Solar at Home

Solar at Home

The trials, tribulations and rewards of going solar

Should You Add Backup Batteries to Your Grid-Tied Solar Array?


My neighbors took a newfound interest in my solar array after Hurricane Sandy. Most of our town in New Jersey lost power for two weeks, and everyone who knew about my panels was asking: Did they keep my lights on? Alas, no. When the grid goes down, our array goes down. The inverter mounted on my basement wall, which converts DC photovoltaic power to AC household current, trips during a blackout. The sun could be blazing and the array could develop a potential of hundreds of volts, but it’s potential in every sense of that word. Without the inverter to complete the circuit, not a single electron flows into my house.

Years ago, stewing over previous blackouts—they call this the Garden State, not the Reliable Infrastructure State—I investigated backup generators, but the expense put me off. I decided just to set up a transfer switch and battery backup for my steam boiler to ensure we don't freeze. But the extended post-Sandy outage and likelihood of a recurrence got me thinking about backups again. Last month, I chatted with Harvey Wilkinson and Phil Undercuffler of Outback Power, based just north of Seattle. Their bidirectional inverters can convert DC to AC (so the array can power the house or feed into the grid) or AC to DC (so the grid can charge a bank of batteries). “If there's a power outage, the system can drop the grid and continue powering the house,” Undercuffler said.

Not only do a bidirectional inverter and battery bank light up the darkness, they give solar homeowners the flexibility of choosing when to buy power from or sell power to the grid, so you can take advantage of tiered rates. In some areas, such as Hawaii, the utility doesn’t allow you to sell power back to the grid at all, because too many people are already trying to do that and the system can’t handle them all. A battery system at least lets you buy when it’s most advantageous to do so. In fact, this is the future we all face, because all grids have only a limited capacity to absorb electricity from panels on people’s roofs and wind turbines in their yards. Not only does solar and wind output fluctuate, distributed power generation can make the network prone to cascading failures. For instance, if the grid frequency wavers, inverters might take themselves offline, which reduces the total generating capacity, which worsens the frequency deviations, which causes other inverters to trip, and so on.

“As you get greater saturation of intermittent renewable supplies, you’ll see less and less grid stability,” Wilkinson said. Undercuffler predicted this would become a major headache when solar and wind produce a fifth of the total power, a goal that New Jersey has set for 2020. Germany has already passed this point and experienced grid imbalances. But if distributed generation creates a problem, it also offers a solution. Batteries in people's basements would even out the power swings. In addition, as I’ve blogged about before, household inverters could help to stabilize the grid by tweaking their AC waveform, compensating for devices such as electric motors that muck things up by momentarily storing energy in their magnetic fields.

I had a vision of a gigantic pile of batteries, looking like a nest of alien eggs, but Wilkinson and Undercuffler said the company’s basic unit is the size of a minifridge (see photo above). It holds up to 12 lead-acid batteries, for a maximum of 600 amp-hours of charge, providing about 24 kilowatt-hours of energy. That’s the punch of the battery pack in a Nissan Leaf, with the crucial difference that the household batteries don’t go through nearly as many charge-discharge cycles. Their battery life is determined mostly by the ambient temperature and, in a typical basement, should be about a decade.

The issue, as always, is cost. The inverter is $6,000 (twice as much as the inverter I have), the empty battery rack runs $2,000, and each battery is $600. Blackouts are a pain, but are they a $10,000 kind of pain? If you really think so, a natural-gas standby generator would probably be cheaper. That said, I suppose tax credits might offset some of the higher cost. Also, the price tag might be justified if you’re stuck with a utility that won't buy your power, or indeed no utility at all. Outback's systems are found in remote African wildlife reserves, Arctic and Antarctic research stations, and other places that are even more infrastructure-challenged than New Jersey.

I’d love to hear what other battery solutions people have tried for their grid-tied systems.

The views expressed are those of the author and are not necessarily those of Scientific American.

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