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Energy Solutions

Getting Power From Solar Equipment When the Grid is Down

A new inverter from SMA allows us to draw some daytime power from our PV system when the grid is down, even without batteries

The 18-kW photovoltaic array on our barn is a group-net-metered system with some of the output going to other houses.
Image Credit: Alex Wilson
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The 18-kW photovoltaic array on our barn is a group-net-metered system with some of the output going to other houses.
Image Credit: Alex Wilson
We have three inverters in our system that are housed in a downstairs room in the 1812 barn. The one with Secure Power Supply is the third from the right. Our Sunny Boy 5000TL-US inverter. The outlet beneath it provides emergency power during outages (when the sun is shining). Comparing the new transformerless inverter (on the left) with the standard Sunny Boy inverter Our PV array being installed. The barn roof was structurally reinforced and covered with standing-seam metal roofing.

One of the biggest complaints I hear about most solar-electric (photovoltaic or PV) systems is that when the grid goes down you can’t use any of the power that’s produced. Consumers have spent thousands of dollars on a PV system, and during an extended power outage on a bright, sunny day when the PV modules are certainly generating electricity, they are disappointed that none of that electricity can be used.

This problem applies to grid-connected PV systems that do not include battery back-up. Off-grid systems work just fine when the grid is down, but the vast majority of the roughly 300,000 PV systems in the U.S. are grid-connected systems without batteries, and most of them lose all functionality when the grid is down.

Given my focus on resilient design (including my founding of the Resilient Design Institute last year), I wanted to install a solar-electric system at Leonard Farm that would have at least some functionality during power outages.

Full islanding capability

I wish we had full “islanding” capability with our PV system. Islanding refers to the ability for a PV system and the loads connected to it to be separated from the utility grid during outages so that no electricity could be fed into the grid and injure utility workers who are trying to repair down lines.

Fully islandable PV systems require specialized inverters along with battery banks that allow them to function off-grid. The battery bank not only provides for functionality at night, but it also establishes the proper waveform during the daytime when the grid is down so that AC power can be delivered to the house.

Some islandable systems, such as the OutBack Radian and Schneider Electric’s Xantrex XW-series inverters, rely on a single inverter that can connect to the grid and a battery bank; these inverters switch back and forth automatically. Such inverters communicate with and draw electricity from the battery bank during a power outage and also send electricity into the grid during normal operation. These are sometimes referred to as bi-modal inverters.

There are other, battery inverters that can be added to a PV system that already has one or more PV inverters. Inverter manufacturer SMA offers such an option, the Sunny Island inverter that switches between the battery bank and SMA’s Sunny Boy grid-tie inverters with fully integrated controls. SMA’s approach is proprietary, in that the Sunny Island battery inverters only talk to Sunny Boy grid-tie inverters.

The MS-PAE inverters from Magnum Energy offer similar functionality, but can be integrated into systems with inverters from other manufacturers. There are various companies that package this type of inverter with a battery bank and the needed controls to provide islanding, or “AC-coupling” when the grid is down. MidNite Solar is one such packager of retrofit kits.

With any of these options, there is a significant cost for this type of islanding capability. For a typical, residential-scale 6-kilowatt (kW) system, the cost ranges from about $8,000 to $16,000, according to Mark Cerasuolo of OutBack Power Technologies, who did an analysis of AC-coupling options. This cost includes the specialized inverter, battery bank, and necessary controls.

A new, low-cost approach

As I said, we didn’t go with full islanding capability, even though I would have liked to do so — and may in the future. The cost of the battery system and other components was just too much for our budget that has been stretched pretty thin with our complex building project — which is finally nearing completion.

What we did do, however, was install a brand-new inverter from SMA that has an outlet that can continue delivering some electricity when the sun is shining during a power outage. SMA calls this feature “Secure Power Supply.” Mounted beneath our 5-kilowatt (kW) Sunny Boy 5000TL-US inverter is an outlet that can deliver 1,500 watts (12.5 amps at 120 volts) during the daytime the power grid is down. Unlike other islanding systems, there is no requirement for battery storage with this option.

This isn’t enough power to operate all the loads in our house that I’d like to power during a power outage, but it’s far better than nothing. The cost is essentially the same as a standard Sunny Boy inverter (though a separate outlet has to be installed). Ours was installed by Integrated Solar Applications in Brattleboro, which installed the  entire 18-kW grid-connected system (with 6 kW of the system being owned by a neighbor).

Like other models in the SMA TL line, our 5000TL-US is a transformerless inverter, which is smaller and lighter than standard inverters, and it offers even higher efficiency: roughly 97%.

Emergency power uses

While 1,500 watts is a significant amount of available power, this Secure Power Supply feature is not really intended for loads that have significant surges as they cycle on, or loads that could be harmed by fluctuating current, such as refrigerators. It’s really designed for charging cell phones and laptop computers.

But I’ll be carefully examining power consumption and surge demand when we shop for a new chest freezer — it would be very nice to be able to power that freezer during the daytime during extended power outages.

There may be a Sundanzer chest freezer, for example (a freezer made especially for solar systems that can work in DC or AC mode), that will work well with the limited output from our inverter. At the very least, we’ll be able to keep our cell phones and laptops charged and power our cable modem and router.

Still in limited supply

I had heard about the new 3000TL-US, 4000TL-US, and 5000TL-US inverters late last year, and heard that they would be shipping in the first half of 2013, but it turns out that we got one of the very first to be installed in the U.S. — or at least in the Northeast. Demand is very high for these systems.

I suspect that within a few years, most grid-tie inverters will include this emergency-power option. I haven’t had to test it out yet, but will be ready for that ice storm this coming winter!

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.


  1. user-1137730 | | #1

    Based on the spec it should have no trouble with a refrigerator. Given the importance of that feature you should have tested before writing the article. To be blunt.

  2. Alex Wilson | | #2

    Powering refrigerator or freezer
    The issue is about the surge when a refrigerator or freezer cycles on. SMA generally recommends against powering such loads with these new inverters, but I believe that the "soft-start" feature of the SunDanzer freezers (when coupled with a conversion kit to use AC that SunDanzer sells) should be fine. As for testing this before writing the article, the inverter has only been out for a matter of weeks, and we haven't purchased a freezer for the new house yet.

  3. Brent_Eubanks | | #3

    nice trick!

    Can you tell us a bit about how they accomplish this technically? The received wisdom among solar sales/installation folks is that any kind of islanding requires some kind of storage. I don't know if this is necessarily technically true, but I'm quite curious. How does the inverter match fluctuations on the generation side and fluctuations on the load side? Is there a small battery (or supercap) built into the inverter? If so, is there a way to replace it when it eventually wears out? Or have they found some way to avoid the need for storage completely?

  4. Alex Wilson | | #4

    How this inverter works
    The short answer is "I don't know." The PV array is generating electricity whether the grid is up or down. I assume that SMA figured out a way to convert some of that DC electricity into AC and do it safely--preventing backfeeding into the grid. There is no battery involved. I am guessing that the relatively low capacity of the attached outlet in islanding mode (15 amps at 120 volts, or 1.8 kW)--significantly less than the peak output I've achieved with the inverter of about 3.8 kW when the inverter is grid-tied--has to do with circuitry to limit fluctuations. I'll try to find out from SMA America.

  5. Ericwest1 | | #5

    Connection to a sub panel
    Is the outlet always energized or only when isolated from the grid? Can a sub-panel be attached so several circuits could be powered?

    This inverter is a great idea. Thanks and please post when you have had more run time with it.

  6. Expert Member
    Dana Dorsett | | #6

    When you have 5-6kw of PV array...
    ... it' doesn't take a rocket science type of inverter to support 1.5-1.8kw of AC plug load without a battery if the loads are somewhat tolerant of minor frequency & voltage fluctuations from switching other loads in/out. It would take a bit of capacitance on the DC side, but not a whole battery.

    But to make a large fraction of the PV output available in island-mode does indeed take a battery (or a capacitance large enough that the distinction between battery vs. capacitor is somewhat academic.)

  7. kevin_in_denver | | #7

    Hey Prius Owners!
    A solar thermal customer of mine came up with a cheap and brilliant solution for his house:

    "I had my electrician install a straight blade receptacle and an interlock switch on my sub panel. I can connect my Prius to a pure sine wave inverter and I connect the inverter to the receptacle and I can run most of the items in my house (no 220 items or microwave).

    Based on my calculations, with a full tank of gas, I can have power for just over 4 days.

    Ironically, the power in my town is very reliable, but it's nice to know that I have a good plan "B".

    * It's only cheap if you already have a Prius!

    Mark Freeman"

    For a little more info:

    But even this $500 idea doesn't pencil out given the historic high reliability of most grids.

  8. heidner | | #8

    Prius solution
    Kevin, your friend should double check the prius setup. It has two batteries. The high voltage battery used for locomotion, and a lower 12volt car battery that is used for starting the small motor and running the low voltage car electronics. It sounds like he is plugging into a cigarette lighter socket. That means the power is coming from the low voltage battery. Run life is not likely to be four days.

  9. Expert Member
    Dana Dorsett | | #9

    You missed the "With a full tank of gas..."
    ...part. The amount of 115VAC you can get out of a ~50 amp-hour lead-acid battery with a full tank of gas is about the same ~0.5-0.6 kwh whether charged by a Prius or a Land Crusher. The peak loads are served by the battery, average loads by the car's alternator, so the capacity of the alternator can be the critical aspect for how much stuff you can actually run for more than a few minutes at a time.

    The powertrain battery on a PLUG-IN Prius is only 4.4kwh at full depletion (to a battery-damaging level) which is only about 10x capacity of an automotive lead-acid battery. The powertrain battery of the non-plug-in is about 1.3kwh, and only half that can be tapped without damaging it, so it's really not a huge improvement in over all storage capacity from the lead-acid battery.

    What you get out of a Prius battery that you don't get out of the lead-acid battery is 10,000+ charge/discharge cycles to half of full-rated capacity. Tapping directly into the powertrain battery of a Prius is NOT a simple hack, and would surely violate the warranty, especially if you were discharging it to below the mid-point. Tapping the battery-charging side of the powertrain system would probably give you substantially more power than the alternators used in other cars, but is that an easy hack? (Methinks not.)

    For emergency backup, (ab)use the lead-acid, battery (any car) not the nickel-metal hydride battery (or lithium ion battery used in the plug-in Prius.)

  10. user-1033003 | | #10

    Splitting ownership
    Someone else owns 6Kw of your 18Kw system? How does that work? Do they somehow get 2/3 of the power or does your local utility company somehow split the compensation or do you pay them some amount for 1/3 of the power or what?

  11. Alex Wilson | | #11

    Group net metering
    We are lucky in Vermont that state law requires utility companies to provide what is referred to as "group net metering," and our largest utility company in the state, Green Mountain Power, has been very supportive of this practice, even before it was mandated. There is only one meter for the electricity feeding into the grid from our system, but one-third of that electricity is apportioned to our neighbor. He will see that solar electricity credited on this electric bill.

    Of the 12 kW that my wife and I own, we will work out contracts to provide a portion of that to one or two other Green Mountain Power customers, since we won't require that much electricity--at least initially. When the farm is fully up and running we may be needing most of that power, depending on coolers, freezers, and other electrical loads.

    You can learn more about GMP's commitment to solar here:

  12. JonathanTE | | #12

    Output relative to array size?
    Alex, you write that your system with this inverter can put out 1.5 kW through the backup outlet. Is this dependent on having a large array such as yours? The typical residential PV array is 1/3 the size of the one on your barn, or even smaller. With smaller PV arrays using SMA's inverter, will the available island power be proportionately reduced?

    On an anthropological note, it's interesting to see you point toward telephone and internet as such important items to maintain, perhaps even above a freezer full of food. I don't disagree with that (I don't know which I would want to maintain during a long blackout), just find it interesting as a change in culture.

  13. Alex Wilson | | #13

    Output of 5000TL inverter
    There are three inverters serving the 18 kW array. Two are standard 6 kW Sunny Boy inverters and each of those has 26 250W modules connected to it (two strings of 13 modules). The third inverter is the specialized unit that offers some power when the grid is down; that one is a 5 kW inverter and it has 20 modules feeding into it (two strings of 10 modules each). SMA doesn't currently make a 6 kW TL inverter. SMA technicians advised our installers, Integrated Solar Applications, on configuring the module strings as they did. (Our total system output won't be affected even though the combined rating of the inverters (17 kW) is less than the rated peak output of the modules (18 kW), since the modules will never provide their peak output, and inverters can actually handle more than their rated output.)

    So, the power we have access to during an outage is only coming from the one 5 kW inverter that has modules with a peak rating of 5,000 watts connected to it. Similar emergency power would be available from a more typically sized array, and we could have had more emergency power available had we had installed three of the TL inverters. (The decision to try one of these new inverters was made late in the process.)

  14. user-1044645 | | #14

    Combine with a UPS?
    Thanks for highlighting this. I work for a solar installer serving Maine and New Hampshire and this has been a dealbreaker on a handful of projects (populous parts of the state have reliable grid power but in the country it's hit or miss). It seems counter-intuitive to some people that they would have all of that power generation capacity on their roof and not be able to use it without the grid... Our typical answer had been - get a generator - or drop $10-12k on a battery backup system with limited utility and shelf life. This TL series set-up is a really nice compromise, for essentially no extra cost to the PV installation.

    I have not tried this, but I've suggested to a few people and it seems logical that you could add some reliability to this system by adding a 120V charging battery like an APC Back UPS in front of the solar output. That way, you have some runtime at night and during clouds, and, assuming the KW output of the APC can handle it, you can have clean sinewave power for your fridge, too.

    While this is still basically a lead-acid battery solution, the convenience of a packaged 120V device with no electrical-side interconnections that a homeowner can just purchase and plug in to their wall, beats us as the solar installer having to design and maintain an elaborate system.

    In Maine at least, a lot of people are on pellet stoves and knowing you can still get heat from biofuel when the power is out, is awesome. Just make sure to still fill your tub so you have some water.

    - Fred
    ReVision Energy - Portland & Liberty, ME

  15. b2j2 | | #15

    Prius as a backup
    References: the site PriusChat and some back and forth on Amazon regarding the AIMS 1000 Pure Sine Wave Power Inverter, which is what I have used for several years though outages of up to three days. Such outages tend to occur in snow storms, when need is highest for various components of our heating system.

    The Prius (ours is 2007 ("second generation") - the current models are similar) does have a 12v lead-acid battery kept charged through a dc-dc converter from the 202v traction battery. The inverter is connected to the 12v at the easily accessible 12v battery, so it is actually fed in a way bypassing the lead-acid. A direct traction battery connection is somewhat more challenging, and several systems are available from in New Hampshire. It would be nice to have power for our well - probably a heavier 120v inverter plus an auto transformer would work.

    - Byron

  16. Expert Member
    Dana Dorsett | | #16

    Thanks Byron!
    So, you also get some of the traction battery storage without hacking the higher voltage side ( which still isn't much, but something.) Combined it looks like you can get about one kwh (derated for DC-DC conversion losses) or a hair more out of it without stressing the batteries, maybe 2.5 kwh out of a plug-in Prius.

    Hopefully the traction battery controls don't allow the lead-acid charger to draw the traction battery down to a damaging level (which is about half-charge for the NiMh battery, not sure where it is for the Li plug-in.)

  17. Expert Member
    Dana Dorsett | | #17

    Group net metering (response to Alex)
    ... is the first step to neighborhood islanding!

    Now that grid-tied PV has dropped below residential retail grid pricing on a lifecycle levelized-cost basis in many areas, the future business model of the utilities will HAVE to change. Those utilities who aren't already planning for it are bound to become road-kill on the distributed generation highway. It's coming faster than most analysts thought even as recently as 3 years ago, but it's an unstoppable train at this point, and a new favorite hot topic amongst the wonks at GreetechMedia. Witness item 1 on today's weekly podcast:

    In 15 years I expect the current controversy surrounding re-licensing the legacy nuke in VT will seem pretty quaint in the midst of the hubbub over net metering, micro-grid islanding, and access to remote green power.

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