backup generators in a grid tied system

Okay so some of you may already know, I am in the design phase of building a new home. I plan on using LG Neon solar panels. with a grid tied system. But I would also like to have a back up generator, for when the power company power goes down. Now most (ALL?) grid tied systems shut down when the power lines go out. This is an obvious safety feature for protecting the linemen. But I know you can buy generators that come on automatically and cut off from the power company lines.
My question is can I do both? Can I have a grid tied system with an automatic shut off, so my gas powered generator can supply power?

Yes, needs something so PV doesn't backfeed generator.
One way to go is interlocks that disconnect PV. You could put a relay in the AC circuit of PV, and open it when generator output has AC.

You can also get yourself a battery backup system that interacts with grid and optionally generator. Sunny Island (my picture) can have grid-tied PV and your house on its output, grid on its input. If power fails it makes a local grid from battery. If batteries get full it raises frequency to signal to grid-tied PV that power production should be reduced. Some but not all grid-tie inverters support that, so get one that does.

Sunny Island can have a transfer switch on the input to select between grid and generator. But it requires that one shut off for at least 5 seconds before transferring to the other, so it doesn't get blown up by a sudden shift in phase. I have a manually interlocked breaker rather than auto transfer switch for that reason.

There are some high voltage Lithium battery inverters for that purpose as well. Tesla, and SMA have them, probably requires a special transfer switch. (without that switch they do peak load shaving but can't run during a power failure)

There are batteryless alternatives. Some hybrid inverters work with or without batteries. You would need to find one that is also grid-tied.

Another option is the new model Sunny Boy. They can produce up to 2000W 120 VAC directly from PV (given enough sun), without the grid. You have to manually switch to that and use a plug on its "secure power" port. I think that could be the best option if you want cheap, no batteries, don't mind manual transfer.

If you are not trying to use your solar when the power is off, you only need to have a transfer switch for your generator. You can wire some critical house circuits to a separate panel and then switch that panel to your main panel, or switch it over to allow the generator to power that panel. Something like this, just add your breakers to it.

DThames said:

If you are not trying to use your solar when the power is off, you only need to have a transfer switch for your generator. You can wire some critical house circuits to a separate panel and then switch that panel to your main panel, or switch it over to allow the generator to power that panel. Something like this, just add your breakers to it.

Click to expand...

That is a great idea. I could have the fridge and TV etc. wired to a separate panel box, with a switch inbetween...

Hedges said:

Yes, needs something so PV doesn't backfeed generator.
One way to go is interlocks that disconnect PV. You could put a relay in the AC circuit of PV, and open it when generator output has AC.

You can also get yourself a battery backup system that interacts with grid and optionally generator. Sunny Island (my picture) can have grid-tied PV and your house on its output, grid on its input. If power fails it makes a local grid from battery. If batteries get full it raises frequency to signal to grid-tied PV that power production should be reduced. Some but not all grid-tie inverters support that, so get one that does.

Sunny Island can have a transfer switch on the input to select between grid and generator. But it requires that one shut off for at least 5 seconds before transferring to the other, so it doesn't get blown up by a sudden shift in phase. I have a manually interlocked breaker rather than auto transfer switch for that reason.

There are some high voltage Lithium battery inverters for that purpose as well. Tesla, and SMA have them, probably requires a special transfer switch. (without that switch they do peak load shaving but can't run during a power failure)

There are batteryless alternatives. Some hybrid inverters work with or without batteries. You would need to find one that is also grid-tied.

Another option is the new model Sunny Boy. They can produce up to 2000W 120 VAC directly from PV (given enough sun), without the grid. You have to manually switch to that and use a plug on its "secure power" port. I think that could be the best option if you want cheap, no batteries, don't mind manual transfer.

Click to expand...

Thanks, I will look into sunny island (it sounds like a nice place). My current thinking is I need to go with manual or spend a large amount of cash. Does that seem right?

Manual interlock between main circuit breaker and adjacent branch breaker is about $50 (for a little piece of metal), allowing a generator or battery inverter to safely feed your house.
Manual transfer switches for the whole house or including a sub-panel start at a few $hundred.
Automatic switches are a few $hundred up to a couple $thousand

Hybrid inverters that have grid input and PV input, with an outlet for plugged in or permanent protected loads start at a few $hundred. These would have their own PV panels but probably don't backfeed the grid.

Sunny Island (120V for single inverter setup) lists for $5000 but can be had for around $2000. Several others from Outback, Schneider, etc. offer similar features and price range and some include 120/240V split phase in one unit. These support grid backfeed and other options. You can run the whole house (avoid excessive loads) or wire select circuits on them as a UPS, then no transfer switch. But I would do a manual transfer switch anyway, to bypass a failed system and use grid directly.

The newer Sunny Boy gives you a manually enabled AC outlet for free other than the cost of wiring an outlet to it. Not sure if any other brands have grid-tie inverters that offer batteryless backup.

You can start with grid-tie and add a compatible backup system later, which is what I did. Get an inverter with frequency-shift power control. Several brands offer that now. Also several brands of battery inverter should work with it.

SMA was early in AC coupled systems; most of the others grew out of PV --> battery charger --> battery --> inverter --> AC which was common for off-grid power.

Many inverters are transformerless these days. They are lighter weight and cost less, without all that steel and copper. Transformer type may be more resistant to damage from noise in the power line (transformers don't pass high frequencies so well), and for battery inverters they may start motor loads better.

There are lots of bargains out there, many posted on eBay. My grid-tie system cost $5/watt when I put it in 15 years ago; today you can do one for $1/watt. It is cost-effective compared to utility rates. Battery systems are not, so probably best to set up battery to run communications and refrigerator, not much more.

Your choice of grid-tie inverter will be limited by permits and rules. My location requires "grid support" features so PV doesn't topple PG&E with excessive power. PV panels on the roof must follow coverage/fireman access rules and new electrical safety rules. (Half a dozen Walmarts got burned by Solar City, so there's a reason for revised codes.)

I am a little confused about how inverters work with micro inverters. I am planing on getting the LG Neon R panels which have built in micro inverters. But I think I still need a separate inverter to tie to the grid, IS that right?

Not a separate inverter. Maybe no additional electronics.

Microinverter is either built into the panel, or mounted on/near the panel and connected to it with MC cables.
Some sort of AC wire connects the microinverters so all are in parallel on the 240VAC line.
That alone might be all that's necessary, with the wire tied to a breaker in your panel.
Or, there might be a box that controls them (possibly by a signal modulated on the AC wire rather than a separate data cable.)
Enphase, for instance, has Envoy, which can do things like measure power export and adjust output so power is only used locally (if export not allowed.) Some newer Enphase support frequency-shift and work with battery inverters.

I like high-voltage string inverters. Anything from 8 to 24 panels connect in series (whatever is guaranteed never to exceed 600 VDC), and the inverter converts that to AC. It is right-sized and higher efficiency, compared to microinverters which typically could handle some percentage more watts than a single panel, and are somewhat lower efficiency due to lower voltage and circuitry duplicated per 300W panel. Microinverters also cost about twice as much.

But if your jurisdiction requires panel-level shutdown for rooftop installations, now string inverters need a shutdown box on every panel. At about $30 to $50 each, there goes the price advantage.

What is the price and wattage of what you're considering?

These days I can buy PV panels for $0.35/watt and inverters for $0.30/watt (current models & code) or $0.15/watt (new older model, for retrofit of existing system). Balance of system like mounting racks and wires is an increasing percentage of the cost.

New home. Now is the time to separate protected loads into their own sub-panel. Plan so you can add a transfer switch easily, have a box with conduit between panels so you can pull wires for the inverter. Oversize conduit, like 2", to make pulling easy.

Typical 200A panel with 200A main breaker is limited to 40A PV breaker at one end (allowed 200 + 40 = 240, 20% over busbar.) Only 32A AC with 40A breaker being 25% oversize, so 7700 W PV. Not bad, but you might want more. A 225A Squared-D QO panel with 200A breaker allows 70A PV breaker.

A tap before the main breaker but after the meter could have a dedicated PV panel, e.g. 225A busbar with 150A main breaker. Now you can have 120A of PV. Or just a fused disconnect, after which you can mount a panel or a battery inverter. And you could have a battery inverter on it, wired to an interlocked backfeed breaker on the house main panel. That way the house can be on solar/battery backup or be switched to the grid.

So I would suggest at least a QO 225A main panel for the house, protected loads on a sub panel downstream from the main, and a 100A or 200A fused disconnect also coming off the meter for PV.

WillyP said:

Okay so some of you may already know, I am in the design phase of building a new home. I plan on using LG Neon solar panels. with a grid tied system. But I would also like to have a back up generator, for when the power company power goes down. Now most (ALL?) grid tied systems shut down when the power lines go out. This is an obvious safety feature for protecting the linemen. But I know you can buy generators that come on automatically and cut off from the power company lines.
My question is can I do both? Can I have a grid tied system with an automatic shut off, so my gas powered generator can supply power?

Click to expand...

Here is the similar question was asked and answered on grid-tie, the diagram will give you how the grid and generator would work, as stated in the earlier posts, you can do manual or auto transfer switch:

Magnum diagram: https://diysolarforum.com/threads/ac-coupling-for-grid-tied-solar.3840/post-36532

SMA diagram page 18 of the doc: https://diysolarforum.com/threads/ac-coupling-for-grid-tied-solar.3840/post-36532

Micro inverter grid tie systems and solar based power during a "grid down" condition are miles/kilometers apart in today's way of doing things. If you want solar based power in an off grid situation, a typical micro inverter grid tie system is not what you want. So the first thing is to decide if the generator solution fits your needs/wants. If no, then you need to stop and start looking for a solution for off grid solar based power.

DThames said:

Micro inverter grid tie systems and solar based power during a "grid down" condition are miles/kilometers apart in today's way of doing things. If you want solar based power in an off grid situation, a typical micro inverter grid tie system is not what you want. So the first thing is to decide if the generator solution fits your needs/wants. If no, then you need to stop and start looking for a solution for off grid solar based power.

Click to expand...

I think the times when there is no grid power will be so few and far between that it isn't really an issue. But I still tend to plan for anything. Especially if it is a situation where the power is off for a couple days and the outside temperature is well below freezing, with a couple feet of snow covering all the roads.
Going completely off grid is a little scary. But not out of the question.

What is the price and wattage of what you're considering?

These days I can buy PV panels for $0.35/watt and inverters for $0.30/watt (current models & code) or $0.15/watt (new older model, for retrofit of existing system). Balance of system like mounting racks and wires is an increasing percentage of the cost.
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I am planing on going with LG Neon R panels. They are rated at 375W with a built in micro inverter. But they cost $400 bucks per panel. SO I guess that is about 1.50 per watt when you figure less than perfect conditions.

WillyP said:

I think the times when there is no grid power will be so few and far between that it isn't really an issue. But I still tend to plan for anything. Especially if it is a situation where the power is off for a couple days and the outside temperature is well below freezing, with a couple feet of snow covering all the roads.
Going completely off grid is a little scary. But not out of the question.

Click to expand...

I agree that it is rare. I also think that if someone were to think solar will get them out of a jam from power outage that is winter storm related (what happens some around here) the fact is that on a cloudy day the solar will make about 5% of what it makes on a good day. So as long as you have fuel the generator is more of a sure thing. I put in a grid tie only system. Later I got some batteries and I already had some extra panels, so I am slowing setting up some standby power in case the grid is down. That plan includes batteries with both generator and solar charging. Enough battery to keep the freezer, frig, and my wife's CPAP going for a couple of days. If the sun can't keep up, the generator is there to charge the batteries.

Hedges said:

New home. Now is the time to separate protected loads into their own sub-panel. Plan so you can add a transfer switch easily, have a box with conduit between panels so you can pull wires for the inverter. Oversize conduit, like 2", to make pulling easy.

Typical 200A panel with 200A main breaker is limited to 40A PV breaker at one end (allowed 200 + 40 = 240, 20% over busbar.) Only 32A AC with 40A breaker being 25% oversize, so 7700 W PV. Not bad, but you might want more. A 225A Squared-D QO panel with 200A breaker allows 70A PV breaker.

A tap before the main breaker but after the meter could have a dedicated PV panel, e.g. 225A busbar with 150A main breaker. Now you can have 120A of PV. Or just a fused disconnect, after which you can mount a panel or a battery inverter. And you could have a battery inverter on it, wired to an interlocked backfeed breaker on the house main panel. That way the house can be on solar/battery backup or be switched to the grid.

So I would suggest at least a QO 225A main panel for the house, protected loads on a sub panel downstream from the main, and a 100A or 200A fused disconnect also coming off the meter for PV.

Click to expand...

You pretty much answered all my questions on this topic. Thanks for your input. I am kind of weighing all my options at this point, as I have until next spring before we start construction. Trust me when I say. I will have many more questions before then.