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What Solar Controller Do I Need?

Sizing the correct solar controller for your set up is critical. Too small and you risk losing the power your panels are creating, too large and you are wasting money on something you do not need. 

Solar controllers (also known as solar regulators) take the voltage produced by your solar panels and convert it into a safe lower voltage to charge your battery. Batteries require between 14 and 14.7V to charge (depending on the chemistry of battery), so this voltage reduction process prevents your battery from being damaged by receiving too much voltage.

Don't waste your valuable sunshine hours with the wrong controller!

So what do you need?

The two types available are PWM (Pulse Width Modulation) and MPPT (Maximum Power Point Tracking). 

PWM Controllers
PWM controllers are an older technology, and work by simply “pulsating” the solar input on and off (multiple times/second). This results in a portion of the voltage produced by the panel being wasted/burnt off as heat energy to regulate the voltage to that required by the battery. 

MPPT Controllers
MPPT controllers, however, work by converting the incoming excess voltage (downwards), and ramping the amps up, in order to find the optimum voltage by which to charge the battery. The result is that you get the right voltage for your battery but also in increase in current throughput and so these controllers can produce up to 20-50% more power to your battery than a comparable PWM controller (all dependant on the VOC of your panel). The higher the output voltage of the solar panel, the more that is wasted with PWM.

As a general rule of thumb, anything over 120w and/or 18v should have an MPPT controller, or else you risk losing a significant portion of your power production. Below this, a good quality PWM controller is sufficient and more cost effective for small installations in most cases as the excess voltage it needs to clip is minimal. 

The next thing to look at is the type of battery you have.Different batteries (AGM/lithium/calcium/gel) all require different charging profiles (check out our article over here on why you shouldn’t charge a lithium battery with an AGM charger). Most solar controllers and DCDC chargers come defaulted with an AGM/sealed setting, so you have a different battery chemistry, make sure your controller settings can be changed to suit. 

Then there is the functionality you require. Do you want a simple set and forget option? Or do you have a larger array and like to be able to keep an eye on how your system is operating? For the “set and forget” type, we recommend (& use ourselves) the EpEver Tracer BP range.These units are bullet proof and come with a 2 year warranty. Fully sealed, they can withstand not only inclement weather (IP68), but also being knocked around a bit should they be used on a portable installation and are also suitable for in engine bay installation. They can be adjusted for different battery types including LiFePO4 (they come pre-set on AGM settings) using either the Bluetooth e-Box attachment (which pairs with an app on your smart phone) or the MT50 digital remote screen. These attachments can also be used to monitor solar input and output.

If you’re wanting to be able to monitor your system more closely (& don’t need it to be portable/water proof), we recommend (& also use) the Victron range of MPPT controllers. These units can be paired with a smart phone app, allowing real time monitoring of your solar input, have 30 days of history and the ability to log performance data. While they are not water proof, they do come with a 5 year warranty.

How do I determine the correct size controller?

To size a controller you need to consider the potential output amps (current) being produced by your controller. To do this you need to take the solar panel wattage and divide it by the output voltage of the controller (Current=Watt/Volt), a general rule of thumb for this value is around 14V ( this makes an allowance for the different output voltages at the various stages of the charging cycle and for system losses etc). If this value is close to the controller size, you might consider going up to the next size controller. 

The other consideration with sizing a controller is the voltage of the panels, especially if you are planning to connect the panels in series (therefore increasing the voltage) or if you are using house panels (which operate at a higher voltage). The value you need to look at is the Open Circuit Voltage to ensure this is within the parameters of the controller.

Let’s run through an example. Here are the specs for our AllSpark 120W Flexible Solar Panel. If we have just one panel, we take the 120W and divide it by 14, giving us 8.5amp. So in this instance you would probably select a 10amp controller. As the Open Circuit Voltage is 24.6v, you could go for either the EpEver 2610BP or the Victron 75/10.

If we had 3 panels together in series, that’s 360 watt ( 3 x 120w ). 360W/14V = 25.71amp, so you would look to use a 30A output controller ( Victron 100/30 or EpEver 7810BP ) . Then if we take the Open Circuit Voltage of the 120w panel ( 24.6v ), multiply this by 3 ( 3 panels connected in series ) you have 73.8V. So for this setup you would probably again go for the Victron 100/30 (100v / 30A) or EpEver Tracer 7810BP (95v / 30A) as they will accept the higher panel voltage.

What if I have a DCDC?

DCDC chargers come with their own solar input (almost all have MPPT), however, unless you have one with solar priority (like the RedArc BCDC models), your alternator will do all the work whilst driving. In this case, depending on how large your fixed solar array is, having a separate solar controller may still be the best way to go.  If you already own a DCDC or would like to add one to your system, the same principles apply as sizing for your solar controller above (in addition to sizing for your batteries) check out our article over here for more information. 

Remember, no two set ups are the same, and asking a mate or on social media will likely get you a whole lot of answers not specific to your needs. If you’ve had a read through here and are still unsure of which way to go, drop us a line and we will walk you through your options.

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