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What Size Solar Charge Controller? (How To Calculate)

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Do you need to know what size solar charge controller you need for your solar system? This post will help you answer this question in the next 5-10 minutes.

When we talk about sizing a solar charge controller, what we really mean is figuring out what “amp-rating” you need to fully leverage the power produced by the solar panels to charge your batteries. The higher the amp-rating, the more power the charge controller can send to the batteries to charge them (assuming the controller receives the same amount of power from the solar array).

If you use an undersized charge controller relative to your solar array, the charge controller won’t be able to utilize the full power of the array to charge the batteries. Conversely, if you buy too large a charge controller for your system, you’re simply wasting money.

In this post, we provide a simple two-step process to easily and quickly help you calculate what size solar charge controller you need.

If you’re ready, let’s get to it.

Top Pick
Victron MPPT 100/30 Solar Charge Controller

We love this Victron controller not only for it's high quality build, but the Bluetooth feature is rock solid. The Victron smartphone app provides an amazing interface to monitor all your solar data. This 30A charger is good for solar systems up to 440W (for 12V batteries).

We earn a commission if you make a purchase, at no additional cost to you.
09/17/2023 02:16 am GMT

Step 1:

Finalize Solar & Electrical Specs

Before you can calculate your ideal solar charge controller size, you will need to finalize two details of your solar & electrical system. These are:

  1. Total wattage of solar array
  2. Operating battery voltage

We’ll go into each of the two details below and explain WHY they determine the eventual size of your solar charge controller.

1. Total Wattage Of Solar Array

Unsurprisingly, the larger the wattage of your solar array, the larger the solar charge controller you will need. A single 100W solar panel will only require a small 10A charger, but a large 600W array will require a 40-50A charger.

Therefore, finalizing the total solar array wattage is a critical first step.

Example: For our camper van, we decided to install a 380W solar array (2x180W panels).

2. Operating Battery Voltage

The solar charge controller will be wired to the auxiliary battery that you are trying to charge with the solar panels. And the battery’s operating voltage will factor into the size of the solar charge controller.

In short, the higher the battery voltage the smaller the solar charge controller you’ll need. This is because the higher the volts, the lower the amps required to push the same amount of power to the batteries.

Most auxiliary batteries operate at 12V. But yours might also be 24V, or even 48V.

Example: Our solar charge controller will be connected to a 3600Wh battery bank operating at 12V.

Step 2:

Calculate Solar Chage Controller Size

Now that you finalized your total solar array wattage AND battery operating voltage, refer to our below table to find the recommended solar charge controller size for your system.

Note: The left-most column is for 12V systems and the middle column is for 24V systems.

Solar Watts
(12V System)

Solar Watts
(24V System)

Charge Controller

(Budget Model)


(Budget Model)













Example: We use the Victron 30A charge controller because our solar array wattage (380W at 12V) falls within the 220-440W range.

With exception to the first row, which is a budget-friendly Renogy PWM charge controller, our table only recommends Victron controllers. This is because we personally use a Victron 30A charge controller and can vouch for their ‘day in and day out’ durability. We also love the Victron app, which pairs to the charger controller via Bluetooth and allows us to view all our solar stats on our phone. Read our Victron solar charge controller review for more information.

If you need a budget-friendly alternative, use our table to first help you identify the charge controller amp-rating you need and then do a search on Amazon for a different brand.

That's It...For Now...

Charge Controller Size Summary

If all you want to know is what size solar charge controller you require, then you already have all the information you need. However, sizing a charge controller is just one of several calculations you’ll need to do.

For additional solar info, we recommend the following articles to help you install the perfect DIY solar system:

If you’re looking for a complete solar charge controller wiring diagram, like the image below, download our free solar wiring eBook to help you with your installation project.

Wiring solar charge controller to bus bars & indicating wire size.
Wiring charge controller to bus bars

For additional solar charge controller information, keep reading below.

Additional Info

What Does A Solar Charge Controller Do?

In simplified terms, a solar charge controller regulates the voltage coming from the solar panel array to keep your batteries properly charged and healthy.

There are a couple of additional important features that we will also go over.

A solar charge controller serves four important functions to your solar array and batteries.

1. What Does A Solar Charge Controller Do?

Solar Charge Controller Reducing Volts Before Feeding Battery
Solar Charge Controller Reducing Volts Before Feeding Battery

When trying to understand WHY a solar charge controller is important for charging batteries, it’s critical to know two important facts.

  1. Batteries can only be charged by being fed power from a small voltage range (usually between 13.6v – 14.4v depending on the battery chemistry and state of charge).
  2. The voltage produced by a solar array is wide ranging and depends entirely on the size of the solar array and whether series and/or parallel connections are used.

To simply feed a battery a wide range of voltages is dangerous and can permanently shorten the lifespan of the battery.

Therefore, a solar charge controller is used to regulate the wide ranging & inconsistent incoming voltage from the solar array into a more consistent and usable voltage for the battery to accept.

2. Prevents Batteries From Being Overcharged

Imagine sitting at a buffet with a table full of your favorite foods. Though you’d be tempted to try and eat everything at the table, your stomach and brain work together to prevent you from overeating and harming your body.

A solar charge controller acts similarly to your stomach and brain.

When a battery has been significantly discharged, the charge controller will allow the safe maximum amount of volts to flow into the battery to quickly recharge it.

When a battery nears it’s fully charged state, the charge controller will gradually lower the voltage to maintain the health of the battery.

And finally, when a battery is fully charged, the charge controller will stop the flow of power into the batteries to prevent overcharging.

3. Prevents Reverse Current Back To Solar Panels

Volts usually flow from high voltage to areas of lower voltage.

This is great during the day time when solar panels are producing power and pushing volts towards the charge controller and battery.

But at night, or in shaded areas, the voltage coming from the panels drops significantly and well below the standing voltage of the battery. When that’s the case, power will want to flow from the batteries back towards the panels.

This is called ‘reverse current’.

But a charge controller blocks the voltage from flowing backwards.


4. Provides Valuable Solar Harvesting Data

Although not critical to the function of a solar panel system, certain charge controllers can provide valuable real-time and historical solar energy harvesting data.

We use a Victron Bluetooth-enabled solar charge controller, which we can pair with our smartphones to see insightful solar data as power comes in.

From our smartphones, we can see the instantaneous watts being sent directly to our batteries, as well as the volts and amps breakdown of those watts.

Solar Charge Controller Energy Harvesting Data On Smartphone
Data from Victron Solar Charge Controller

Additional Info

PWM vs. MPPT Solar Charge Controllers

There are two different charge controller technologies on the market today:

  • PWM (Pulse Width Modulation) Controllers
  • MPPT (Maximum Power Point Tracking) Controllers

The technical difference between the two is how they manipulate the volts coming from the solar array in order to charge the batteries. It can get quite technical.

But for the purposes of this post, you only need to know that:

  1. PWM controllers are considerably cheaper than MPPTs, as much as 5x cheaper.
  2. MPPTs are roughly 20% more efficient at charging the batteries from the same amount of sunlight.
PWM vs MPPT: Solar Harvesting Data
PWM vs MPPT: Solar Harvesting Data (source:

In the PWM vs MPPT post from MarineHowTo, the author experimented between the two types of solar charge controllers and found that MPPT controllers were 20% more effective than PWM controllers.

The graph above from MarineHowTo shows how many amps an MPPT and PWM controller harvested in different lighting situations across a 7-day span during the spring season in Maine.

  • MPPT 7 Day Total = 220.44 Ah
  • PWM 7 Day Total = 182.48 Ah
  • MPPT = 20.8% Boost In Efficiency

Thank You For Reading!

We’re Yuko and Eric! We both grew up in Asia ( Japan & Hong Kong ), we left our jobs and homes in 2018 and started traveling full time from Canada to Argentina in our self-converted camper van since end of May 2019. “Asobo” means “Let’s play” in Japanese. We named our site “Asobolife” because the life is always uncertain and we live only once so it’s important to always keep positive, playful mind and enjoy the moment you are in the present. We also want to use this website to share our road trip and van building experiences to inspire our readers. We hope you enjoy!

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