There are three basic types of solar power systems: grid-tie, off-grid, and backup power systems. Here’s a quick summary of the differences between them:
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Each system type requires unique equipment that is compatible with the application, so understanding which one you need is the first step in the process of going solar.
Let’s take a closer look at the different types of solar power systems and make a comparison between them.
Grid-tie solar is, by far, the most cost-effective way to go solar. Because batteries are the most expensive component of any solar system, but grid-tie solar owners can skip them completely!
So how do grid-tie solar power systems work?
First, let’s define what we mean by the “grid”. The grid is the utility company’s network of equipment that brings electricity from the power plant to your home or commercial building. If a building is getting electricity from the power company, it is connected to the grid.
Grid-tie solar systems send the energy they generate into the grid, where it is stored for later use. Under a net metering agreement, the system owner receives credit for anything they generate, and they can make use of that energy at any time.
It’s kind of like a bank account: sending energy into the grid is like making a deposit, and using electricity is like withdrawing against your account balance. If you overdraft i.e. use more energy than you produce in a given month, the utility bills you for the difference. No added fees, thankfully.
Grid-tie solar is the best option if you want to offset your electricity bill and save money over the life of your system.
Most grid-tie systems pay for themselves within 5-10 years. With solar panels warrantied for 25 years, grid-tie solar is the only option that reliably turns a profit for the system owner over the life of the panels.
Another advantage is that grid-tie systems can be smaller — you don’t need to generate 100% of your power each month. The grid can supply additional power beyond your production, which is useful when bad weather hampers the output of your panels, for example.
Some people choose to size a grid-tie system for a partial offset of their bill, with plans to expand the system later once their budget allows for it. Design requirements are less demanding than in an off-grid environment, where you are fully responsible for your energy needs.
The main disadvantage of grid-tie systems is that they are still vulnerable to power outages.
"But wait," you might say, "if I’m generating power from sunlight, why does it matter if the grid goes down?"
Unfortunately, grid-tie systems are wired into the utility company’s infrastructure. In case of an outage, utility workers need to troubleshoot and fix the problem, and they can’t do that if connected solar systems are still energized and feeding power to the grid. For that reason, grid-tie solar systems are switched off during outages to allow utility workers to safely make repairs.
The solution? A hybrid system that connects to the grid, but draws on a battery bank in case of outages. We’ll cover those at the end of this article, but first...
Off-grid solar is best for delivering power to remote locations where there is no access to a utility line.
Folks who live off the grid are solely responsible for generating their own electricity. This is usually accomplished by building an off-grid solar system that can cover a day’s worth of electricity usage, with a backup generator to supplement production during long stretches of bad weather.
The main draw of off-grid solar is the freedom to live wherever you want. It doesn’t matter if your property is 100 miles from civilization: if you have sunlight, you have a reliable way to generate power.
Although off-grid solar components are more expensive, there can be some hidden financial benefits to living off the grid that can offset those higher costs. Undeveloped plots of land located far off the grid will naturally cost less than a prime grid-tie location. In many cases, the lower land costs do more than enough to offset the higher cost of going solar off the grid.
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Pretty simple, really: the need for a battery bank makes off-grid solar significantly more expensive.
However, it’s often wiser to invest in an off-grid solar system than it is to run a power line to a remote location. While an off-grid system may cost more than a grid-tie system, it is still more frugal than other remote power solutions, like running a new utility line or relying on a gas generator.
One way to keep costs down is to use propane appliances where possible to reduce your demand for electricity. Opting for a propane stove, clothes dryer, wall heater and on-demand water heater means you can get away with a smaller inverter and smaller battery bank.
It also helps to stagger electricity usage — for example, running laundry and the dishwasher at different times — to reduce your peak power consumption and relieve some of the costs of energy storage.
If you live on the grid, but you want protection from power outages, your best bet is a battery backup system.
Backup power systems connect to the grid, and function like a normal grid-tie system on a day-to-day basis. However, they also feature a backup battery bank that takes over in case of outages.
When grid power goes out, your inverter automatically disconnects from the grid and draws on energy stored in your battery bank, which will keep your appliances running when the grid goes down.
Battery backup systems have been gaining popularity recently, especially in light of news stories covering grid failures in Texas and wildfires interrupting service in California. They are also favored in climates that are vulnerable to fierce storms and natural disasters like hurricanes and tornadoes. The backup battery bank offers peace of mind to shield the owner from blackouts.
Lastly, battery backup is valuable if you have appliances which require uninterrupted power. If you are running a well pump, for example, service interruptions can be a massive headache. Adding backup power to your grid-tie system will keep these critical appliances running during a blackout.
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.
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.
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 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. More 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. More 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.
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