Would batteries benefit your grid-tied system?
Most grid-tied solar-electric systems are “batteryless” and require the utility grid to function. The grid provides a “signal” for a grid-tied inverter to follow, creating an AC waveform from the DC PV system output. Once that signal disappears or goes too far out of voltage or frequency specifications, the inverter stops operating. That is the most efficient way to produce PV-made energy—more watts can be converted.
Conceptually, a PV array could produce useful energy any time the sun shines. So why can’t we use it if the grid does not operate? It’s because a PV array is a constant-current energy source—it cannot cut back or increase the energy available depending upon how much the household needs at any moment. For example, there might be a fridge motor that draws 9 amps while running. A PV system putting out 9 A could keep up with that, except for one problem. When the fridge starts running, the motor surges to possibly double or triple its running amperage. Since the PV system is limited to 9 A, it can never start the motor.
When hooked up to the utility, this same PV system has the grid available to make up for any deficiencies, like during appliance start-up surges. The grid also provides a place to go for any excess energy produced by the PV-powered home.
Including a battery in the system adds a source of energy that can vary according to the needs of the home. Special battery-based, grid-tied inverters are designed to disconnect from the grid, instantaneously switching internally to draw needed energy from the battery instead of the grid. These inverters still disconnect themselves from the grid when the grid goes down, so that they don’t inadvertently energize the grid while utility workers are working on it—a potentially shocking hazard.
High frequency or long durations of electrical outages are the most common reasons to have battery backup. Many rural homeowners want battery backup with their systems because they live with a low-quality grid or have affecting circumstances—trees near power lines, a long power line extension, wind or snowstorms—that make outages more frequent or of longer duration.
If you do suffer consequential outages, or if there are appliances that absolutely must run all the time, you’ll need backup energy during an outage. You may have medicine or food that needs to be kept cold; lights and computer equipment for work; an oxygen generator for your health; radios and TV for access to news; or a lift for alter-abled access.
A detailed worksheet listing loads and their usage is necessary to size a battery-backup system (see “Sizing a Grid-Tied PV System with Battery Backup” in this issue). Any improvements in load efficiency that can be made prior to sizing the system will reduce costs. Sizing a battery backup system usually takes several attempts—you need to weigh the cost of backing up loads with the loads’ importance, while paring down the list or increasing efficiency to stay within your budget.
Battery backup system sizing is much more critical than grid-tied sizing, so details are a must. If you size a grid-tied system too small, the extra energy needed comes from the grid—no problem, except for a higher bill. If you size your battery backup system too small, you may run out of electricity during an outage, defeating the point of having battery backup.
A typical batteryless grid-tied PV system’s cost is $6 to 8 per rated watt. A grid-tied system with battery backup can cost $10 or more per watt, because of adding batteries and the extra equipment needed to charge them. The total size of the critical loads and duration of outages dictates the size of your battery, inverter, and charging source (PV array, wind generator, micro-hydro generator, etc.)—and your system’s cost. Rewiring your service panel adds another cost, since backed-up loads must be separated from non-backed-up loads and placed in a dedicated service panel. This work can lead to remodeling, further adding to the labor and cost involved.
A battery-based grid-interactive system requires a specialized inverter. Most of the time, the inverter operates in its grid-tied function, converting DC energy (from a PV array, for example) into grid-quality AC energy that can be consumed on-site or—if the loads do not use all the energy—exported to the grid. The inverter’s second function is as a backup power supply, which handles battery charging when the grid is available and seamlessly switches to battery backup mode when grid power drops out.
In the more-common grid-interactive mode, the inverter is exporting energy from the charging sources to the household loads and any excess energy is pushed on to the grid. If the home consumes more energy than the charging sources can supply, that energy is pulled from the grid. Specifically, if the backed-up load center demand exceeds what the charging source can supply, the inverter will pass grid electricity through its internal transfer switch to its output circuit (i.e., to the separate backed-up loads subpanel).
During a utility outage, the inverter switches to converting DC from the battery to AC for the backed-up loads panel. The means of feeding the grid during grid-tied mode is disconnected when the grid is down, and all loads in the main panel are de-energized. Once grid power returns, the inverter will wait five minutes to make sure the grid is stable and then switch the backed-up loads panel from inverter power to grid power and start recharging the battery with whatever power sources are available—grid, solar, etc. Then the inverter resumes sending excess energy to the grid.
When the grid is out-of-spec but not out, the inverter will disconnect from the utility and send power to the backup panel. The loads inside the main panel will continue to run as long as the loads are getting enough voltage from the grid. Inverters have a very tight spec and may disconnect from the utility even when there isn’t a major problem. In this scenario, the inverter wouldn’t be able to send power back to the grid if the PV array was producing excess power.
While the complexities and design factors for a battery backup system may be daunting, the rewards can make up for it. Few battery-backup customers ever say they regret the system choice—while some batteryless grid-tied customers fantasize about using their appliances while the grid is down. Usually, the financial outlay for a battery-based system is the determining factor. These systems, like all home RE systems, are becoming more affordable thanks to falling module and other PV equipment prices—and the generous 30% federal tax credit that even applies to battery-based systems.
Flint Richter (flint at rockygrove.com) lives and writes off-grid in Arkansas’ Boston Mountains. He is a partner and NABCEP-certified PV project manager with Rocky Grove Sun Company and a contracted instructor for Solar Energy International. He is teaching his young daughters the difference between a solar module and a solar panel.