While many of us fantasize about freeing ourselves from a lifetime of utility bills, the reality is that the grid is usually reliable and accessible. Most of us are already on-grid, so taking our homes off-grid doesn’t make much environmental or economic sense. Today’s modern grid-tied inverters make having renewable energy a snap. These systems are increasingly affordable and allow you to meet as much of your energy needs as you want—even annually zeroing out your electric bill.
Grid-tied (GT, utility-interactive, or UI) inverters actively “test” the grid and make operating decisions based on grid conditions. Even though some inverter manufacturers have GT inverters that resemble their stand-alone (off-grid) inverters, they function differently. There are two main types of GT inverters: batteryless and battery-based.
Batteryless grid-tied inverters
These inverters need the grid present to operate—except when they are installed in an AC-coupled system (see “Battery-Based AC Coupling” sidebar). These inverters make the simplest systems: the main components are the inverter and PV modules, plus safety disconnects (see “Simplicity” diagram).
This does not mean that the inverters themselves are simple—quite the opposite. Grid-tied inverters continuously perform complex tasks to keep the entire system operating efficiently and safely. The operation for these inverters is straightforward: when the sun is shining on the modules and the grid is present, the inverter accepts DC from the array, converts it to AC and synchronizes with the grid, pushing this energy into the electrical distribution system.
For a typical residential system, the inverter is connected to the main breaker panel along with all the home’s loads. The PV system simply supplements the grid. The inverter pushes energy toward the grid, even while the loads in the home are drawing energy. If the PV array and inverter are producing less than the loads are consuming, the remainder comes from the grid. If the array produces enough energy to exceed the draw of the loads, the inverter sends the excess energy into the grid, “spinning” the utility kWh meter backward. Without sunlight, the array is not producing, so the utility resumes as the sole provider of electricity for the loads, running the meter forward again.
If the grid voltage or frequency goes outside UL1741 specifications, the inverter immediately disconnects itself from the house electrical system, and therefore from the grid. This process is extremely quick—the inverter will recognize the problem and disconnect before you even know there is an issue. Once the grid is within specification again for five continuous minutes, the inverter reconnects and resumes operation.
Batteryless inverters are the most popular choice because of their high efficiencies and low maintenance. There is very little user interaction with these inverters since they don’t incorporate any components that require diligent maintenance, like batteries. This also helps keep the overall system costs lower. For most of us in locations where the grid is very stable and outages are short in duration, batteryless inverters are a great solution.
Batteryless systems’ inability to operate without the grid can be a drawback. Some people have loads that need continuous power—like medical equipment, refrigeration, or lighting. For those folks, battery backup inverters are necessary. These inverters are connected to batteries, which store energy to back up the grid. Although batteries are not the only option available, they are the most reliable and economical choice for these systems. Devices like flywheels and supercapacitors are being developed and tested. For now, deep-cycle batteries are the best solution for PV systems that need energy storage.
Battery-based inverters are subject to the same safety regulations as batteryless inverters so they can be interconnected to the utility grid. These inverters and the systems they are connected to are more complex than batteryless systems. In addition to all the components in a batteryless system, a battery-based system requires a charge controller, batteries, and a backup load center. These components, plus the addition of a maintenance item—the batteries—add cost and complexity to these systems. (See “Grid-Tied…with Backup” and “Sizing a Grid-Tied PV System…with Battery Backup” in HP139.)
The operation of a grid-tied, battery backup inverter is very similar to a batteryless inverter. The only difference is that the inverter uses a small amount of energy to keep the batteries fully charged in case of an outage.
In the event of a utility outage, day or night, the inverter recognizes the outage and, like the batteryless inverter, immediately disconnects itself from the utility grid. At the same time, the inverter will begin to draw DC energy from the batteries (and PV array in daylight), converting it to AC for the backup loads. This happens very quickly (within 34 milliseconds) and, if already on, computers and all but the most sensitive electrical backup loads will continue to operate without a hiccup. Once the grid is back in operation, the inverter will reconnect to it and begin normal operation. Then the inverters can start recharging the batteries, readying them for future power outages.
For prolonged outages, one of the benefits of these systems (as opposed to a simple UPS with no PV) is the ability to use the PV array as a battery-charging source. If the utility is out for multiple days (and the sun is shining), the PV array can recharge the battery bank during that time. Of course, if the power outage happens during a winter storm and there is little or no solar resource during that outage, the PV array will contribute very little to the battery bank.
The extra system cost and added maintenance of batteries are the two biggest drawbacks for this type of system. For people who endure prolonged power outages, the peace of mind may well be worth the cost and maintenance time.
Knowing basic system operations and your particular requirements will help decide which type of GT system is right for you. This can come down to answering a few questions about your needs: How often does the power go out? How long is the typical outage? What loads are absolutely necessary during outages? How much maintenance are you willing to perform on the system? How much can you budget to cover system costs?
If outages are more than a minor annoyance and you can handle some battery maintenance, such as checking battery cables, exercising the batteries, and regularly checking electrolyte levels (not necessary if using sealed batteries), a battery backup system may be the best choice. For most, the utility service in their area is reliable and outages are short and rare. In these situations, a batteryless inverter will result in overall higher energy yields and a simplified system.
Ryan Mayfield is a NABCEP-certified PV installer and ISPQ Affiliated Master Trainer. When he isn’t trying to absorb all things solar, he is busy trying to influence the next generation by helping his kids solarize their backyard forts.
California Energy Commission • www. gosolarcalifornia.org • Listing of California-eligible RE equipment, including the state’s additional inverter specifications