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

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

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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.

George Musser About the Author: is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

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

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  1. 1. jbairddo 8:34 am 03/19/2013

    Hence the push to have electric cars act as buffers to the system and use their batteries as storage.
    How much current needed to at the inverter to allow it to work? Couldn’t one use their car batter and a cheap 400w inverter into an electric outlet in the house so they could use their solar. I guess this was a system for prepping.

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  2. 2. sethdiyal 5:59 pm 03/19/2013

    The car would have to be home daytime – ain’t happening.

    The average home uses 12 Mwh annually of power or 30kwh daily so this rig would only last a day.

    The best AGM batteries can only be discharged to 50% 1500 times so they’d have to be replaced often.

    Finally you have to store for summer to winter.Add a lot more batteries.

    Cost is best case a buck a kwh.

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  3. 3. peterkienle 8:06 pm 03/24/2013

    I would think that in case of an outage, running on backup power you’d only leave the essentials on and be using less than 30kwh a day.

    Since 2008 we have a 2.6kw PV array in our front yard. It’s grid-tied and if we schedule showers, laundry and other power hungry activities during the day (with good sunshine) we can save quite a bit.

    But if there is an outage our PVs go offline and it would be nice for that not to happen. $10,000 for a battery backup would be too steep for me but if there is a simple solution to use the power from the array in an outage that would be great. We have a handful of computer backup power supplies around the house which are enough to provide light and limited power for a while.

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  4. 4. solarhome 1:59 pm 04/4/2013

    If you’re paying $6,000 for a grid tie battery backup inverter then you’re paying far too much, in fact about $2,000 too much. The same goes for the battery rack and the batteries mentioned in this article. Outback is not a bad brand but you can buy a higher efficiency model from Schneider Electric called the XW series. A 6.0kW XW series inverter sells for under $3,000. Although maintenance free AGM batteries are nice, you can buy wet cell 6 volt 220 amp hour batteries for a little over $100. Eight wired in a series string will give you 48 volts at 220Ah for 10.56 kilowatt hours of storage during an emergency. Two series string will double your storage. And who on Earth would pay $2,000 for a simple battery rack. Build your own out of 2x4s and marine grade plywood. for under $200. And using your car battery and a cheap 400 watt inverter that’s plug into an outlet so you can use your solar array ? Don’t even think about it. First of all most of the cheap 400 watt inverters on the market produce a modified sine wave output (really a modified square wave) this will probably not be identified by your grid tie inverter as a viable waveform due to the missing sine wave peaks and will not “wake up” your gridtie inverter during an outage. Actually you better hope that it doesn’t because if it did, your would not only begin powering your home but you would also energize the utility company’s power lines creating the potential for electrocuting a lineman who is working to restore the power.

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  5. 5. fafcosolar 3:05 pm 05/22/2013

    The key is to understand what your real battery backup needs are. I’ve run into people that expect to have battery backup for their whole house, only to learn that they typically consume 150 kilowatt-hours of energy per day. It’s probably not going to happen! On the other hand, some people just want to charge their iPhone or have some basic lighting. You probably already have a battery or two in your garage (in your car). A small inverter might just meet your needs in a real emergency!

    Again, understanding how much energy you *really* need is key to your battery backup strategy.

    Jason Szumlanski
    VP/GM, Fafco Solar

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  6. 6. novelremodeling 7:39 pm 06/5/2013

    This is something to consider if you think about it. For instance, if you have a customer that is paranoid about natural disasters then this is idle for them. For most, especially californians, this isnt something to be to concern about.

    Jim Griffen
    Novel Remodeling

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  7. 7. Jennifer Bakker 6:39 am 08/2/2013

    Hi George,
    Take a look at the powerrouter solar inverter. It even manages the battery discharging. 2020 Solar

    Link to this
  8. 8. Jennifer Bakker 4:14 pm 08/26/2013

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  9. 9. radartech 8:45 pm 03/12/2014

    Why can’t you connect a grid tied system with a battery backup so that you have power during a blackout? I understand that if your system is always hooked up it would definitely experience difficulties due to excessive loads at a power failure BUT if you installed a break then make panel between your battery backup, solar panel, and the grid there is no chance of overloading your system when the power suppliers that be decide to fail. Your system won’t try to feed the entire grid because you are forced to disconnect before you enable your panels. It took me all of 5 min to dream up a schematic and 1 hour to put it on Power Point but I don’t think I can post it here. With this schematic even panels with built in inverters could be used, the batteries and a separate full sine wave inverter would initially provide the power to start up the panels and voila! POWER! You could install a small UPS in front of critical loads to take up the slack while switching over.

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  10. 10. mayanktanwar 4:45 am 05/12/2014

    Solar batteries and flat plate batteries at

    Link to this
  11. 11. Michael8310 3:22 pm 06/26/2014

    Great article. We utilize Outback equipment for much of the off-grid work that we do. In such situations, the high cost of running grid power to the home offsets the initial cost almost completely.

    For battery backups, it remains an expensive solution. The prices have come down considerably,though (see Edison Solar) The Outback 4000W Radian is less than $3,000.

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  12. 12. sheilagonzalez 5:09 am 08/16/2014

    Solar Power Supply was totally unknown for me. I visit a site here i first seen this topic but thanks to you shared details on that.

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  13. 13. chetanyadav 5:57 am 09/2/2014

    I can post it here. With this schematic even panels with built in inverters could be used, the batteries and a separate full sine wave inverter would initially provide the power to start up the panels and voila! POWER! Read more….

    Link to this
  14. 14. sherrizahir 6:16 am 09/11/2014

    Choice for a Power bank is totally depends upon your daily consumption.It is good to have a better power bank in such a blackout.You can visit on given link for more information for power banks…..

    Link to this
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  18. 18. Jonathan Cole 11:31 pm 02/21/2015

    This article is not very helpful in describing a solar photovoltaic system that can continue to work even when the grid goes down. It describes very expensive systems that require the owner to invest in extra equipment that mostly benefits the electric company. There is a standard type of inverter called an inverter/charger (also available from Outback) that can automatically use the grid as backup but does not feed anything into the grid. Therefore the electric company has no say as to whether you can hook up to the grid. As long as the system is sized properly for the loads, the user/owner consumes all of the solar electricity it produces.Less than 1% of the total electricity provided comes from the grid. Additionally some large-draw but infrequently used or non-essential appliances can remain connected to the grid which means a smaller investment in solar generating capacity is required. I have designed and deployed many such systems and they are much more reliable than the grid. In thirty years of working with this technology the only power outages that occurred (less than 5) resulted from faults that had nothing to do with the solar PV system or its electronic power electronics. This technology is dramatically more reliable than the electric company.

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