Charge controller power factoring.

Let me start by saying this applies ONLY to MPPT solar charge controllers or and any buck converter.
Many many people find this confusing and is an essential part of buying a solar charge controller.
In this article, I’ve made the “standard” information and down a bit is the simpler information and generally how I would do my calculations.

How to select a solar charge controller

To calculate the output current of a solar charge controller using a buck function for a 12V system with a 98V solar array, you can use the power balance equation:

Pout​=Pin​×η

where:

• ( P_{out} ) is the output power in watts (W),
• ( P_{in} ) is the input power from the solar array in watts (W),
• ( \eta ) is the efficiency of the buck converter (typically between 0.85 to 0.95).

Assuming you know the power rating of your solar array (in watts), you can calculate the input power. The output power ( P_{out} ) will be the power required to charge the 12V system.

The output current ( I_{out} ) can then be calculated using the formula:

Iout​=Vsystem​Pout​​

where ( V_{system} ) is the voltage of your system, which is 12V in this case.

For example, if your solar array is rated at 500W and the efficiency of the buck converter is 90% (0.9), the output power would be:

Pout​=500W×0.9=450W

Then, the output current would be:

Iout​=450W/12V=37.5A

Here is another example:

Given the input current of 13 amps from your 98V solar array, the output current for your 12V system can be calculated using the power balance equation, considering the efficiency of the charge controller.

First, calculate the input power (( P_{in} )):

Pin​=Varray​×Iin​

Pin​=98V×13A

Pin​=1274W

Assuming an efficiency (( \eta )) of 90% for the buck converter, the output power (( P_{out} )) would be:

Pout​=Pin​×η

Pout​=1274W×0.9

Pout​=1146.6W

Now, calculate the output current (( I_{out} )) for the 12V system:

Iout​=Vsystem​Pout​​

Iout​=12V 1146.6W​

Iout​=95.55A

Power Factoring Maths – the easy way

A simple way to do the math is to power factor the voltage.
Array voltage / battery voltage.
The resulting factoring number for the same is 98/12 = 8.166
Our current from the array is 13 amps x 8.166 = 106.166 amps output.
You could drop 10% for losses giving you 95.55 amps.

The Solar panel sticker will give you VOC and ICS (the voltage and the amps).
the charge controller as above is 100v ( its maximum input voltage) and the current output (30 amps)

We can do the math backwards because of the conservation of energy and ohms law.
If we have a 12v battery this x 30a is 360 watts of solar.
for 24 volt x 30a that will be 720 watts.

Typically solar panels are around 38 volts. so we can examples from this voltage.
a single panel has a factor at 12 volts of just 3.166, The charge controller is 30 amps.
So we can divide 30 by 3.16 which gives us 9.47 amps.

So our panel can be 9.47 ISC with a 38 volts.
If we look at the math forwards, that’s 38v x 9.47 =360 watts.
for a 24v system the amperage of the panels would be half.

Fitting the solar charge controller for the battery

For the most part people use LFP or lead acid, and for the purpose of this article we will use a 40 amp charge rate, we will buy a 50amp charger so we have a margin.

12 x 50, = 600 watt solar panel.
You wont find a 600watt solar panel so you will use two. You can install them in series ( double voltage) and 300w, but the voltage changes and therefore the factoring changes.
38v x 2 = 76v
76v / 12v =6.33

13a x 6.33 = 82.29 amps this is above the limit of the charger but a 300w panel would have a current of 7.9 amps x 6.33 which is 49.97 amps, and within the 50 amp limit of the charger.

Note, the panel current on the sticker is the peak current at STC, this does happen however you are likely to see NOCT results, which are a bit lower. The SUN makes current and therefore the season will change as well the current. This drop should be considered for winter when sizing the array and depending how critical your charge requirement is.
Could you switch in a parallel array for winter? yes. but dont forget to turn it off or you can order our smart array controllers.

What’s a BUCK converter? what’s this got to do with solar?

The MPPT is one side of a charge controller, the idea is that the voltage in and current in is maximized giving you the maximum power. This is DC power and the charge controller will take some input and make it an output voltage for your battery. the Solar voltage has to be higher than the battery voltage in order to charge, and therefore the voltage is bucked (dropped).
In a hybrid or inverter charge controller, there may be a boost ( added) conversion from the DC to a higher voltage DC called a “System Bus Voltage”.

if you take 240v AC and make it DC through rectification, you would see around 379volts and you will usually see the SBV is that voltage within the inverter, as it will need this to convert the DC to AC power.

Here is a video on Boost and Buck circuits. its a little technical, but you will now know how these things work: