Grid Neutral: How this DC transformerless inverter brings affordable off-grid power to the grid

sol-arkThe latest advancements in solar + storage systems have been a departure from the history in this category — mainly, doomsday preppers and hippies getting off the grid, man. No, the solutions of tomorrow are grid-tied but built to reduce a home’s dependency on the grid, using time of use strategies to shift loads to avoid the most punitive electric rates.

The sexiest solar + storage inverter advances in this area are DC transformerless options — a sole inverter capable of handling the PV, grid and battery connections. Because these inverters will be grid-connected, they prioritize continuous power efficiency instead of peak power. This is fine unless a customer is looking for an on-grid system that also can handle battery backup capabilities because that requires high peak power. There’s also the efficiency loss when converting from PV to battery and back to AC. An inverter starting at 97 percent efficiency could be at 92.2 percent or lower depending on the brand by the time the energy finally makes it to its destination.

So, yes, DC transformerless is a streamlined, future-proof architecture, but you will be making a compromise in efficiency somewhere. Well, except with the Sol-Ark inverter.

Peak vs. continuous

Sol-Ark is a new inverter on the block (or grid, I suppose). It is the brainchild of U.S. veterans looking to engineer a solution to help families be less dependent on the grid in an affordable way. They wanted to take the autonomy and top end abilities of the off-grid inverter without losing continuous power efficiency. To do this, Sol-Ark beefed up its hardware to minimize conversion losses. The result: Sol-Ark can deliver 96.5 percent efficiency in on-grid and 93 percent in off-grid and time of use scenarios with minimal conversion losses. On average, Sol-Ark needs 10 to 15 percent fewer solar panels and 5 to 30 percent less storage. That is serious savings.

“On our system, we have an internal 400-volt bus, and we convert high voltage solar panels to that 400 volts, and then that’s converted to AC,” says Tom Brennan, engineering manager for Sol-Ark. “When we go to the batteries, we focused on a super-efficient conversion method that allows us the highest efficiency directly into a 48-volt battery.” The end result is 95.5 percent efficiency from battery to AC.

Where many inverters in this space lag is in AC to battery efficiency. Outback, for example, is rated about 82 percent efficiency. Sol-Ark hits 96 percent efficiency because of a different methodology for pumping power in and out of batteries — a powerful 185 amp DC-to-DC charger.

Other DC transformerless options out there like Pika and SolarEdge are designed for newer 380-volt Lithium batteries. But Brennan notes these batteries are still 50 volts internally and require a double conversion every time going in or out of the battery. It’s why Sol-Ark has focused on just making 48 volt batteries more efficient.

Your on-grid customers might want storage as backup and to have power when the grid is down. This again is when peak power is crucial to handle motors starting up. For example, SolarEdge and Outback Skybox are not off-grid solutions. They only deliver 5 kW on the battery and little additional peak power means they can’t start up A/C or well pumps. Pika has a per string optimizer, and if you don’t use it, it won’t work off grid because it’s not an AC-coupled system.

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The reason Sol-Ark has so much peak power is it packs in twice as many components as its competitors. It is literally designed to survive a solar flare or EMP attack, which was the goal for these U.S. vets at the outset.

“We didn’t just focus on the person who wants to save on an electric bill but focused on emergency responders and state government, but in an affordable manner,” Brennan says. “To do this, we basically built a 20-kW inverter and throttled it down. We don’t have heating issues because we are not taxing the system.”

Providing the most powerful, efficient inverter for on or off-grid solar + storage does come with some new school drawbacks, at least right now. For example, Sol-Ark does not meet California’s Rule 21 criteria.

“That has not been an issue because we just don’t export power in California,” Brennan says. “We use the solar during the day, batteries at night and grid as a backup.”

On-grid extras

Sol-Ark comes ready to perform the typical on-grid capabilities like time of use and grid sell back but also has a few unique options:

Limited home mode. CTs or current sensors are placed on the mains of the house, and instead of just full grid selling or only powering critical loads, limited home mode is an in-between option. The CTs sense when any other circuit not on the critical loads panel kicks on and ramps up the solar power as much as it needs to zero the meter if it can.

“Maybe you don’t have a net meter agreement to sell back power to the grid, but you can push power to your whole house as long as the grid is up,” Brennan says. “If it’s down, you can only run critical circuits. We designed Sol-Ark 8K for seamless on-grid capability.”

A lot of people in rural areas take advantage of this because they either don’t want to deal with the electric company or have no incentive to deal with it, so they use that to push as much power as they can to the house.

Smart loads. This is a programmable load that’s not based on time but rather the batteries’ state of charge or how much PV power is being produced. Brennan explains: “We can turn on A/C or the hot water heater at 100 percent battery and solar is producing 2,000 W during the 9 a.m. to 3 p.m. time range when I’m producing excess solar. And if the solar stops producing [or batteries get to 95 percent], they shut off automatically. It’s perfect for on-grid time of use and off grid for preserving battery life.

Hot water heating takes about six panels of energy a day, and air conditioning can take about 12 panels a day, so if just those can be run from PV alone and without the batteries, that will extend the life of the batteries by at least 50 percent or more and probably reduce the battery bank size by 30 percent. The Sol-Ark system will also account for and adjust to the degradation of battery health over time.

What’s old is new again

Another question Brennan asks: Why are solar + storage customers waiting for lithium prices to drop when they will be connecting to the grid anyway? Sol-Ark’s goal is make an off-grid approach to solar + storage work on the grid without doubling the cost of a system.

“We don’t think customers need lithium if it’s just sitting there for backup; it’s a waste of money,” he says. “If you are on the grid, then take advantage of it. You can go with lower cost AGM batteries that last 10-plus years. If you have the grid where you are, get AGM batteries and use them as backup or slightly used and don’t necessarily deep cycle them. If you’re totally off grid, then we recommend lithium or carbon-based AGM that have four to five times the cycle life. You can minimize your usage of the grid, but do it to keep your battery bank at a reasonable cost/size. If you were to try going completely off grid with a traditional size home, the battery cost would make you cry.”

Chris Crowell is the managing editor of Solar Builder.

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