In Solar Power World’s recent webinar “Battery Backup for Grid-Tied PV Systems,” Doug Pratt, senior engineer at SimpleRay Solar, discussed whether battery backup is a viable option for everyone, adding battery backup to existing systems and implementing it with new systems.
Is Battery Backup For Everyone?
Doug Pratt tells it to you straight: A backup battery isn’t for everyone. The additional cost to add a backup battery to a standard grid-tie system, including materials, labor and parts, can run anywhere from $6,000 to $8,000 at least.
However, battery backup is a good option for people who need instant, reliable backup (data needs, computer support, medical needs, etc.). For everyone else, it’s more of a convenience factor, says Pratt.
“In all honesty, [the] average residential client is going to be better served with a generator and large tank of propane, and that’s going to get them through the typical three- to five-hour utility failure,” he says.
Battery backup systems are meant to cover only critical loads, meaning the fridge, freezer, furnace, water pressure, lighting, security system and occasionally air conditioning.
Can You Add Battery Backup To An Existing System?
The most common scenario is one in which the customer already has a grid-tie system installed and wants to add on a battery backup.
“We saw a lot of people after Hurricane Sandy inquiring about these sorts of systems to add on to their existing grid-tie system,” Pratt says.
When the add-on system senses utility failure, it will disconnect and send an AC signal into the system, tricking the conventional grid-tie converters into thinking that the grid is still up. So, in the event of a grid failure, if the sun is shining, the inverter will keep running.
The output from a conventional PV array going to a conventional grid-tie inverter normally goes directly into the household’s main AC panel. The backup system is installed between the inverter and the main AC panel, and now the output goes to a critical loads sub-panel and lands on it before continuing on to the main panel.
Now, in the event of a utility failure, the battery-based inverter will isolate itself from the main panel, open a contactor and shut off any access to the utility. Then the inverter will power up using stored energy from the batteries and continue supplying power to the critical loads sub-panel where all the supported loads are located.
Rest assured, the add-on kits will work with new or existing conventional grid-tied systems, and no alterations are necessary on the existing grid-tied systems.
The add-on kits include an inverter, AC/DC power center with input and output breakers, DC cabling, a sealed battery pack with approximately 9.5 kWh worth of stored energy and battery boxes. All you will need to install is a locally supplied critical load AC breaker panel.
What About Battery Backup Installations On New Systems?
According to Pratt, when there is no preexisting PV system on a house, and a client wants to install one with a backup battery, those direct battery-based inverter systems usually cost about 50% less than add-on systems, depending on the size of the PV system and how much battery backup is requested. Additionally, direct battery-based systems can be either off-grid or grid-tied.
With battery-based systems, a charge controller is used between the PV array and the battery; power goes from the PV array to the charge controller, and then on to the battery pack. On the other side of the battery pack sits the battery-based converter where the AC input connects to the utility grid and the AC output goes to a critical loads supported panel.
Here’s how it works: The charge controller receives enough charge for the batteries to reach 56 volts, while at the same time, the inverter is programmed to prevent the batteries from getting above 53 volts. The charge controller is now in maximum power point tracking mode and forcing all the wattage into the battery pack to raise the voltage to 56 volts. The inverter is then pulling all that power (excess electrons) off the battery, converting it to AC and dispensing it into the household loads, and/or the utility grid, on the supported loads side of a built-in automatic transfer switch.
During normal circumstances, when the utility grid is up, the internal transfer switch is closed and power can flow out of the inverter to supported loads, then flow back to the utility grid.
If the grid fails, the transfer switch opens, the inverter is still running and sending AC power to the supported loads, the supported loads inside the house are still receiving power (either from PV input or from the batteries’ stored energy), the utility grid has been cut off and nobody in the house has noticed a power outage.
All battery-based grid-tie converter systems use this kind of system with an internal transfer switch, referred to as a “bottleneck,” that opens to cut off the utility grid. For most manufacturers, it’s a 60-amp unit, which Pratt says isn’t enough to run a household.
“This is why these battery-based systems are not good at running the whole house,” says Pratt. “They really are for critical loads.”
For those clients who want to run everything, Pratt suggests a 15-20 kW generator and a large tank of propane for a better and cheaper solution, adding that propane makes a great backup source because it doesn’t degrade over time.