Solar Integration: Inverters and Grid Services Basics
An inverter is one of the most important pieces of equipment in a solar energy system. It’s a device that converts direct current (DC) electricity, which is what a solar panel generates, to alternating current (AC) electricity, which the electrical grid uses. In DC, electricity is maintained at constant voltage in one direction. In AC, electricity flows in both directions in the circuit as the voltage changes from positive to negative. Inverters are just one example of a class of devices called power electronics that regulate the flow of electrical power.
Fundamentally, an inverter accomplishes the DC-to-AC conversion by switching the direction of a DC input back and forth very rapidly. As a result, a DC input becomes an AC output. In addition, filters and other electronics can be used to produce a voltage that varies as a clean, repeating sine wave that can be injected into the power grid. The sine wave is a shape or pattern the voltage makes over time, and it’s the pattern of power that the grid can use without damaging electrical equipment, which is built to operate at certain frequencies and voltages.
The first inverters were created in the 19th century and were mechanical. A spinning motor, for example, would be used to continually change whether the DC source was connected forward or backward. Today we make electrical switches out of transistors, solid-state devices with no moving parts. Transistors are made of semiconductor materials like silicon or gallium arsenide. They control the flow of electricity in response to outside electrical signals.
A 1909 500-kilowatt Westinghouse “rotary converter,” an early type of inverter. Illustration courtesy of Wikimedia.
If you have a household solar system, your inverter probably performs several functions. In addition to converting your solar energy into AC power, it can monitor the system and provide a portal for communication with computer networks. Solar-plus–battery storage systems rely on advanced inverters to operate without any support from the grid in case of outages, if they are designed to do so.
Toward an Inverter-Based Grid
Historically, electrical power has been predominantly generated by burning a fuel and creating steam, which then spins a turbine generator, which creates electricity. The motion of these generators produces AC power as the device rotates, which also sets the frequency, or the number of times the sine wave repeats. Power frequency is an important indicator for monitoring the health of the electrical grid. For instance, if there is too much load—too many devices consuming energy—then energy is removed from the grid faster than it can be supplied. As a result, the turbines will slow down and the AC frequency will decrease. Because the turbines are massive spinning objects, they resist changes in the frequency just as all objects resist changes in their motion, a property known as inertia.
As more solar systems are added to the grid, more inverters are being connected to the grid than ever before. Inverter-based generation can produce energy at any frequency and does not have the same inertial properties as steam-based generation, because there is no turbine involved. As a result, transitioning to an electrical grid with more inverters requires building smarter inverters that can respond to changes in frequency and other disruptions that occur during grid operations, and help stabilize the grid against those disruptions.
Grid Services and Inverters
Grid operators manage electricity supply and demand on the electric system by providing a range of grid services. Grid services are activities grid operators perform to maintain system-wide balance and manage electricity transmission better.
When the grid stops behaving as expected, like when there are deviations in voltage or frequency, Smart inverters can respond in various ways. In general, the standard for small inverters, such as those attached to a household solar system, is to remain on during or “ride through” small disruptions in voltage or frequency, and if the disruption lasts for a long time or is larger than normal, they will disconnect themselves from the grid and shut down. Frequency response is especially important because a drop in frequency is associated with generation being knocked offline unexpectedly. In response to a change in frequency, inverters are configured to change their power output to restore the standard frequency. Inverter-based resources might also respond to signals from an operator to change their power output as other supply and demand on the electrical system fluctuates, a grid service known as automatic generation control. In order to provide grid services, inverters need to have sources of power that they can control. This could be either generation, such as a solar panel that is currently producing electricity, or storage, like a battery system that can be used to provide power that was previously stored.
Another grid service that some advanced inverters can supply is grid-forming. Grid-forming inverters can start up a grid if it goes down—a process known as black start. Traditional “grid-following” inverters require an outside signal from the electrical grid to determine when the switching will occur in order to produce a sine wave that can be injected into the power grid. In these systems, the power from the grid provides a signal that the inverter tries to match. advanced grid-forming inverters can generate the signal themselves. For instance, a network of small solar panels might designate one of its inverters to operate in grid-forming mode while the rest follow its lead, like dance partners, forming a stable grid without any turbine-based generation.
Reactive power is one of the most important grid services inverters can provide. On the grid, voltage— the force that pushes electric charge—is always switching back and forth, and so is the current—the movement of the electric charge. Electrical power is maximized when voltage and current are synchronized. However, there may be times when the voltage and current have delays between their two alternating patterns like when a motor is running. If they are out of sync, some of the power flowing through the circuit cannot be absorbed by connected devices, resulting in a loss of efficiency. total power will be needed to create the same amount of “real” power—the power the loads can absorb. To counteract this, utilities supply reactive power, which brings the voltage and current back in sync and makes the electricity easier to consume. This reactive power is not used itself, but rather makes other power useful. Modern inverters can both provide and absorb reactive power to help grids balance this important resource. In addition, because reactive power is difficult to transport long distances, distributed energy resources like rooftop solar are especially useful sources of reactive power.
A worker checks an inverter at the 2MW CoServ Solar Station in Krugerville, Texas. Photo by Ken Oltmann/CoServ.
What is a hybrid inverter?
A hybrid inverter is an intelligent inverter that enables the storage of excess solar energy in a solar battery. Hybrid inverters function like a regular grid-connected inverter for solar panels, but can operate in different configurations.
Hybrid inverters can operate in a battery backup mode, storing unused power in the battery. This power can be used at a later time when the solar panels are not generating power, or in the event of a power outage.
Most hybrid inverters can also operate without a connected battery system and then function the same as a regular grid-connected inverter, delivering excess solar power to the grid.
Why a hybrid inverter?
A hybrid inverter is an all-in-one plug-and-play inverter that is ready for different configurations. A solar panel system with a conventional inverter requires a separate inverter to convert AC to DC, back and forth. A system with a hybrid inverter does not require a separate battery inverter. Especially if you want to be ready for the future or add batteries later, a hybrid inverter is the best choice.
This type of inverter is already prepared for different configurations and thus ready for any expansions. However, they also have limitations and are generally not recommended for off-grid solar power systems. In that case, a specific off-grid inverter is the best choice.
Types of hybrid inverters
Basic hybrid inverters
Basic hybrid inverters ensure that excess solar energy generated is fed back to the grid and power is withdrawn from the grid when needed.
However, a basic hybrid inverter also allows you to connect a battery system. Therefore, this simple hybrid inverter is a good choice only if the power grid is stable.
Setup of hybrid inverter with battery storage
A hybrid inverter features multiple modes, making it an advanced energy storage solution. A hybrid inverter can power a household, charge connected batteries and feed excess power back to the grid. In the event of a power failure, the unit switches to battery power and operates independently of the grid.
All-in-one hybrid energy storage system
Modern hybrid inverters often come with a battery system to form a complete system. Such an integrated system is also called a Battery Energy Storage System (BESS). A fully integrated system of a hybrid inverter with battery storage can be connected to any existing solar system.
Off-grid systems
Off-grid inverters are much more advanced and powerful than regular hybrid inverters. They can operate with or without a grid connection and there is an immediate backup in the event of a power failure. These inverters are powerful enough to power even equipment with very large peak loads, such as air conditioners, pumps and heating equipment.
Off-grid systems are able to operate fully independent of the grid. In this case, the hybrid inverter is the link between the solar panel setup and the battery system, and no exchange with an external network takes place. The disadvantage of such a system is that there is complete dependence on the energy available and stored at the time.
Which hybrid inverter?
When choosing a hybrid inverter, it is important to consider what the situation is. Is there an existing solar panel setup? Do you want to add solar batteries? Do you want to be fully independent of the grid? How much electricity do you use on a daily basis and at what times? What is the budget?
ESTG is a wholesaler of solar panels and other sustainable electrotechnical products, such as solar inverters, EV chargers, solar batteries and thermal heating. With a wide range of A-brands, ESTG supplies installers across Europe. See all SMA inverters.
Hybrid Solar Inverter Basics: Introduction, Functions and Advantages
Hybrid solar inverters are a new solar technology that combines the benefits of traditional solar inverters and the flexibility of battery inverters into a single device. Hybrid inverters are an emerging solution for homeowners looking to install a solar power system that has room for future upgrades, including a battery storage system.
A hybrid solar inverter is a single device that combines a solar inverter and a battery inverter to intelligently manage power from solar panels, solar batteries and the utility grid at the same time.
A conventional solar grid-tie inverter converts the direct current generated by a photovoltaic (PV) system into alternating current to power your house, which also allows excess power generation to be transferred to the utility grid. A battery inverter handles the process of converting the DC power stored in the solar battery into AC power that can be used in your home.
By combining these functions into a single device, a solar hybrid grid-tie inverter streamlines and improves the operations of the classic solar inverter. Even better, since the amount of solar power available may depend on weather and seasonality, the hybrid inverter can draw power from the grid to charge your battery storage system if needed.
What are the functions of hybrid solar inverter?
The basic purpose of an hybrid solar inverter is to convert the DC input from a solar panel into an AC output that can be used by the home. The ability to use hybrid solar inverters takes this process to a new level and includes the following benefits.
- Bidirectional DC to AC power conversion: Typically, solar cells are charged by either DC power received directly from the solar panels (DC coupling), or DC power converted from AC power by the solar inverter (AC coupling). Then in order to release this power, the inverter needs to convert the stored DC power to AC power. With a hybrid inverter and battery, one device can fulfill both roles. A hybrid grid-connected inverter can convert DC to AC to power your home, but it can also take AC power from the grid and convert it to DC to be stored in a battery for later use.
- Power Regulation: Depending on the time of day and weather patterns, solar power fluctuates as sunlight levels increase and decrease. The hybrid inverter regulates this power to ensure that the entire system operates within the desired parameters.
- Power monitoring: Solar hybrid grid-connected inverters can be fitted with solar monitoring software to measure and monitor your PV system via a display or connected smartphone app to help identify any faults.
- Power maximization: A hybrid inverter with a maximum power point tracker (MPPT) checks your solar power output and correlates it with the voltage of the batteries. This allows for optimal power output and converts DC power to the optimal voltage for maximum battery charging. MPPT ensures that your solar power system works efficiently under any conditions, such as varying amounts of solar light, solar panel temperature and electrical loads.
How does a solar hybrid inverter differ from a regular inverter?
Where a regular inverter, whether it be string (most common) or battery (used for energy storage) can only convert energy for one type of system, a hybrid inverter combines these two functions to allow for use in multiple systems.
A hybrid inverter can still generate power in the same way as any other kind of inverter. Its standout feature however, is that it already has built-in connection for energy storage systems. This means that if a household already has a battery, it can connect to that system straight away, as well as the panel system.
Why do you need an inverter?
An inverter is needed to help convert the electricity generated by solar panels into energy compatible for household use and storage ability. This is because solar panels create direct current – DC – energy whereas household appliances generally run off alternating current – AC – electricity. Therefore, without an inverter, the energy generated from a solar panel is practically useless in the home.
What can you do with a hybrid solar inverter?
Since a hybrid solar inverter is combining the functions of two separate systems, there are a few notable things it can do that a regular inverter can’t. Some functions you may find in a hybrid inverter include: grid-tie mode, hybrid mode, backup mode and off-grid mode.
Please note, not all inverter manufacturers will include these specific features in each of their models.
- Grid-tie Mode: This is a function that allows the hybrid inverter to default to simple solar inverter operations. In this mode, battery features are switched off.
- Hybrid Mode: Under this option, the inverter defaults to battery operation, working to help support the storage of excess solar energy during the day for use in the evening.
- Backup Mode: This allows the inverter to act as a backup power source during grid outages. Generally, this feature allows the system to automatically switch itself to this mode if an absence of grid power is detected. This is a less common feature among hybrid inverters.
- Off-grid Mode: As the name suggests, this feature allows the inverter to operate as its own power source and provide independence from the grid. It uses its own power to generate solar energy, power the home and charge any battery systems. Only a select few hybrid inverters come with this feature.
Should I get a solar hybrid inverter?
Ultimately, this decision comes down to the needs of your solar system and home energy usage. If you’re installing a solar power system for the first time, it might be worth looking into a solar hybrid inverter as a means to future-proof your panel system. This way there is some flexibility should you decide to add a battery system later on.
If you aren’t interested in battery storage you can still get everything you need for your solar system from a standard string inverter. Going with this option however, may just mean that you’ll need additional inverters or equipment, should you decide to add a battery system down the track.
Before making any purchase decisions it is best to get in contact with a licensed solar installer who can evaluate the energy needs of your home and help you choose the inverter that’ll make the most of your solar energy. They can also take into consideration your budget and solar energy goals and suggest an inverter that meets these needs.