Much has been made of the interoperability problem with home energy storage systems. Existing solar customers need battery systems that can connect easily with their legacy equipment. AC-coupled storage has proliferated to solve that problem.

When Maine-based Pika Energy got to work designing power electronics for a new residential storage platform, the team decided to go in a different direction.

"AC coupling is attractive because you can retrofit it. However, actually getting the new battery inverter to talk to the legacy PV inverter is not a simple thing to do," said company President and co-founder Ben Polito. "Do you want to send your best technician to figure out how to interact with that 2011 inverter in an optimal way? It becomes more like a science project than a scalable solution."

The desire for easily scalable deployment and reduced installation time led Polito to a plug-and-play platform approach. That product is now available for order as the Harbor Smart Battery, a collaboration with Panasonic Eco Solutions North America.

The system will ship later this year in 10- and 15-kilowatt-hour configurations, offering up to 6.7 kilowatts of continuous power.* It integrates Panasonic lithium-ion battery modules and is designed for easy pairing with solar generation using the same inverter for both. It can handle islanded backup as well as solar self-supply.

The different elements link up through Pika's REbus nanogrid platform, which Polito describes as "USB for clean power." It provides a ready-made hookup so that different home energy assets can all communicate with each other, coordinating system performance based on market price signals. 

The Harbor system has built-in electronics to connect the Panasonic batteries into the REbus system. That makes it easy to slide the battery modules into place with minimal electrical work. The individual components weigh less than 75 pounds, so they don't require large teams to carry.

"The whole 15 kilowatt-hour battery solution can be installed by one guy or gal in less than an hour," said Polito, whose company recently raised $2 million.

The design concept reflects a theory of what the residential storage market will look like in the future. 

Storage companies have chased existing solar customers as a logical early market. The first adopters have demonstrated they have money and interest in investing in clean energy products. Since they already paid to have solar, they're liable to invest in storage to protect that investment if compensation for surplus solar power falls.

That dynamic has driven demand for storage in Australia and Germany, as generous feed-in tariffs have started to phase out.

"Right now, the size of the Australian residential retrofit market is larger than the whole U.S. residential market in terms of pent-up demand and potential for actual economic deployments for PV self-consumption," said Brett Simon, a behind-the-meter storage analyst at GTM Research.

Companies like Enphase and Sunverge are chasing that market with AC-coupled products. That architecture connects a self-contained storage-inverter pair with the existing solar-inverter pair.

A downside there is the additional conversions needed to store solar energy and extract it for use. Instead of one DC-to-AC conversion for discharge, the AC architecture goes DC to AC (solar to inverter), AC to DC (inverter to battery) and DC to AC (battery to load). Some energy gets lost in each jump, and that adds up over a system's lifetime.

The AC-coupled versus DC-coupled debate is complex and by no means settled. In the meantime, Pika is betting on a growing audience for residential storage that doesn't need to bother with those extra conversions.

"Our equipment AC-couples happily, but we look at a U.S. market characterized by 1 percent penetration of PV and see that as 99 percent non-penetration," Polito said. "What's the right architecture for the 99 percent of folks who don't have a PV system and will be incentivized by market dynamics to add storage?"

Moreover, most solar customers will have their rates protected through grandfathering in the event of future changes to net metering, he noted. If customers can earn money on the surplus generation, they have less incentive to buy a battery.

"It’s hard to argue with an infinite battery for free," Polito said, referring to the grid.

That situation isn't guaranteed, but a decision to take away grandfathering in Nevada, for instance, was met with swift popular resistance that led to a reversal of the change within a year.

As utilities move beyond net metering and adopt more granular time-of-use tariffs, the economic case for storage is likely to grow. In Hawaii, new rooftop systems already need self-supply capability. Pika's system also allows a household to install solar now with the knowledge that it can easily add storage later on, if the economics cross into more favorable territory. The REbus system can work with other storage devices too -- it's not limited to Panasonic.

Outside of select geographies, the storage moment has not yet arrived for the U.S. residential market. Pika can use that in its pitch, though.

Lithium-ion prices are dropping fast, electric vehicles are reaching wider audiences and revenue opportunities like blockchain and virtual power plant aggregation may soon transform from buzz into reality. Amid those trends, Pika would like to sell power electronics for solar that are designed from scratch to incorporate storage, so it doesn't have to be figured out after the fact.

*Correction: The article originally stated this number as instantaneous power rather than continuous.