Power From Solar During Grid Power Failure

[woltnj]

I am getting a new 8.8kW Solar installation done over the next couple months. There is one problem I am trying to solve. When the system is installed with SMA inverters, it will have a DC and AC cutoff so when the grid power goes the solar will be shut down. What I am trying to figure out is how to keep the solar generating power for my house while cutoff from the grid during the power failure. Is the cutoff done purely as a safety measure for the power companies or is it something technical with the way the inverters are designed that they need the grid power to function.

I was thinking about getting a 220v UPS to plug into the house AC to provide power to the inverter while the grid power is disconnected thus keeping the solar generating power to the house. I know there are expense battery solutions that people use but I do not want to store enough power to run the house, I just want to generate power during the day if I lose power from the grid.

Thanks

[indalecio.feng]

This is a special function for the inverter, in case someone does some work on the powerline which connect to your home and will not hurt by your SHS AC output to the powerline.

[woltnj]

I understand why the power is cut off, I want to keep the power cut off from the grid but want to have the solar panels still power my house.

[JustinSolarGuy]

You are talking about trying to Trick the inverter into thinking the grid is still on. I don't think this is a good idea. If your UPS is a modified sine wave type then the SMA will probably have problems recognizing this power; it may damage the inverter. You will need to talk with an Electrical Engineer. I am no Engineer but I know this basic truth. Its always a bad idea to use the wrong tool for the job.

The Grid tied inverter you are purchasing is specifically designed to not do what you want it to do. If you really want power from you solar panels during an outage you need to buy a battery based grid tied inverter. This will cost you about 10% in system efficiency and you will need to buy batteries, a battery box, and a charge controller plus you will need to separate you critical circuits onto a smaller sub panel for energizing when the grid goes down. You will pay a lot of money for the luxury of power during the odd outage.

Another option if you want to mess around with a grid tied system that you already purchased is to put in an A/B switch and steal your DC solar power before it gets to your Grid tied inverter. You can have a qualified Electrician tap into the feed from the roof. Now you have to figure out what to do with 200-500VDC. Look for a battery based inverter or charge controller than can handle this voltage not sure if they exist. You can run DC water pumps or other motors. Possibly certain light bulbs. That's about it.

Or you could put in a few A/B switches in the array wiring and steal your power from the individual modules or pairs of modules before they get strung together in the array. Now you have the option to just steal two, four, or six panels and you can definitely find a charge controller and inverter that will run on the voltage of two panels in series. Of course these systems are manual and you would only implement this if you already had a grid tied system and just couldnt stand the thought of wasting all of your solar energy during an outage. Also, you will need to somehow get this power to the appliances, the safest way would probably be to roll out some extension cords or to have a licensed electrician separate some circuits in the appropriate manner.


If you are worried about the end of civilization as we know it, and you will want to take advantage of your solar energy at that time then you only need to buy some batteries, a float charger, a cheap non grid tied inverter, some extension cords and a charge controller. Throw the batteries on a float charge somewhere safe and leave the rest in a box in your garage. When the the big whatever it is hits you can rewire your panels to charge the batteries and run whatever you want off of the inverter. But this would not be practical for the odd utility outage.

But honestly the importance of instant automated backup power for your home is greatly overestimated by the average consumer in most cities. Just buy a UPS for your computer and get a Generator to run your fridge. Plus you can take the generator camping or bring it along on the exodus to the promise land after the big whatever it is hits. You aren't going to bring your 800 square foot array along... that's for sure.

[woltnj]

Thank you for a very thorough explanation.

[slade1977]

There is a relatively simple way (and safe and legal) way to accomplish this.

Go get a manual bus transfer switch. It's the same kind of switch board used for generators that are attached to houses on the grid. You can get an automated one, but I've had no problems with my manual one on my generator.

Since I don't know exactly how your system is wired, I'll try to be vague enough to give you ideas where to put it.

First, you'd need to install an A/B switch upstream of the gridtie inverter. Then you'd need another, non-gridtie inverter. That would then need to be tied into the manual transfer switch. When the lights go out, you would have to switch the A/B switch, and then the manual transfer breakers.

The way a manual transfer switch works for home systems is this. It basically replaces the breakers in your circuit panel. You wire the actual home circuit to the transfer switch. The transfer switch has two power inputs, one from the gridtie system and one from the non grid tie system. It is completely safe and legal. What happens when you switch the breaker from gridtie to nongrid tie, it completely isolates the circuit from the grid and there is no way power will flow out of your house to the grid and possibly killing a lineman somewhere when your 110VAC becomes 440VAC going upstream through the transformer.

When the switches are in the grid tie position, power is coming from the applicable circuit breaker in the main panel into the transfer switch, and then powering the home circuit.

When you throw the switch into the non-grid position, power is drawn from the non-grid source and then into the home circuit.

And then, for added insurance, kill your main breaker until you see your neighbor's lights go on. I only do this because my best friends dad is an electrician and I wouldn't trust his life on a $200 bus transfer switch.

I know it may be hard to understand how I wrote it out, but if you look at a wiring diagram for a generator transfer switch, it'll make it perfectly clear (as mud). Basically, the bus transfer box acts as a bunch of A/B switches. You'd need to identify 'vital' circuits you'd want to keep running when off the grid. They make some really large boxes that can power the entire house, but you'd likely be better served to identify only those circuits you truly need.

[JustinSolarGuy]

That is nearly identical to my first solution except that you forgot to address the voltage issue. Most off grid inverters are 24 or 48 volt input. A grid tied SMA will probably be running 250-350 volts. Good luck finding an off grid inverter that will take that kind of voltage. If someone on here knows of a high input voltage off grid inverter please let me know. I would recommend this option to my customers as something they could add on later if desired. You may be able to find some way to step that voltage down using a Charge controller or DC to DC converter. But you will probably have to get creative and usually those are also designed for 24 to 48 volts.

[Mike90250]

There are Industrial DC-DC converters that will convert 400VDC to 48VDC. You can charge batteries from that, to keep a fridge going off a small inverter.

Expect to pay about $300 for a 200W converter

[JustinSolarGuy]

That's good to know. That's a lot of dough for a few hundred watts. You won't be able to run your whole array on one of those. You would need to run a bunch in parallel or use a small inverter. This just illustrates how things sound easy until you actually try to implement them. If you don't have a lot of experience building things and putting projects together you will seriously underestimate what is really involved getting something put together that seems so simple.

[Mike90250]

I got a 5 pack on ebay a couple of years ago, 200W each, $50 lot. Still have to make
a heatsink plate for them, and wire it up, but I "could" downconvert my array voltage
to something useable.

[slade1977]

I sometimes forget that the electrical systems I work with are usually at an industrial capacity, so I regularly see inverters capable of going from 400VDC to 220VAC aren't hard to come by, but do run around $1000.

[Arthur]

TerraWatt Power ecoJoule 2500 inverter

Has anyone heard anything about the TerraWatt Power ecoJoule 2500 inverter? It supposedly solves the problem that was being discussed on this thread. In a power outage, it prevents solar power from going into the grid but does NOT prevent solar power from going to your house.

Arthur

[greenHouse]

I'd have serious doubts about how well that product can work in real-world scenarios. Cloud passage will drop all DC input and cause an AC power outage. A few seconds later, AC power returns. Then another cloud and another AC power outage.

If you want to ride through an AC power outage, battery backed is the only way to go. If not, pure grid-tie. But batteryless AC seems like it would be far more trouble than it is worth.

[netttech]

I just read about an Grid-tie/battery backup set-up. It didn't list the inverter by manufactor, but apparently they exist.

Having an automatic switch to cut-off the Grid would be necessary though. I know locals that have those switches installed when they use their gas generators, during an extended outage. It may even being required here.

Good luck
Jeff

[greenHouse]

I just read about an Grid-tie/battery backup set-up. It didn't list the inverter by manufactor, but apparently they exist.

Having an automatic switch to cut-off the Grid would be necessary though. I know locals that have those switches installed when they use their gas generators, during an extended outage. It may even being required here.

Good luck
Jeff

Well, yeah, once you add batteries to the equation all sorts of things are possible. I have a pair of OutBack GVFX3648's and they run without the grid just fine. Them and 25KWh of batteries