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Knowing how many solar panels you can use with a charge controller is critical. If the controller is overloaded there is a good chance it gets damaged permanently. If you are planning to buy a charge controller, this guide can help.
The voltage of a solar array should not be greater than the maximum input voltage (VOC) of a charge controller. If the controller VOC is 100 volts, and 3 solar panels with a VOC of 22 volts each are connected in a series, the controller can handle it because the total is 66 volts.
Calculate How Many Solar Panels Per Charge Controller
In these examples we will be using an MPPT charge controller because it provides better performance with high powered solar panels compared to PWM. PWM controllers are best left for small scale PV systems.
Most MPPT charge controllers can handle 3 solar panels in a series per string. The total PV voltage in a series cannot exceed the charge controller maximum input voltage or open circuit voltage (VOC).
You have three 24V solar panels with a VOC of 46V each and a 60A 150 VOC MPPT controller. The panels are connected in a series, which combines the voltage of each solar module.
46 x 3 = 138
The solar array requires 138 volts. Your 60A charge controller has a maximum capacity of 150 VOC so you can run the solar array.
Here is another example.
You have three 48V 300W solar panels with an open circuit voltage of 44V each and a 50A 100 VOC MPPT controller.
If you connect one of these solar panels to the controller, the VOC is well within the controller limits. But if you connect the three panels in a series:
44 x 3 = 132
The VOC increases to 132 volts, which exceeds the controller’s capacity. You have to reduce the panels to just two or get a more powerful charge controller.
For reference here are the most common solar panel voltages and their maximum input voltage / open circuit voltage.
|Solar Panel Voltage||VOC|
These figures are for information purposes only. Always check the specifications on your charge controller and solar panels.
Some high end charge controllers can work with up to 5 solar panels in a series per string. They also have a higher VOC limit and are ideal for large scale solar systems.
How to Add a Safety Margin
The calculations given above work fine, but if you want to be extra cautious, add a safety margin to the figures. Usually it is 25%, but it can be any number you like.
The formula is similar except you add 25% to the results. So if you have 3 solar panels in a series with 38 VOC each, that would be 114 volts in total.
Using our regular calculations, a 114V solar array will require a charge controller with a 120 VOC capacity (the nearest size available to match 114).
But if you add a 25% safety margin you need a 150 VOC controller:
114 + 25% = 152
Again the 25% is arbitrary and can be any number you choose. And for many solar power users a safety margin is not necessary. In fact many opt to oversize their solar array.
What is Solar Array Oversizing?
In another post we explained why solar panel outputs are often lower than their rating. A 300 watt panel may only produce 270 watts due to dirt, shading, cloudy skies and other factors.
This is why some solar controllers can be oversized. That is, you may use a solar panel that has a higher capacity than what the manufacturer recommends.
For example, a 12V battery and a 20A MPPT controller might be designed for a 275W solar panel. But it can also be used to charge a 300-330W solar panel. How?
Due to the various ways solar power is lost, a 275W panel may only produce 250W, wasting the capacity of the controller and battery. With a 300W panel, the output will be closer to the controller maximum capacity.
Caution: not all charge controllers accept oversizing. The array VOC must not exceed the controller maximum input voltage.
When oversizing, it is permissible to exceed the nominal maximum operating power, but not the maximum input voltage or VOC That will damage the controller. .
If the controller manual says oversizing is allowed, you may do so. Oversizing can be by 25% or as much as 150% depending on the controller.
Do not oversize the array unless the manufacturer expressly says it is possible. Doing so will void the warranty and could damage the controller and solar panels.
How Many Solar Arrays Can a Charge Controller Handle?
The battery size determines what solar array size can be used with the controller. The higher the battery voltage, the more solar panels you can use.
Charge controller amps x battery voltage = solar panel size in watts
30A x 12V = 360
30A x 24V = 720
Again this should only be done if the controller VOC is not exceeded. And if you live in a cold climate, add at least 5V to the solar array VOC.
Charge Controller Size Guide
Besides making sure the controller VOC is large enough for your solar array, you also have to make sure the controller amp size is right for the solar panels and battery bank.
The calculation is simple
Solar array watts / system voltage + 20% safety margin = charge controller size
You have solar panels connected in a series at 41V each. Multiply by 3 and that is 123V. Add 20% and you get 153.
A 123V solar array needs either a solar controller with a 150 or 160 VOC capacity. Most 60A MPPT charge controllers can handle this output.
Charge controller capacity is determined by the maximum charge current and maximum input voltage.
The max charge current indicates the battery size that can be charged. The max voltage current indicates how many solar panels can be connected in a series.
How Temperature Affects Voltage
The preceding calculations are suitable the typical day, with the sun out. But if you live in a cold area or it is winter, the solar panel VOC could jump by up to 8 volts or more. The PV voltage could exceed the charge controller VOC and damage it.
For example, you have a 40A MPPT charge controller with a 100 VOC. It is connected to 3 x 200W solar panels with a 31 VOC each. The panels are connected in a series so that is 93 VOC, which is still within the controller capacity.
But if the temperature drops, the solar array VOC might exceed 100 volts. Even if it is just for a few minutes this could damage the controller.
One way to avoid this problem is to connect the solar panels in parallel. Doing so will increase the power but not the voltage. Even if the VOC increase by 8 volts, that will only be 39 VOC.
However it can be a hassle if you have to reconfigure your solar panels every winter. A more practical solution is to buy a charge controller that has a higher VOC limit so it can handle the voltage spike.
Tips For Using a Charge Controller
- Plan ahead. Calculate how much solar power you will need. This will make it easier to find out what charge controller size to get.
- MPPT is better. The greater your solar power needs, the better off you will be with MPPT. The limitations of PWM controllers will cost you in a large solar system. But if you only need small amounts of power, PWM will do.
- Follow the operating manual. The instructions will tell you how the controller can be used. If you are not sure, contact the manufacturer.
- Factor in your local weather. Solar power is all about the weather. The more you know about the sun hours available in your area, how cold it gets, when it rains etc. the more you will get out of your solar power system.
- Always go for the next largest size. You can never go wrong with a bigger charge controller, but going for a smaller will be problematic.
One of the things that you quickly learn with solar power is the amount of math you need to do, and it is not fun. However it is a necessity to ensure you get the best performance without compromising the safety of your system.