Solar Academy Lesson 4: How inverters and charge controllers work

An inverter is a component in a solar system that converts the DC power generated by solar panels into AC power for use in the home or electrical grid. Freedom Forever primarily installs Solaredge inverters. This article will explore the capabilities of the Solaredge inverters. Charge controllers are components that are used to manage charging and discharging of batteries connected to the system. Solaredge inverters meant for use in battery-equipped solar systems are called StorEdge inverters.

How an inverter converts DC power to AC power

A simple way to understand how an inverter converts DC to AC power is to look at the inverter circuit as being 2 pairs of 2 switches for a total of four switches. The switches are paired so that when switch 1 and 3 are closed, switches 2 and 4 are open. Then, when 1 and 3 are open, 2 and 4 are closed. Each pair of switches will cause the current to switch directions when closed.

Switches aren’t actually used in an inverter circuit. Instead of switches, transistors such as insulated-gate bipolar transistors (IGBP) or Metal-oxide semiconductor field-effect transistors (MOSFET)  function as switches. 

Those transistors also enable the current to rise and fall gradually as they open and close. This is necessary for the current output to be in the form of a sine wave. If the transistors opened and closed instantaneously, the output from the inverter would be a square wave, which will not work safely as AC current for many devices.

Lastly, the inverter will need to step up the voltage level to 120 VAC. An inverter uses a step-up transformer to accomplish that.

Grid isolation, a mandatory function of solar inverters

All grid-tied inverters are required to isolate themselves from the electrical grid if power fails or falls outside of acceptable limits. This isolation is required so that solar systems don’t send current into power lines when they may be being worked on by line crews. As with the inverter circuit, the isolation circuit uses IGBPs or MOSFETs as switches to connect or disconnect the inverter to/from the grid.

Inverters for systems with batteries

If an inverter is to be used as part of a solar system with batteries, then an additional component called a charge controller will be part of the inverter. A charge controller is a device that regulates voltage and/or current to keep the batteries from overcharging. Batteries get damaged if they are overcharged, the charge controller prevents that from happening.

How charge controllers work

Charge controllers do two different things. In solar systems equipped with Maximum Power Point Tracking (MPPT) charge controllers, they adjust the input power from the solar panels to ensure the maximum possible power output. If the voltage being generated by a panel is less than the voltage coming from other panels, the MPPT controller will reduce the current coming from that panel so the voltage output is raised. A panels’ voltage output is reduced when that panel is partially shaded.

In solar energy storage systems, charge controllers regulate the voltage being sent to the battery to prevent overcharging the battery. As the battery gets closer to its rated voltage, the charge controller will gradually reduce the amount of current going to the battery. The excess power is fed into the grid. When the battery reaches its capacity, the charge controller will maintain the battery’s charge with a trickle charge.

Solaredge inverters

Many of the solar systems installed by Freedom Forever come with Solaredge inverters. One of the main reasons Freedom installs Solaredge inverters is their reliability. Many inverters come with only a 10-year warranty. But Solaredge inverters can be warrantied for up to 25 years, so they are the logical choice to include with our systems that are covered by Freedom’s 25-year production guarantee.

MPPT and voltage management are handled separately for each solar panel in the solar system by the Solarege inverter. Thanks to that, the inverter is only responsible for converting the DC current produced by the panels into AC current. This separation of functions leads to a simpler and more reliable design.

Digital signal processing produces high-efficiency DC to AC conversion

Solaredge uses a proprietary HD-Wave technology that relies on digital signal processing (DSP) to produce a clean sine wave. Thanks to the DSP technology enables DC to AC conversion at a record-breaking 99% efficiency. Thanks to that high efficiency, the inverter produces less heat, which is the key to the Solaredge inverters’ long life.

StorEdge solar energy storage system inverters

StorEdge inverters are Solaredge’s solar energy storage system inverters. These inverters use DC coupling to provide DC power to the batteries without having to convert the power to AC first. This enables the StorEdge inverter to charge the batteries more efficiently since some power would be lost if the inverter had to convert the DC from the solar panels into AC power, and then back to DC power to charge the batteries. The StorEdge inverter can be installed with up to two LG Chem Cell RESU 10H batteries for a battery capacity of 19.6-kilowatt-hours. The StorEdge SE7600A-USS2 inverter is rated for a maximum power output of 7.6 kilowatts when the solar system is connected to the grid. During a blackout, the SE7600-USS2 can produce up to 3.3 kilowatts for single battery systems, of up to 5 kilowatts for dual battery systems. It’s important to note that most homes use an average of 5 kilowatt-hours of power. Thus if you manage your power usage wisely during a blackout, you should have enough power to operate major appliances in your home.

Managing your power usage during a blackout

The amount of power that you’ll have available during a blackout period will depend on how much power you are getting from your solar panels during the day, and how much power you have stored in your batteries at night.

With a two-battery system, you can get a maximum of 5 kilowatts of power from your system. During daylight hours, you’ll want to monitor how much power your system is producing. Any amount of power that you use over that which your system is producing would have to come from your batteries. LG recommends that the Chem Cell batteries be charged at the rate of 3.3 kilowatt-hours for the longest battery life, so you should plan to spare as close to that amount as possible during the day while your batteries are charging.

Nighttime power management

At night, with a two-battery system, you have a maximum of 19.6 kilowatt-hours of power. You should plan to keep your power usage low enough to make it through the night. For example, if you used an average of two kilowatt-hours of power, the batteries would be able to produce nearly 10 hours of electricity. Thankfully, a household’s power usage declines later at night, so most homes will use less than two kilowatt-hours of power for most of the night.

Managing your power usage wisely can help you make it through an extended blackout like the ones that hit northern California last year since your batteries get a fresh charge each day when the sun comes up. Just be sure to provide enough power for your system to charge the batteries during the day, and manage your power usage wisely so you make it through the night.

Freedom Forever can upgrade your existing system to battery storage

If you are a Freedom Forever customer, we can upgrade your system to include batteries. Batteries are eligible for the federal tax credit, even if you’ve already claimed it for your solar system. California customers are also eligible for the state’s Self Generation Incentive program, which pays up to $400 per kilowatt-hour of installed battery capacity. Pacific Gas and Electric predicts that public safety power shutdowns will continue for the next 10 years. So the ideal time to install batteries is before late summer, which is when the state usually starts getting the high wind conditions that cause the blackouts.

You should contact a qualified tax expert and consult with them. Neither Freedom Forever’s family of independent authorized dealers nor Freedom Forever itself can give you tax advice. A qualified tax professional will understand your situation and be able to give you the most relevant advice.

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