Solar Charge Controller Calculator

Graham Bogie

This solar charge controller calculator will help you correctly size this important component in your camper solar setup.

It’s simple and straightforward to use. We’ve included a section below to answer some questions you may have.

This calculator forms part of our camper solar setup series

And it’s just one of our electrical calculators for RVs and campervan conversions too.

Once you’ve sized your controller, head on over to our detailed step by step solar system installation guide and you’ll be living off-grid before you know it.

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How do I use this solar charge controller calculator?

  1. Select the battery voltage – 12v or 24v
  2. Enter the total watts in your solar array 
  3. Enter the number of solar panels in the array
  4. If you know it, enter the Open Circuit Voltage (Voc) of your panels
  5. If you know it, enter the Short Circuit Current (Isc) of your panels

How do I find the battery voltage?

Batteries usually have a label on the size and somewhere on that, it will state the voltage – usually 12v or 24v.

In the unlikely even that the battery doesn’t show the voltage, use a multimeter to test it.

Check out our complete guide for how to use a digital multimeter in your camper.

I don’t know how many watts or solar panels I need. What should I do?

Avoid buying a solar charge controller until you know exactly what size solar setup you need.

Solar charge controllers are one of the most expensive components of the entire system so it is best to size the system before spending your hard earned cash.

Use our solar calculator to help size your panels.

What watts should I use?

If you’re installing as much solar wattage as you have room on your roof, just enter the total watts.

But if you want to add more panels later, even a portable panel, include the extra wattage now.

This will allow you to size a solar charge controller large enough to handle the larger setup.

MPPT charge controllers are quite expensive. It’s more cost effective to buy a large controller now than have to replace it later.

How do I find the Voc or Isc of my solar panels?

Every solar panel has a detailed specification including the Open Circuit Voltage (Voc) and Short Circuit Current (Isc).

This is usually printed on a sticker on the back of the panel. 

I don’t know the Voc or Isc of my panels. What should I do?

For accuracy, it’s best to enter the Voc and Isc of your chosen solar panels into the calculator.

If you haven’t already bought your solar panels but know the model you’re getting, you can usually get the detailed specification from the manufacturer’s website.

Our solar charge controller calculator will work without them though, using an estimated Voc and Isc. 

Just leave the fields set to zero in the calculator and we’ll do the rest.

It’s pretty accurate for panels upto 200w. 

Most solar panel of the same wattage have more or less similar specifications, so the calculated result is a good indication of the size of the charge controller you need.

For more assurance though, try to get hold of the specification for your chosen panels.

I have mixed solar panels. Which Voc & Isc should I use?

When sizing solar charge controllers, we need to understand how the array will be wired – in series or in parallel.

When a solar array is wired in parallel, the lowest voltage is used to calculate the solar charge controller.

If wiring is series, the lowest current is used.

So if you have mixed solar panels, use the lowest Voc and the lowest Isc from all panels. This may mean using a Voc from one panel and an Isc from a different panel. 

What is the difference between wiring my panels in series or parallel?

A solar array wired in parallel produces a much higher current than when wired in series. 

When wired in series, the solar array produces much higher voltages.

Each of the scenarios has their advantages and disadvantages, especially when installed in a campervan for off-grid living.

You can read more about wiring solar panels in series and parallel here.

What is the difference between a PWM and MPPT charge controller?

The main differences between a PWM and MPPT charge controllers is price and efficiency ratings.

A PWM controller is low cost but even when performing at its peak, isn’t especially efficient. 

If you want (or need) a solar charge controller to squeeze as much juice out of the wattage on the panels, a highly efficient MPPT controller is the way to go.

You can read more about the differences between PWM and MPPT charge controllers in this post.

My solar setup produces too much current. What does this mean?

The largest MPPT solar charge controller today can handle upto 100 amps or current. 

If your setup is any larger than this, you’ll need to split your configuration into multiple array, each with a dedicated solar charge controller.

To size the new configuration, enter the revised details of each array into the calculator. You’ll need to calculate each separate array in turn.

My solar setup produces too high a voltage when wired in series. What does this mean?

Solar arrays wired in series produce high voltages. The largest MPPT solar charge controller today can handle upto 250 volts.

If your setup is any larger than this, you’ll need to split your configuration into a mix of series and parallel.

If you received this message from the calculator, we have recommended a correctly sized solar charge controller to handle the mix of 2 even strings wired in parallel.

You can see our solar panel wiring diagrams for more detail about how to do that.

The calculator has suggested I speak to the local energy company. Why?

This solar charge controller calculator is aimed at RV, campervan, motorhome and caravan owners.

It’s unlikely even the largest set up would produce so much current and voltage as to need more than 2 MPPT charge controllers.

It’s likely you either have entered incorrect numbers, left your finger on the keypad for a little too long or are trying to generate enough electric to power the Blackpool illuminations. 

Our little calculator just isn’t up to it.

Need More Help Calculating Your Campervan Electrical Components?

Here’s the links to our other calculators:

Graham Bogie

Graham Bogie - Expert in RV & Campervan Electrics - Mowgli Adventures

Graham is a seasoned marine electrical engineer with two decades of experience designing customized electrical systems for plant machinery and converting campers and overland vehicles. His expertise has led him to author the reputable Campervan Electrics Handbook and become the chief designer of our Campervan Electrical Design Service. As a knowledgeable figure in the field, his YouTube channel, blog, Facebook group, and newsletter, offering electrical advice and product reviews, reach more than a million users each year.

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