PV Grid Tied Power Outage

Vic99

In the next few months I'm going to pull the trigger on a 5.6 kW grid tied, roof mounted PV system. Each panel will have MPPT and will all be tied to a central inverter. At the same time, the company will upgrade my electrical from 100 Amp to 200.

They tell me when the grid goes down, my solar goes down. Short of not being grid tied, what can I do to keep my system productive during a power outage?

ggunn

Re: PV Grid Tied Power Outage

Vic99 wrote: »

In the next few months I'm going to pull the trigger on a 5.6 kW grid tied, roof mounted PV system. Each panel will have MPPT and will all be tied to a central inverter. At the same time, the company will upgrade my electrical from 100 Amp to 200.

They tell me when the grid goes down, my solar goes down. Short of not being grid tied, what can I do to keep my system productive during a power outage?

The bad news is that you cannot have a PV system that powers AC loads which does not incorporate batteries and another type of inverter (either additional or alternative) in your system. The worse news is that doing so can double the cost of the system, or more, depending on which and how many loads you want to supply with power when the grid goes down. A whole house backup is for most folks prohibitively expensive.

The reasons for this are somewhat involved, but it is not just because it's illegal to interconnect with the grid with a GT inverter that does not shut down when grid power goes away, although it indeed is.

If you peruse this forum and others that are about solar, you will (not surprisingly) find this question asked and answered many, many times. It is disingenuous on the part of any solar installer who does not make this clear to a prospective customer very early in the sales process.

Vic99

Re: PV Grid Tied Power Outage

What about running off of a gas powered generator when grid goes down? Does this prevent me from running my array off of a generator, or running some of my circuits off of a generator or car adapter?

I currently have neither, but figured it might make sense to do everything possible at once.

ggunn

Re: PV Grid Tied Power Outage

Vic99 wrote: »

What about running off of a gas powered generator when grid goes down? Does this prevent me from running my array off of a generator, or running some of my circuits off of a generator or car adapter?

I currently have neither, but figured it might make sense to do everything possible at once.

The problem with that is if you could get it to work (which isn't assured because the output waveform from many generators is too dirty for a GT inverter to synch to), if the current demand from the loads you are running ever falls below the output of your inverter, then you will backfeed the generator, which has a high probability of doing major damage to it. If you have a generator, set up your transfer switch such that the PV inverter is disconnected (i.e., shut down) from the loads when the generator is running.

And forget about a tiny battery and inverter just to provide a synch pulse for the GT inverter to read; that will not work, either. In an off grid system (or battery backup when the grid is down), you must power the loads from batteries; the only function of the PV is to recharge the batteries. Furthermore, depending on the magnitude of the lumped load you want to supply, the PV system has to be large enough to replace (during the time when the sun is shining) all the energy you are pulling from the batteries, else you will fall behind and run your batteries dry.

These scenarios have been discussed at great length and the answer is always the same. To run PV off grid, you must have batteries and a battery inverter and your batteries must be large enough to supply your loads. You can be AC coupled (such as with a Sunny Island/Sunny Boy system), or DC coupled (Xantex or Outback), but you have to pony up; there's no free lunch.

Too many installers do not make this clear because they are afraid of losing the sale. They depend on the customer to think to ask the question, and many don't until after they have bought a system. It gives the industry a black eye when an irate system owner calls an inverter company's tech support demanding to know why they don't have power during an outage; I was in a SMA workshop yesterday and they told me that it happens all the time.

BB.

Re: PV Grid Tied Power Outage

As ggunn says--"Simple" GT Inverter+Solar panels is "off" during a power outage and there is no "cheap" way of getting back online (even with a backup AC generator--at least reliably/safely).

There are "hybrid" inverters that can do both Grid Tied and Off Grid... This inverters still need a full sized battery bank (think the size/weight of 24-36 [fixed] car batteries for a 6kW inverter/solar array or ~600 AH @ 48 volts). And these batteries typically need to be replaced every 5-10 years.

The hybrid inverter has an internal AC transfer switch that cuts your "protected" loads (sub panel) from the grid and then supplies power to your loads. And they support the connection to the AC2 input for a backup generator too...

The hybrid inverters are not that much more expensive, but the batteries and interconnections do.

If you have reliable grid power--then a backup generator is usually the most "cost effective" solution.

If you have unreliable power, or power that is down for 2+ weeks at a time, then, possible a true hybrid solar system may make sense for you (or if you are disaster planning).

-Bill

ggunn

Re: PV Grid Tied Power Outage

BB. wrote: »

As ggunn says--"Simple" GT Inverter+Solar panels is "off" during a power outage and there is no "cheap" way of getting back online (even with a backup AC generator--at least reliably/safely).

There are "hybrid" inverters that can do both Grid Tied and Off Grid... This inverters still need a full sized battery bank (think the size/weight of 24-36 [fixed] car batteries for a 6kW inverter/solar array or ~600 AH @ 48 volts). And these batteries typically need to be replaced every 5-10 years.

I know you know this, but FYI to the OP, you cannot use car batteries. Car batteries are designed to deliver high current for short periods of time, while batteries for off grid or grid backup operation are designed to deliver (relatively) low current for long durations. It calls for a different design and using either type of battery for the other application will destroy it in short order. As you might anticipate, the type of battery needed for off grid/grid backup is more expensive.

BB.

Re: PV Grid Tied Power Outage

Yea--just trying to give an idea on size...

If you seen electric forklifts in a warehouse type store--A couple of those batteries to support a ~6kW system.

-Bill

solar_dave

Re: PV Grid Tied Power Outage

These guys are right on, this is why a nice fuel efficient inverter type generator for the occasional outage on you critical loads is a better deal when your grid is relatively reliable.

mike95490

Re: PV Grid Tied Power Outage

well, there is the high voltage charge controller from Xantrex
http://www.solar-electric.com/xaxwmp80amp6.html

That will charge batteries off your HV PV array, and then a 48V battery bank can drive critical loads via a small 48V inverter

ggunn

Re: PV Grid Tied Power Outage

mike90045 wrote: »

well, there is the high voltage charge controller from Xantrex
http://www.solar-electric.com/xaxwmp80amp6.html

That will charge batteries off your HV PV array, and then a 48V battery bank can drive critical loads via a small 48V inverter

That might work, but he would still need another inverter and enough batteries to power his loads, and a switch to take his PV off his GT inverter and put it onto the charge controller (you cannot connect PV both to a CC and a GT inverter; that would interfere with the MPPT function of both the CC and the GT inverter), and an AC battery charger to keep his batteries topped off. It would also mean he would have to learn a whole lot more about how this stuff works to design that system.

It would be better, IMO, just to add a Sunny Island and batteries if his GT inverter is SMA. Either way, it wouldn't be cheap.

Vic99

Re: PV Grid Tied Power Outage

Thanks for all the replies. If it is not cheap, I won't do it. The system and labor are going to be expensive enough as it is. Thought it would be easiest and least expensive to do it while the system is going in.

The company was not evasive. I've had two different estimates and both told me I'd have no power if the grid went down . . . I did have to ask the question.

I've lived here 9 years (Massachusetts) and have lost power for more than a day only twice. We don't even have a back up generator, although I have a wood stove that is my primary source of heat anyway.

ggunn

Re: PV Grid Tied Power Outage

Vic99 wrote: »

Thanks for all the replies. If it is not cheap, I won't do it. The system and labor are going to be expensive enough as it is. Thought it would be easiest and least expensive to do it while the system is going in.

The company was not evasive. I've had two different estimates and both told me I'd have no power if the grid went down . . . I did have to ask the question.

I've lived here 9 years (Massachusetts) and have lost power for more than a day only twice. We don't even have a back up generator, although I have a wood stove that is my primary source of heat anyway.

Well, they weren't evasive when you asked, but you had to ask. Many folks (one might even say most) folks new to solar just assume that their house will have power from their PV when the grid goes down and are unpleasantly surprised when they find out differently, especially so if they find out only after the system is already installed and they experience a power outage. Solar installers are well aware of this fact and if they do not volunteer the information up front, IMO they are taking advantage of their customers' ignorance. When I was working in residential PV, I always made sure that this was understood by all parties concerned long before any papers were signed.

You are correct that it would be easier and less expensive to make your system capable of off grid operation if it were done before the system got installed, but it wouldn't be at all cheap in any case.

jaggedben

Re: PV Grid Tied Power Outage

ggunn wrote: »

he would still need ... a switch to take his PV off his GT inverter and put it onto the charge controller (you cannot connect PV both to a CC and a GT inverter; that would interfere with the MPPT function of both the CC and the GT inverter),...

You probably know this, but charge controllers and GT inverters usually call for pretty different voltages, and therefore very different string wiring. So it would have to be a very complicated switch arrangement. I'm rather doubtful anyone has actually done this.

AC coupling is a way around this but is probably more expensive than a typical hybrid system setup.

BB.

Re: PV Grid Tied Power Outage

That was probably why the Xantrex 600 Volt MPPT 60 amp charge controller was recommended... Just a DC Transfer switch should be enough (without complex series/parallel re-wiring).

But, it still makes for a fairly expensive off-grid solar power system. And why we suggest doing the research first (installation and maintenance costs) and confirm (or not) that the backup solar power is a cost effective solution for "your" situation.

I keep drooling about some sort of backup/hybrid solar power for my home--But a couple hours of lost power every few years (for the last 50+ years), it just did not make economic sense.

I chose a small backup genset and 20 gallons of fuel (+stabilizer, change once a year). Figured it gave me options. Fuel for genset and cooking, or fuel for bugging out--If needed. Plus the 50 lb genset and 20 gallons of fuel was way more portable than 20x 175 watt solar panels+a thousand lbs of battery bank.

If I was in the middle of no-where, and had ice/fire storms, perhaps my choices would have been different.

-Bill

jaggedben

Re: PV Grid Tied Power Outage

Suggestion:

There should be a sticky thread on this subject. Not sure which forum to stick it in (maybe more than one?). ggunn's response at the top of this thread would make for a good starting point.

Cariboocoot

Re: PV Grid Tied Power Outage

jaggedben wrote: »

Suggestion:

There should be a sticky thread on this subject. Not sure which forum to stick it in (maybe more than one?). ggunn's response at the top of this thread would make for a good starting point.

Uh-oh: I think I know what's coming!

niel

Re: PV Grid Tied Power Outage

you should discuss this with the installer as he is the one who put in the original system and will put in anything else you desire in changes. there aren't any cheap easy answers for you to diy. note here that i did not say he can't diy so don't jump on me.