83 percent of Puerto Ricans remain without power three weeks after Hurricane Maria hit the island. The goal is for 25 percent of customers to regain power by the end of October, but it could be months before the territory’s grid is fully operational again. Meanwhile, 36 percent of the island still does not have water service. Since energy is required to treat and deliver water, presumably the lack of power is standing in the way of getting some of those water systems back online. (Water, of course, is also needed to generate energy, but that’s a topic for another time.)

As of this past June, Puerto Rico had 215 megawatts (MW) of solar power installed, split roughly 60-40 between utility-scale and distributed or residential scale installations. That translates into powering about 19 average American homes for a year. Even though that number is small, many people are asking, why aren’t these solar arrays helping generate power and get things back up and running faster in Puerto Rico?

Since solar panels generate energy from the sun’s rays, it seems like the panels should be generating any time the sun is shining. But unfortunately, it turns out that solar systems are more complicated than that. The majority of solar system designs require power from the electric grid to power the inverter, the piece of equipment that converts the DC power produced by the panels to AC power for consumption. Not only that, but the inverter is usually purposefully designed to shut the system down when it senses that the electric grid has gone down. This is in order to prevent dangerous conditions and equipment damage that can result when a solar system continues to pump energy onto the grid during an outage. So we can expect that the majority of solar arrays in Puerto Rico are currently not operating, by design. In the meantime, families who can afford it are relying on expensive, dirty generators for light and power.

There are solar arrays, however, that are designed to operate in conditions where the electric grid has gone down. This is called islandable solar power, referring to the idea that during an outage the arrays form “islands” of power. The main difference in these systems is that they require a special (usually more expensive) inverter. The inverter has a controller that disconnects from the grid when it detects that the grid has gone down, and directs all of the solar energy being generated to power the local load. Islandable systems do not need to include batteries, but ones with storage will be able to continue supplying energy when it becomes cloudy or at night.

Disaster situations aside, an islandable solar array with battery storage can be an effective replacement for a backup diesel generator. While the solar option is likely to be more expensive compared to a backup diesel generator, if a utility is considering adding or replacing a backup diesel generator and also considering installing solar panels, installing islandable solar with storage alone could be more cost-effective.

When an entire set of facilities or campus is islandable together, this is referred to as a microgrid. A microgrid may include one or more distributed energy resources, such as a solar array. A community might choose to link all of their critical services together in a microgrid to jointly be more reliable during emergencies. This is one of several options recommended by the EPA for water utilities seeking to be more resilient.

Whether the future of power in Puerto Rico is islandable solar, solar plus batteries, microgrids, or even a return to the previous status quo, overhauling the territory’s grid will require an enormous amount of investment. Fixing the island’s distribution system alone is expected to cost $5 billion. Meanwhile, before Maria hit, the Authority was operating plants an average of 44 years old, many of which were outdated oil-fired plants (whereas the average is 18 years on the mainland). Calling its equipment “‘degraded and unsafe,’” the Puerto Rico Electric Power Authority (PREPA) filed for bankruptcy back in July with $9 billion in debt. As Congress considers a $29 billion aid package, the need to rebuild is a unique opportunity to build a stronger, cleaner, and more resilient grid for the island.