoring to prevent overcharging
  • Auto shutdown in case of voltage spikes to protect your devices
  • MPPT Technology (Maximum Power Point Tracking)

We’ve gathered five tips to keep you informed and help you avoid oversizing. As we delve into each idea, you’ll also learn:

1. Determine Your System’s Energy Needs

To determine a system’s energy needs, you need to compare the maximum power output (wattage) of the solar panels relative to the nominal voltage of your batteries.

The division of these values (wattage/nominal voltage) results in a system’s power rating and an effective way of sizing your charge controller.

This equation accurately predicts the energy level your charge controller will have to cope with, thereby guiding your choice in terms of size and preventing a mismatch.

The best-sized device should be equally rated with an additional 25 percent safety factor for unexpected conditions, such as current-raising temperature drops.

Let’s put this into perspective.

Suppose your panels have a maximum output of 300 watts and your battery has a 12V nominal voltage. What is the optimum size for your charge controller?

  • The system’s energy rating will be 25 Amps (from a division of 300 watts by 12 volts).
  • The best-sized controller will be equal to 25 Amps plus a 25 percent safety factor.
  • A 25 percent safety factor of 25 Amps is 6.25A (from the multiplication of 0.25 by 25 Amps).
  • As a result, the optimum size for your charge controller will be 31.25 Amps (from a sum of 25 and 6.25 Amps).

solar charge controller tips to avoid oversizing photo snippet

2. Take Into Account the Operating Temperature

As mentioned earlier, temperature can affect a solar panel’s output current, hence the 25 percent safety factor when calculating the energy level a your controller will be handling.

However, a device’s rated operating temperature range is also important.

When conditions exceed this temperature range, a charge controller will suffer in its ability to regulate panel output.

The transistors within the device that acts as switches malfunction from the heat and experience current leakage. This results in improper energy regulation and charging inefficiencies as is the case with an oversized charge controller.

Take the instance of a 50A charge controller regulating a 10A solar panel. The unutilized 40A charging potential means a longer charge time for your battery and an overall inefficient solar setup.

Similarly, when conditions fall below the specified operating temperature, the components within your controller malfunction from the cold.

The transistors lose conductivity and experience delayed on-off switching times. This creates sensor inaccuracies and impacts the voltage regulation ability of your charge controller. As a result, your solar charging system becomes ineffective.

Choose a solar controller with a wide operating temperature range for optimal results. This ensures resilience and enables your solar system to work efficiently in various environments and conditions.

The Victron BlueSolar MPPT Charge Controller, for instance, has an impressive operating temperature of between -30 to 60°C.

This device can therefore withstand the scorching conditions of the Pilbara region of Western Australia, known to hit 50°C in the summer. You can be confident that it’ll put up with any condition you throw at it.

Victron BlueSolar MPPT Charge Controller

“It rapidly finds the maximum power point and seems to build local profiles to swap back to in rapidly changing conditions, from direct sun to dark rain and in the middle.”
Neil F. reviewing Victron BlueSolar MPPT

3. Choose the Right Type of Charge Controller

There are two main types of charge controllers: Maximum Power Point Tracking (MPPT) and Pulse-Width Modulation (PWM).

Both prevent overcharging and undercharging, but they have distinct technologies with size implications that must be considered to avoid oversizing.

A PWM controller uses a simple on-off transistor technology to regulate the voltage sent to your batteries. As such, it’s cheaper and highly robust, but better suited for smaller solar setups.

PWM controller from Off Grid

Its simplicity is such that a PWM controller will always limit energy to its rated voltage, regardless of panel output.

So, a 13V PWM controller will limit charge flow to 13 volts even if you connect it to an 18-volt panel output. The extra energy is simply dissipated as heat, making this controller inefficient for larger solar setups.

An MPPT controller, on the other hand, uses a more sophisticated voltage monitoring technology to extract maximum power from your panels. This makes it pricier, but more efficient for larger solar setups.

When connected to a large solar array, an MPPT controller will drop the panel output voltage to match your battery while increasing the charging current in the same ratio.

This adjustment allows it to draw maximum power from all your panels, speeding up charge time and enhancing efficiency.

The following table highlights the major differences between PWM and MPPT charge controllers to help you make the right choice.

Solar Charge Controller Application Price
PWM For smaller systems (under 200W) where efficiency isn’t critical such as trickle chargers From $50
MPPT Essential for larger systems (over 200W) where the extra energy yield from efficiency is worthwhile such as house panels. Still beneficial in smaller systems From $150

4. Consider Your Battery Chemistry

Batteries have different chemistries and a distinct charging profile with implications for your solar charge controller and its size. Therefore, it’s crucial that you understand your battery and its charging requirements to avoid oversizing.

Lead-acid batteries have a complex multistage charging profile requiring various current levels. These stages include a bulk charge, taking up 70 percent of battery capacity, and the absorption and float charge, using the remaining 30 percent.

This chemistry requires a multistage charge controller with a matching current rating (size) to deliver the right charge at each stage—preferably an MPPT controller because of its current adjusting capability.

Lithium-ion phosphate batteries (LiFePO4) have a simpler charging profile and don’t require voltage adjustments. However, they’re incapable of overcharging and only take what they can absorb.

Therefore, a simple on-off PWM controller of the right voltage will be effective at delivering a constant charge and switching off once your battery is full. However for Maximum charge efficiency a MPPT should be used.

5. Get Professional Advice

Getting professional advice will help you size your charge controller based on the critical aspects discussed above. It also ensures you pick the best model for your specific system and location.

But the biggest advantage is that a solar expert will help you get the most out of the properly sized components through proper installation.

The DIY videos certainly make it look easy, but proper installation goes beyond standard screws and wrenches. It takes specialised equipment to ensure each component is primed to deliver maximum efficiency. For example:

  • A solar pathfinder to assess the shading on your rooftop and determine the best placement for maximum solar exposure
  • A multimeter to test the overall efficiency of your system by monitoring current flow from panels to batteries and by showing readings on electrical resistance
  • A tiltmeter to test the angle placement of your panels to ensure maximum sun exposure
  • An infrared camera to spot temperature differences across your panels before they cause system failure

When it comes to solar charge controllers, it’s easier and faster to consult a professional.

Get the Right-Sized Charge Controller From Off Grid Direct

So there you have it. Five tips to help you avoid oversizing your solar charge controller. With these ideas in mind, here’s why we think you should shop at Off Grid Direct.

We’re passionate about bringing value to the marketplace and excited when the customer (you) gets the best deal for bringing sustainability to their home or business.

As such, we offer:

  • Free shipping for orders above $300
  • Price match guarantees if you can find a better offer out there
  • 30-day easy returns if you’re not happy with your purchase

Contact us today or check out our solar solutions and take advantage of our incredible offers.

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