Backup Power Without Batteries: Page 3 of 3

Intermediate

Inside this Article

Jeff and Kathy Ball
No batteries? No way! SMA America’s new inverter can provide some backup electricity without batteries when the sun is shining, but the grid is down.
SMA America’s new batteryless grid-tied inverter
SMA America’s new batteryless grid-tied inverter allows users to access up to 1.5 kW from their PV array—even when the grid is down.
SMA’s Sunny Webbox performance meter
SMA’s Sunny Webbox performance meter displays when the system is in stand-alone operation. Here, the system is supplying 42 W to the SPS load.
SPS powers a router and a laptop
Here, the SPS powers a router and a laptop, which displays the PV system’s cumulative data.
shading on one string of the array will not compromise the performance of the other string.
Thanks to the inverter’s two separate MPPT circuits, shading on one string of the array will not compromise the performance of the other string.
Jeff and Kathy Ball
SMA America’s new batteryless grid-tied inverter
SMA’s Sunny Webbox performance meter
SPS powers a router and a laptop
shading on one string of the array will not compromise the performance of the other string.

Once grid power is restored, the SPS switch must be manually turned off for the inverter to restart its grid-tied operation. Otherwise, the SPS circuit will keep the inverter in “stand-alone op” mode, preventing grid-tied operation, and therefore preventing the rest of the array’s solar electricity from being used in the home or any of it from being fed to the grid.

Is the SPS Right for You?

Even though the SPS technology has limited applications in comparison to a battery-based grid-tied system, the ease and low cost of adding it to a grid-tied design, including the cost of the SMA inverter and the associated wiring, makes it hard to imagine why anyone wouldn’t want the added benefit of accessing otherwise unused solar power during a grid outage.

Deciding whether or not the SPS suits your needs as a backup power source is another question. Because the Balls don’t have critical loads that need to run during a grid outage, or frequent power losses associated with their utility service, the SPS circuit is adequate for their needs. On the other hand, the SPS circuit may be too limited for those who want a backup power system that can run at any hour of the day and is not limited by the solar irradiance available at the time of the grid outage. For those who already have a grid-tied PV system, but have always lamented over not being able to use their solar energy during a grid outage, it may be possible to upgrade their system by replacing their current inverter with the new TL model. The feasibility of this upgrade will depend on the system capacity, array configuration, and, of course, cost considerations.

Whether it’s the 3, 3.8, 4, or 5 kW model, each TL inverter can redirect 1,500 W of solar power. If a system owner wants more backup capacity, they may want to consider dividing their array into subarrays and using separate TL inverters. The Balls’ 5 kW array is wired into two strings of 10 PV modules. Instead of installing one 5 kW TL inverter, with each string feeding a separate MPP input, each series string of 10 modules could have been tied into a separate 3 kW TL inverter, giving them two 1.5 kW SPS circuits. Of course, the cost of installing two 3 kW inverters would be more than the cost of one 5 kW inverter, but, given the right circumstances, it is a design option worth considering.

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Orion Thornton is the owner of Onsite Energy, a Montana-based solar-electric design and installation company. He is a NABCEP-certified PV installation professional and a contracted instructor for Solar Energy International (SEI). Orion lives in a net-zero energy home powered by solar electricity, solar thermal, and wood heat.

Comments (42)

Judy Merry's picture

Thanks for your thoughtful replies. I will be investigating what wiring and equipment an 'ac backup system' will require...starting with the Homepower archive of course!

Judy Merry's picture

My new grid tied micro-inverter solar panels (5kw) will be installed soon. I want to be able to use power from them when grid is down, or when I want some. What wiring can I have done now to be ready to use this new type inverter when it becomes available here in Orange County southern Cal.?

SonicSolar's picture

With any grid tied PV system , micro inverter or string inverter, you can use a AC coupled backup system to allow you to operate when the grid is down. Their is always the downside to a small battery bank, but if you have micro inverters or AC panels installed it may be your only option. It would be a serious undertaking to rewire your whole roof to convert your system to SMA's new string inverter product. I love SMA's new product and will be offering it to all my new customers and existing string inverter customers once it comes out.

Greg Smith_4's picture

Hi Judy, I would agree with the other posters that a retrofit, while electrically possible, would cost a lot of money and time. I misunderstood your original post and edited this response. However, if the installation hasn't started, and if you really want to use this technology, then it might be worth a call to your installer.

1500W might not sound like much but considering you can get a little bit of power MUCH cheaper than installing a battery system, anyone who has ever suffered through an outage will immediately see the benefit of this feature. Those who trash it just don't get it.

Keith Hertz's picture

Before making the trade-off between this String Inverter and your current Micro Inverter setup I'd ask your installer for the benefits of each. Micro Inverters offer some very clear cut advantages over string inverters so to switch to this type of technology makes sense only in select situations… like you lose power all the time. You'll end up sacrificing system efficinecy for the convenience of having 1500W when the grid goes down.

Greg Smith_4's picture

I must disagree with your definition of 'system efficiency", Keith. Because a micro inverter architecture uses “n” times more inverters than a string inverter system, then that imposes a reliability requirement on the inverter to be “n” times MORE reliable than a string inverter in order to offer the same system level reliability.

Assuming both inverters have the same failure rate of 0.22% (documented SMA Sunny Boy reliability), then that means the micro inverter system reliability would have to be .22/n where n= the number of micro inverters.

Have twenty micros on your roof? Then each of those micros better have a reliability factor of .011% to beat SMA.

Keith Hertz's picture

Oooo… I don't know if I'd go down that route. I'm not married to either technology but I can say we've had a lot more failures with string inverter systems over micro inverter systems. Westinghouse did a 3rd part independent study for 2 years on this very subject. They used 10,630 micro inverters and 3,373 string inverters. The failure rate for the micros was at .21% where the string inverter was at 9.43%. So… even throwing your nth degree information in the mix makes for a lousy case. Field-proven the number comes out to micro inverter being 45x more reliable. NOT 45 percent… 45 times.

Add to the mix that when a micro goes out it only effects one panel and the rest of your system is still operational. Compare this to the string inverter that wipes out the entire array and you have a very one sided argument.

We use both technologies but when I designed my home system a few years ago, I went with the micro inverter and a 25 year warranty right out of the box. For those who have zero shading issues, perfectly oriented roofs and don't want to invest the extra upfront costs, then the choice may be string inverters.

My 2 Cents. Not trying to argue… just inform.

Greg Smith_4's picture

Ah, the infamous Westinghouse study. We are very familiar with that piece. The industry would hardly call a 2-year study by one company a hearty, field-proven case study. But adjusting that report for reality indeed informs. I have heard your comment about experiencing string inverter failures more than micros; perhaps offline you can tell me which inverters you were using?

I would be happy to take this offline if you want to break the numbers down from the Westinghouse/Akeena study. But on the surface, comparing ~2.1MW of micro inverters over an 18 month period to ~16.9MW of string inverters over an ~8 YEAR period hardly seems fair. Add in the fact that Akeena had exposure to the 25,000 Xantrex inverter recall during that time and it becomes clear where the high number of string inverters came from. But those charts sure did make the study half-way convincing.

I guess it depends on whose Kool-Aid pitcher we drink out of. Is that even a relateable analogy anymore? Maybe we should go with Red Bull...

Michael Welch's picture

Hi Judy. It is either/or. If you want this new type of inverter, you will have to redo your project to include it instead of microinveters. You should contact your installer immediately.

In4mation's picture

Out of curiousity, I have a 200A manual transfer switch between the meter and my primary panel. Is it possible/likely the inverter would turn on when generator power is supplying power to the house?

Greg Smith_4's picture

Michael gives some very sound advice. If the generator has an AC waveform good enough to fool a PV inverter then the inverter could connect to this little micro grid, not knowing the difference between that waveform and the utility. If the loads cannot handle all the PV then it will backfeed into the "grid". I would contact the generator manufacturer and see what their feelings are about back-feeding into their generator.

My experience with the genny manufacturers is that they will give you a response similar to, "We do not recommend connecting a solar inverter to the load side of our generator." Not very helpful, but it boils down to risk/liability.

SonicSolar's picture

The inverter pushes power by increasing the voltage. The inverters are allowed by UL1741 to increase the voltage to about 263 before they must shut off. The generator produces power and tries to keep the voltage at 240V. A circuit would be needed to disconnect the solar in the case where it produces more the a max set voltage. Limiting the solar voltage based on the generator set voltage would not hurt the generator. The generator would need to have a super smooth output in frequency and voltage or the inverter would not start up. A inverter based generator might do the trick. I have a customer that may be interested in testing a system like this.

Michael Welch's picture

Very unlikely that the generator would stay within spec long enough to allow an inverter to start. But really, the generator transfer switch should be wired to feed a subpanel instead of between the meter and the mains.

In4mation's picture

@ Michael, Good catch!!..and further proves I cannot multi-task! Let me correct my initial statement. I have a 200A transfer switch between the main 200A distribution panel and a full size 200A sub panel. (Don't ask what I was thinking earlier). The main panel houses large, non-essential 220 VAC loads. (HVAC, Oven, etc..). The subpanel includes all 120VAC lighting, outlets, well pump, furnace, etc.. Regarding the generator, it is an industrial (Kubota) diesel generator. (Not a $500.00 Home Depot special) Would that make a difference? I don't know.

Michael Welch's picture

Then your inverter should feed into the mains panel and not the subpanel, in which case it will be isolated from the generator.

I have heard of broken generators resulting from feeding a grid-tie inverter into them. Don't do it.

myself248's picture

This seems to confer none of the advantages you'd expect from having it integrated into the inverter. It's manual transfer, it's low-wattage, and it's only a single outlet? I could do exactly that with an external inverter clipped across the array terminals. Already do, with a low-voltage array.

By integrating it into the house's inverter, I'd also hope to gain some transfer-switch-like functionality, and the ability to run house loads during an outage without extension cords and without widowmaker cords. I'd also want automatic transfer, so if an outage happens while I'm not at home, at least the fridge has the opportunity to do some chilling while the sun's shining. (I'd add a datalogger to assess safety when I return.)

But what really baffles me is that, if you've got a 5kW array, and the inverter's obviously capable of handling that power, why limit SPS to 1500 watts and a single outlet? This is a step in the right direction, to be sure, but it's a very small step indeed.

Jim and Elaine Stack's picture

Using your plugin car as a battery system with V2G Vehicle to GRID would provide much more power. 1500 watts is not much. A LEAF with 24 Kw battery pack would be a lot better and even work over night. The method in this article would on;ly cover when the Sun was out during the day. Read www.V2G-101.webs.com for more details on V2G

Michael Welch's picture

Definitely part of the future. But not a viable option now or any time soon.

Keith Hertz's picture

I like where this is going but 1,500W isn't much and is certainly not enough for those who are considering a battery back-up system. This has a wall plug on the inverter itself so… in the event of a power outage are they suggesting simply running an extension cord to the inverter? I can't imaging creating a dedicated circuit for 1500W.

Greg Smith_4's picture

Hi Keith, so far our biggest challenge is to get people to understand that this is opportunity power and not a full scale power-a-well-pump system. We are already hearing success stories from the east coast after the winter storms where people had phones, small heaters, some refrigerators and laptops successfully powered up through the daytime using this SPS. The dedicated outlet is am ust so that UL would approve it. We don't backfeed this power to the grid.

The location of the outlet can be anywhere. One family ran it all the way into their kitchen. It just takes a little imagination.

Brian Letourneau's picture

You people are just figuring out how to do this????? I thought this magazine was on the cutting edge.

SonicSolar's picture

A transfer switch would be one part of the needed equipment to operate a on/off grid AC coupled system. Once the transfer switch has isolated the solar inverter and loads from the grid, another voltage source would be needed to wake up the solar inverter. You would also need a circuit to disable the solar inverter if it starts to supply more power then your home requires.

Brian Letourneau's picture

I can see people on here need to do more research/learning. I wish everyone the best of luck!

Greg Smith_4's picture

Hi Brian, I am not aware of any other PV inverter that is able to operate independently of the grid and without the need for a battery inverter micro grid. This is a cutting edge feature that provides a cost effective solution for opportunity power from a solar inverter.

Do you know of another inverter that can already do this?

Brian Letourneau's picture

Put in a transfer switch like you would use for a generator with a 5kw inverter. You could go as far to put in a sub power panel after the inverter for the circuits you want to power. Not rocket science. People just have to think out side the box of what the industry wants you to think and buy. When you get a way from the narrow mind set of the PV "big box thinking" the options are endless.

Michael Welch's picture

Hi Brian. With any other batteryless grid tied inverter, that won't do the trick. But if you ADD an inverter that takes advantage of battery storage, it will. We do recommend thinking outside of the box, but when it comes to equipment that is available, this is the first time a batteryless grid-tied inverter has been available that will supply AC from a PV array to a home.

myself248's picture

But a grid-tie inverter won't start up without seeing a nice stable 60Hz from the utility, which it then syncs up with. They're specifically designed not to cold-start, so as not to create energized "islands" that would surprise line workers.

So, the inverter and transfer switch would need to know about each other, which is the missing concept in this product and every other.

Brian Letourneau's picture

A transfer switch goes from line power to a back up power supply like a generator. So the transfer switch does not put power out on the line.

Deane Mitchell_2's picture

Sounds like a possible "game changer". We have a PVR grid connected array in little old Adelaide in Australia. Pays our bills, as we get a great feed-in tariff.......but when we have a power failure (often!!!)....those PVR's are just dumping their output. Brilliant idea. Wonder if they'll look to International Markets - with 220 - 250V AC outputs? I'd certainly "be in the queue" for one!!!! Cheers from "Downunder"

Greg Smith_4's picture

Hi Deane, it has been a long time since I was in Australia. In my previous life as a US Navy submariner we finally grabbed that dangling carrot in front of us and made it to Perth, Brisbane and a short (but sweet) 3-day stay in Hobart, Tasmania. So much fun! It wasn't so fun playing around with your (much quieter) diesel subs, but the mooring after-parties were awesome!

I don't know why it took the industry so long to come up with this, but I'm glad SMA paved the way for this type of energy independence. So far, there are only 2 countries that have this SPS feature- the US and Japan. The US version is country configurable, meaning we can change the frequency output to 50Hz, however we can only provide an output to L1 and L2. This made the Jamaican's very happy since they have a very uncommon grid type of 220V/50Hz!

Japan is a 50Hz model but the voltage is at ~202VAC (single phase), however I'm sure that voltage window could be widened using our free software tool and a direct connection to the inverter using a laptop. Adrian Ho is the SMA AU Solar Academy trainer, however I'm sure their Service Line could help as well. We trainers are a busy bunch!

Here is the SMA Australia contact page link: http://www.sma-australia.com.au/en_...

Deane Mitchell_2's picture

Thanks Greg.....most appreciated detail.....maybe you'll make it back here to our wonderful Country again.....we're a "friendly lot"!!! Thanks for the detail on the Inverter....I shall try contacting SMA Australia & see what gives :->)) Cheers

Deane Mitchell_2's picture

Thanks Greg.....most appreciated detail.....maybe you'll make it back here to our wonderful Country again.....we're a "friendly lot"!!! Thanks for the detail on the Inverter....I shall try contacting SMA Australia & see what gives :->)) Cheers

Robert Bridges's picture

This is very good news. I have been considering a grid tied system but wanted battery backup incase of power outage due mainly to a southern California earthquake. So all I am looking for is basic electric to power a light bulb and my chest style freezer that has been converted into a beer fridge. We would use it to store perishables in an emergency (and my home brew of course).

Wimberley Wind and Solar's picture

Hey Robert and Michael

We have installed a few of these systems both in Southern Cali and in Texas. Mike is totally right about the possibility of start up amps knocking out the Daytime Emergency Backup.. We devised a solution using the TL Line and it works just fine. We implement a UPS battery pack of a certain kind and set it all up on a telecommunications rack.. We also have run multiple UPS units and generally go commercial or telecom style. This also allows you to have night-time power.

Michael Welch's picture

Hi Robert. It is possible that this system would not be able to start your chest freezer. Many motors can surge over 1,500 W upon startup.

Greg Smith_4's picture

Great point, Michael. Surge is an important part of backup calculations, however we have already seen instances in the field from customers who are able to plug in large refrigerators- all of which were Energy Star Compliant, which probably has something to do with the ability of the SPS to run.

There are also some things that the homeowners will learn, lesson that off-gridders have known for decades, like starting the largest load first and then plug in all the other stuff. If the SPS trips off then unplug something to get under the 1500W capacity. the SPS will restart on its own 20 seconds later and reconnect if the loads are <1500W.

Greg Smith
Senior Technical Trainer, SMA America

Michael Welch's picture

Greg, are you saying the surge capacity is beyond 1.5 kW? That would be great.

Greg Smith_4's picture

No, not at all and I apologize if it came across that way. The message I was trying to convey is that you get 1500W and if there is a surge above that then the SPS turns off and then reattempts to connect 20 seconds later.

If you have a device that does not surge above 1500W then you can power that load. Installers have relayed this information to me about these ESC fridges not tripping the SPS when the compressor kicks on, which leads me to believe that there are some appliances out there with surges below the 1500W of the SPS.

Again, this SPS provides opportunity power at no additional cost to a PV inverter.

Michael Welch's picture

Thanks for straightening that out.

Greg Smith_4's picture

Hey Michael, this is one of the few times that I am actually happy to eat my own words. I pressed our German colleagues on the surge capability and it appears that they have been holding out on us!

Looking at the surge curve, it looks like this little guy can withstand a 19A surge for 500ms and 23.5A for 250ms.

Michael Welch's picture

Thanks, that is interesting and 2,280 & 2,820 W surge, respectively. I wonder if a fridge startup surge stays within a half second.

Greg Smith_4's picture

The 6-year old GE Profile Energy Star Compliant fridge in the Solar Academy will start, but my 8 year old Frigidaire 16.5 cu.ft. will not. It really depends on the manufacturer. I saw some new LG refrigerators at Lowe's a few weeks ago that had some new kind of compressor start-up method. Sounded interesting, but the thing was $3300. I wonder if they would let me use our SMA van with the TL22 for testing. Someone from the east coast gave me a link to a "CyberPower OR2200LCDRT2U 2000VA 1320W Smart App LCD UPS" that actually allowed his fridge to start.

Pretty cool what people are doing.

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