An inverter is a critical component of any solar energy system. The basic function of an inverter is to convert the direct current (DC) power that solar panels create to alternating current (AC) power that is usable in homes and businesses or fed directly into the grid in front-of-the-meter projects (utility-scale solar arrays). Solar inverters are classified majorly into Standalone and Hybrid Inverters. Hybrid inverters usually come with their charge controllers while standalone doesn’t.
Batteries used for solar energy storage are available in two main types, lead acid (AGM & Gel) and lithium-Ion. There are several other types available such as redox flow batteries and sodium-ion but we will focus on the most common two. Most modern energy storage systems use rechargeable lithium-ion batteries and are available in many shapes and sizes which can be configured in several ways. Battery capacity is generally measured in either Amp hours (Ah) for lead-acid, or kilowatt hours (kWh) for lithium-ion. However, not all of the capacity is available for use. Lithium-ion based batteries can typically supply up to 90% of their available capacity per day, while lead-acid batteries generally only supply 30% to 40% of their total capacity per day to increase battery life. Lead-acid batteries can be discharged fully, but this should only be done in emergency backup situations. Different Battery sizes are 12V 200A, 2V 500A etc.
A solar charge controller manages the power going into the battery bank from the solar array. It ensures that the deep cycle batteries are not overcharged during the day and that the power doesn’t run backwards to the solar panels overnight and drains the batteries. Some charge controllers are available with additional capabilities, like lighting and load control, but managing the power is its primary job. A solar charge controller is available in two different technologies, PWM and MPPT. How they perform in a system is very different from each other. An MPPT charge controller is more expensive than a PWM charge controller, and it is often worth it to pay the extra money.
A PMW solar charge controller stands for “Pulse Width Modulation”. These operate by making a connection directly from the solar array to the battery bank. During bulk charging, when there is a continuous connection from the array to the battery bank, the array output voltage is ‘pulled down’ to the battery voltage. As the battery charges, the voltage of the battery rises, so the voltage output of the solar panel rises as well, using more of the solar power as it charges. As a result, you need to make sure you match the nominal voltage of the solar array with the voltage of the battery bank.
An MPPT solar charge controller stands for “Maximum Power Point Tracking”. It will measure the Vmp voltage of the panel, and down-converts the PV voltage to the battery voltage. Because power into the charge controller equals power out of the charge controller, when the voltage is dropped to match the battery bank, the current is raised, so you are using more of the available power from the panel. You can use a higher voltage solar array than battery, like the 60 cell nominal 20V grid-tie solar panels that are more readily available. With a 20V solar panel, you can charge a 12V battery bank, or two in series can charge up to a 24V battery bank, and three in series can charge up to a 48V battery bank. This opens up a whole wide range of solar panels that now can be used for your off-grid solar system
- Multistage charging of battery bank– changes the amount of power set to the batteries based on its charge level, for healthier batteries.
- Reverse current protection– stops the solar panels from draining the batteries at night when there is no power coming from the solar panels.
- Low voltage disconnect– turns off attached load when battery is low and turns it back on when the battery is charged back up.
- Lighting control – turns attached light on and off based on dusk and dawn. Many controllers are configurable, allowing settings for a few hours or all night, or somewhere in between.
- Display– may show voltage of battery bank, state of charge, amps coming in from solar panel.
- The solar charge controller is typically rated against Amperage and Voltage capacities. Select the solar charge controller to match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your application. Make sure that solar charge controller has enough capacity to handle the current from PV array.
- For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration).
- According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array, and multiply it by 1.3
- Solar charge controller rating = Total short circuit current of PV array x 1.3
Remark: For MPPT charge controller sizing will be different. (See below)
- select nominal rated voltage
- divide solar panel array total wattage by system voltage
- add 20% as safety margin
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