Let's say you've decided whether you need a PWM or MPPT controller. Now it's time to discuss the size of your controller. The voltage and the amperage are usually the first things that you have to check about a controller. Regulators can have 12, 24 or 48 volts and their amperage varies between 1 and 60. However, it's safer to choose a controller with slightly higher (about 25% more) amperage than the current from solar panels to ensure the safety of the system.
PWM-controllers do not control or limit the output and just use the array current. As a result, you can't be using PWM-regulators, whose amperage is lower than the amperage of your system — it will simply break. Let's set an example. Imagine that you're setting 3 100-watt panels in parallel and their rated voltage is 12V. On a good day one of them produces around 5.5 A. Multiply by 3, you get 16.5 A. Sometimes it gets really hot, and irradiance rises higher than standard 1000W/m2. This is why you need to add these 25%, so 16.5 A * 1.25 = 20.625 A. For this case you need a controller whose amperage is 20 A or better even above that.
We've already talked about how PWM controllers perform well when the voltage of the solar panel system is slightly above the voltage of batteries. PWM regulators will be as efficient (or almost) as MPPT controllers if you are using:
- 30-cells panels and 12 V batteries
- 60-cells panels and 24 V batteries
- 120-cells panels and 48 V batteries
While the rated voltage of panels in these cases is equal to the voltage of batteries, their open circuit voltage is usually slightly higher. That makes them perfectly suited for a PWM controller, which is significantly cheaper than MPPT, but will perform just as fine.
MPPT controllers, on the other hand, can be considered as smart DC-DC converters, and can handle the current higher than its amperage. However, it means that you are losing the energy you've gathered, because if the amperage of your controller is, let's say, 60 A, it will produce 60 A even if panels give out 100 A altogether.
Another thing to look at while sizing a charge controller is a maximum solar output. Whatever the type, the maximum voltage of the controller must be higher than the voltage of the system. Let's say you have 2 60-cells panels, and their open circuit voltage goes up to 36 V. Then 2 * 36 = 72 V, so the maximum voltage of the controller must be higher than that.
Finally, you need to take a look at battery Amp/hour rating. The amperage of the controller should be 10%-20% of it. For example, a lead-acid battery with 100 Ah rating, a 15 A regulator will do just fine. To generate 10 A you would need two 100 W panels or just one 150-200 W panel. Of course, the size of your batteries, power and number of your panels and therefore the properties of the charge regulator depend on the size of your house and your energy needs.