Grid Tied w/Switch to off-grid

[coulin]

The convenience of a grid-tied system is important to me and it would be nice to pay for my system with the savings from my energy bill. I don't like that because I'm grid tied, in a blackout I don't have power (I understand it's for the safety of the line workers).

So I've been looking up a smaller grid-tied system (that will really only augment my bill, not erase it) that can be hard-switched to an off-grid system in an emergency. Essentially I'd have the same micro-inverter panels powering either one or the other but not both.

My question is, in the off-grid system...what are my options? It would only be for emergencies and only need to run, at most a well pump. I'm wildly OK with having 8 hours of sunlight energy and just going dark at night. My ignorance stems from how the batteries function in the system. With the fluctuation in power output from the panels (weather etc) I think the batteries sort of...level out the output.

If I'm producing FAR more power than I'm actually using, do I need that battery 'buffer'?

For example, if I buy a 6000w system. Using the above system, I switch the whole system to an outlet and I use that outlet to charge my kindle. I'm using so little that the entire hour it's charging I never get CLOSE to the power produced...do I still need that battery?

 

[MurphyGuy]

You can do what I do and turn your grid-tied solar system into an off-grid solar system with a flip of a switch.

Our grid tied solar uses standard SMA Sunny Boy SBxx series inverters and we regularly pump energy into the grid. When the grid goes down, as it often does out here in the rural countryside, I simply go into the basement, flip off the main breaker, and flip on what is called a "generator interlocked" breaker.
A generator interlocked breaker allows you to legally backfeed your home's electrical panel. There is a mechanical device ($40 at hardware stores), that prevents the generator breaker and your main breaker from being turned on at the same time.. this is for the safety of linemen working on the lines.

Now the cool part.
You need to purchase a special type of battery inverter capable of a technique called AC Coupling. Normal off grid systems use solar panels to power a charge controller which charges the batteries.. then you attach an inverter to get power.
With AC Coupling, the battery inverter that makes power from the batteries also causes your solar system to think the grid is back on.. this causes the solar system inverter to start making power.
If the solar system makes more power than the house is using, then the special battery inverter uses the extra juice to charge batteries.. if the solar doesn't make enough, then the inverter draws from the batteries and adds that juice to whatever the solar is making..
If the solar makes more power than the house uses AND the batteries are all charged, then the AC Coupled battery inverter performs a neat trick.. It raises the frequency above 60hz.. this is called "Frequency Shift Power Control" (FSPC).
If your grid tied inverters are capable of FSPC, when the grid hits 61hz, they will begin to throttle back their power output by 10% for every 0.1hz above 61hz.. When the frequency hits 62 hz, the grid inverters will shut down..

Even if the grid tied inverter isn't capable of FSPC, it will still shut down at 62hz as they all do.

Not many battery inverters are capable of AC Coupling. SMA Sunny Islands can do it, Outback Radian's can do it.. and I think Victron can do it but you'll have to check on that.
You'll have full 120/240 power to your entire house.. All you need is the battery inverter capable of AC Coupling, a battery bank, and to install an extra breaker in your panel so it can be put on an interlock device.

Yes, the battery inverters are expensive.. You can get the Sunny Island inverters on ebay for about $3000 right now, or you can purchase an Outback 4kW radian for around the same price.. You need two Sunny Islands to make a full 120/240 system, or you can go with a single unit and use a transformer for the 240 (well pump, etc).

No need for a second solar array. Here's a photo of my setup.. 11kW solar with 2 grid tied SMA Sunny Boy inverters, and the box in my basement with the battery bank and the two Sunny Islands.

 

[MurphyGuy]

$2200 for a single Outback 4kW Radian inverter
or
$4200 for a single Outback 8kW Radian inverter
Both of these are capable of feeding your home with full 120/240 power like the grid.

A step up is the SMA Sunny Islands (120 Volt only) You need two of them to make the 120/240 volts for your home or you can get a single 120 unit and wire in a transformer for 240 loads. Sunny Islands are 6kW inverters capable of large surges.

Forgot to address this part:
Yes, you need a battery.. However, some normal grid tied inverters have a feature that can provide a small amount of off-grid power. With my SMA Sunny Boy grid tied inverters, this feature is called Secure Power Supply, and it will provide 120 volts through a 20amp circuit so long as the solar array is making enough juice.
I believe Fronius inverters have this feature, but you'll have to check on others.

 

[philatio]

Sorry to hijack the thread, but I am going through what MurphyGuy is talking about at the moment. Can you shed some light for me MurphyGuy?

I have an SMA Sunny Boy 7.7 grid tied inverter attached to 10.4kw of solar. I jumped the gun and got a Bluetti 2 x AC330, 2 X B300 and the fusion box to hook them up in parallel for split phase functionality.

I may have bought the bluetti's to quick, but wanted to get the pre order pricing. I have already consulted an electrician to install a 10 circuit transfer switch and inlet box etc.

I assumed that during grid down, I could charge the bluetti's from the SPS. No so says Bluetti, At least not is split phase mode.

If I read your post above correctly, are you saying a couple Sunny Island would allow me to power my whole main panel?

I would appreciate anymore info you could provide.

 

[meetyg]

You can do what I do and turn your grid-tied solar system into an off-grid solar system with a flip of a switch.

Our grid tied solar uses standard SMA Sunny Boy SBxx series inverters and we regularly pump energy into the grid. When the grid goes down, as it often does out here in the rural countryside, I simply go into the basement, flip off the main breaker, and flip on what is called a "generator interlocked" breaker.
A generator interlocked breaker allows you to legally backfeed your home's electrical panel. There is a mechanical device ($40 at hardware stores), that prevents the generator breaker and your main breaker from being turned on at the same time.. this is for the safety of linemen working on the lines.

Now the cool part.
You need to purchase a special type of battery inverter capable of a technique called AC Coupling. Normal off grid systems use solar panels to power a charge controller which charges the batteries.. then you attach an inverter to get power.
With AC Coupling, the battery inverter that makes power from the batteries also causes your solar system to think the grid is back on.. this causes the solar system inverter to start making power.
If the solar system makes more power than the house is using, then the special battery inverter uses the extra juice to charge batteries.. if the solar doesn't make enough, then the inverter draws from the batteries and adds that juice to whatever the solar is making..
If the solar makes more power than the house uses AND the batteries are all charged, then the AC Coupled battery inverter performs a neat trick.. It raises the frequency above 60hz.. this is called "Frequency Shift Power Control" (FSPC).
If your grid tied inverters are capable of FSPC, when the grid hits 61hz, they will begin to throttle back their power output by 10% for every 0.1hz above 61hz.. When the frequency hits 62 hz, the grid inverters will shut down..

Even if the grid tied inverter isn't capable of FSPC, it will still shut down at 62hz as they all do.

Not many battery inverters are capable of AC Coupling. SMA Sunny Islands can do it, Outback Radian's can do it.. and I think Victron can do it but you'll have to check on that.
You'll have full 120/240 power to your entire house.. All you need is the battery inverter capable of AC Coupling, a battery bank, and to install an extra breaker in your panel so it can be put on an interlock device.

Yes, the battery inverters are expensive.. You can get the Sunny Island inverters on ebay for about $3000 right now, or you can purchase an Outback 4kW radian for around the same price.. You need two Sunny Islands to make a full 120/240 system, or you can go with a single unit and use a transformer for the 240 (well pump, etc).

No need for a second solar array. Here's a photo of my setup.. 11kW solar with 2 grid tied SMA Sunny Boy inverters, and the box in my basement with the battery bank and the two Sunny Islands.

I must say that I've heard about AC Coupling a few times, and even seens some videos on YouTube, but reading your explanation really helped me get a better idea if what it is and how it works!

Thanks