Charge Controller Sizing
How to choose the right size charge controller for your solar energy system.
Charge controller sizing is best understood when you grasp what a charge controller does.
Solar Charge Controller Functions
The function of a charge controller is to regulate the charge going into your battery bank from your solar panel array and prevent overcharging and reverse current flow at night.
It does this by using a transistor to shunt the PV charging circuit. This means, if your battery is full, it stops the charging and if your battery is reaching an unhealthy discharge point, it stops the discharging.
By using a pv charge controller you minimize the use of utility power and maximize the chances of your batteries and other photovoltaic components lasting longer, thus increase the life expectancy and efficiency of your entire solar system.
More sophisticated solar charge controllers make sure the battery is charged by utilizing pulse width modulation (PWM) or maximum power point tracking (MPPT).
By inputting pre-set high and low voltage cut off settings, you can help keep your batteries healthy and efficient, automatically.
Charge Controller Sizing
Sizing charge controllers is pretty simple really. PV charge controllers are rated and sized depending on your solar array's current (amps) and the solar systems voltage (push).
Therefore solar charge controller sizing basically involves "getting a charge controller big enough to handle the amount of power and current produced by your solar energy system".
The most common pv charge controllers come in 12, 24 and 48 volts. Amperage ratings can be between 1-60 amps and voltage ratings from 6-60 volts.
So if your solar system's volts were 12 and your amps were 14, you would need a solar charge controller that had at least 14 amps.
However due to factors such as light reflection, sporadic increased current levels can occur, so you need to factor in an additional 25% bringing the minimum amps that our pv charger controller must have to 17.5 amps.
So we'll need a 12 volt, 20 amp charge controller (rounded up).
It won't hurt anything if your charge controller's amps are higher, in fact it's a good idea just in case you increase the size of your solar energy system in the future.
MPPT Charge Controllers
MPPT stands for Maximum Power Point Tracking and a MPPT charge controller is used in the very common case where your solar array's voltage is higher than your battery bank's voltage.
This is the case with the example solar arrangement diagrams we use in our Solar Panel Wiring Diagrams section, so pay attention to this if you choose to copy any of those arrangements. MPPT charge controllers also work great with systems that have panels with odd voltage ratings, for instance: 56V.
When a MPPT solar charge controller notices a difference in voltage, it will automatically and efficiently convert the higher voltage to the lower voltage so your panels, battery bank and PV charge controller can all be equal in voltage.
So if you had a 900 watt solar array with 48 volts, and your battery bank's voltage was 24 volts...
...you can determine the amps your PV charge controller needs to have by dividing the watts by the lower of the two volts.
Watts / Volts = Amps
So 900W / 24V = 37.5 Amps
Plus you still have to add an extra 25% for unexpected current increases due to factors such as light reflection... and you get 46.87 amps.
So you'll need a 24 volt, 50 Amp MPPT charge controller (rounded up). Learn more about MPPT charge controller sizing.
PV Charge Controller - Upper Voltage Limit
All charge controllers have an upper voltage limit. This refers to the maximum amount of voltage they can handle from the solar array. Make sure you know what the upper voltage limit is and that you don't exceed it or you may end up burning out your solar charge controller.