As the aging U.S. power grid experiences more catastrophic blackouts due to fires, ice, snow or other natural disasters, more Americans may be considering the merits of disconnecting from it all and living fully off-grid. The concept seems quaint and simple, but the reality of ensuring a standalone electric system operates as it should for as long as possible is much more complicated.
Although it’s important to properly design and install the actual solar panels on an off-grid system, the sensitive power electronics must be all the more carefully planned out. This starts with choosing the right inverter for the environment.
Manufacturers have worked to design inverters that can withstand even the harshest off-grid conditions. Perhaps the biggest design choice is whether to create a vented or sealed inverter. Typical string inverters installed in residential garages are vented to ensure adequate air flow is available to keep them cool. Cooling is important because when inverters overheat, they shut down to protect themselves. But internal fans that assist in the ventilation process can draw in unwanted debris when inverters are installed in off-grid installations.
“You’ve got roads that are basically dirt, so they kick up a tremendous amount of material in the air. An inverter that’s going to draw straight atmosphere into the system would happily suck all that dirt in, and the next thing you know, when you shake the box, you got a pound of dirt that comes out of the bottom,” said John Webber, international sales manager for OutBack Power.
The larger ventilation openings in typical residential inverters can also be an invitation for rodents and bugs to nest inside and cause damage. For installations near an ocean, venting can let salt enter the inverter and corrode metal parts.
“My rule of thumb is if you can smell the ocean, there’s salt in the air. That’s a pretty easy thing for people to get their brains wrapped around on choosing that system. If you can smell the jungle, then the jungle is going to come for you,” Webber said.
The solution to these off-grid environmental issues is choosing a sealed inverter design. OutBack’s FXR/VFXR inverter series is built using a sealed design, with only a small spiral on the bottom to allow the inverters to breathe. That means it takes in as little of the surrounding atmosphere as possible.
Sol-Ark also manufactures sealed hybrid inverters that CTO Tom Brennan compares to microinverters in their long warranties and lifespans.
“Most outside-rated hybrid inverters are indoor electronics inside a steel umbrella, but our outdoor electronics are 100% isolated from rain, dust, humidity, bugs that gives it a 20+ design life,” Brennan said.
The downside to sealed inverters is they don’t cool themselves as well as vented versions, so their overall power rating is lower as a result. But that tradeoff could be worth it for a longer life span and less required maintenance.
OutBack has added design elements that help sealed inverters keep cool such as internal fans to keep air moving and an external fan on top that blows air across fins on the outside of the box.
Morningstar Corporation is in favor of forgoing fans entirely in its off-grid inverters. The manufacturer has one small off-grid inverter on the market today, the SureSine, but is developing a larger product called the MultiWave, said Ezra Auerbach, consultant for product development and application.
“If you don’t have a fan, you don’t bring dirt and dust in from the environment,” Auerbach said. “The sealed aspect of the unit will also make it more resistant to salt intrusion and, of course, without a fan there’s one less thing to break.”
Morningstar must ensure its products can operate at higher temperature ratings since the typical cooling elements of fans and large vents are absent. Doug Grubbs, Morningstar engineer, said most of the company’s equipment is rated to at least 45°C (113°F). The company uses passive ventilation to keep its yet-to-be-released off-grid inverter from overheating, in the form of heat sinks on top.
Choosing mountaintop inverters
Off-grid homes are also commonly in very cold areas, which require a whole different set of inverter design characteristics. Aaron Mandelkorn, president of Colorado-based Renewable Energy Outfitters, specializes in off-grid solar + storage projects in cold temperatures and high altitudes.
The first thing Mandelkorn looks for when he’s choosing an inverter is its temperature operating range.
“At 12,000 ft, 11,000 ft in the middle of winter, these inverters are seeing temperatures of -15°F, not to mention batteries and everything else. Its ability to even really function sometimes gets compromised,” he said.
It’s imperative that the inverters he chooses can operate in those temperatures not just for performance concerns, but also to ensure they are installed “to spec” and thus still qualify for warranty protection.
“If you have a problem, you’re not going to get support from the manufacturer if you’re too cold or too high,” Mandelkorn said. “With complicated off-grid systems, we kind of rely on the support of the manufacturers to help deal with a variety of technical problems.”
He and his team still do plenty of site visits to repair off-grid installations themselves, but they aren’t cheap.
“We come out to places where everything is frozen, we come out to places where everything is just cooking, and just see all sorts of electrical failures because they just can’t handle it,” he said. “A lot of times, to service the site, excavators and snowcats need to be used to literally clear roads in winter, to make roads.”
That process can cost around $10,000 to the customer, but there are some relatively simple steps that can be taken in the installation and O&M process to largely avoid those costly visits.
Installing off-grid inverters to last
If it’s possible, installers should protect inverters by building a robust structure around them, ideally made of metal since rodents can’t get through it, according to OutBack’s Webber. Inverters should also be mounted on treated wood that’s raised off the ground and not easy for animals to enter from underneath. Webber also advises installers to use wire mesh to keep critters away from insulation and wiring.
“At the end of the day, Mother Nature will always try to find her way into your system. Every barrier that you create slows that progress significantly,” Webber said.
In hot climates, the structure should be vented to again ensure the inverter doesn’t overheat. Holes can be drilled, then covered up with wire mesh to act as another barrier against animals. Morningstar’s Grubbs also suggests installing external fans to keep the structure cool.
Since off-grid inverters will usually be paired with batteries, Auerbach said it’s important to make sure there’s adequate air space between an inverter and lead-acid batteries to account for off-gassing. He also encouraged installers to invest in a good temperature sensor and voltage sensor to keep tabs on the system and avoid overheating or other shutdown surprises.
Webber said it’s also important to clear any large trees or branches that could fall on the system during installation and to set a regular O&M schedule to make sure the system isn’t compromised by nature or anything else.
In cold climates, REO’s Mandelkorn stressed the importance of housing the inverters and batteries in conditioned, insulated spaces. He typically installs power electronics and batteries in “power sheds” that are insulated and heated using direct vent propane heaters.
“A little bit of insulation and a little bit of heat goes a very, very long way,” he said.
To keep systems running and minimize O&M site visits, Mandelkorn said he chooses inverters like Sol-Ark’s that have comprehensive remote monitoring capabilities. If the system notifies him of a fault, he can remotely look into the problem and determine if it’s an issue that does not require a trek into the mountains, like DC overvoltage due to an extra-sunny day, or if it’s a critical issue that can only be resolved by him visiting that site in-person.
Sol-Ark’s Brennan said the manufacturer monitors its entire fleet remotely and emails installers if they notice an error.
“Although problems are typically settings we can remotely change, we can catch install mistakes before it leads to hardware damage,” Brennan said.
Mandelkorn said he prioritizes “usability” over serviceability of inverters in his projects, meaning he wants his customers to be able to understand what’s going on with the system using a comprehensive monitoring platform, but he does not want them to have to personally service their systems if it requires any electrical work.
However, Webber said OutBack works to make its inverters easily serviceable by the end user because the manufacturer understands installers are sometimes many miles away from these off-grid systems.
“If they have the skills to put a new graphics card inside their computer, they have the skills to be able to swap those boards out and get that inverter up and running again,” Webber said.
Off-grid solar + storage projects can be complex, but with the right design and installation considerations as well as robust monitoring, installers can ensure their customers enjoy many electrified days with no reliance on power lines.