Can you do battery backup with a grid tie setup? I know there are inverters made to do this automatically but I don't have one.

Can you do it manually if you had a 400vdc battery bank?

Tesla's second-generation PowerWall with the internal inverter is designed to work as backup independently of how your pre-existing inverter works, or even if you don't have PV at all. They have a DC version that can work with many inverters and would likely be more efficient, but you'd have to get more information.

https://www.tesla.com/powerwall

They're pushing these aggressively in the Australian market, where net-metering at retail is never an option, and battery manufacturers from all over are scrambling for market share, now that it's financially rational in a 10 year analysis to use batteries for both back up and to inhibit export to the grid (at a paltry feed in tarriff.)

http://reneweconomy.com.au/tesla-launches-powerwall-2-says-all-solar-homes-will-have-storage-53696/

I wonder if getting the DC power wall would be more efficient. Otherwise you're going DC from panels to AC through inverter back to DC to charge the batteries(?), unless the Powerwall 2 supports DC charging also.

Also I don't know if my inverter (solaredge 6000) can handle direct loads without the grid tie. There are a few different diagrams out there that suggest you just need a "StorEdge interface" to tie the battery pack into the DC supply side of the inverter. I'd have to limit my load but that is OK. As long as it can support both 240v and 120v loads I'd be fine with it.

Here is one:
https://loadsheddingsolutions.wordpress.com/storedge-solaredge-tesla-powerwall/

I reached out to my solar installer but they said they're working on a solution.

Here is an article that talks about how the new 2.0 PowerWall is actually available in both DC and AC output.

https://www.greentechmedia.com/articles/read/The-New-Tesla-Powerwall-Is-Actually-Two-Different-Products

My thought is with my SolarEdge inverter, they would use a DC PowerWall and use the SolarEdge to provide AC power to the house.

Somewhere in there needs to be an automatic or manual switch which shuts down the backfeed to the grid. Also something needs to tell the SolarEdge inverter to keep power going.  The way it is currently configured will not pass power when the grid is down.

Green Mountain Power has been selling & supporting Tesla Powerwall installations for nearly a year now, and should be willing/able to discuss the particulars with you (especially if they happen to be YOUR utility):

http://products.greenmountainpower.com/product/tesla-powerwall/

I contacted them also - waiting to hear back, but $6500 for the 7 KW Powerwall (according to their website) doesn't seem good to me. Maybe it is just outdated info, or, maybe this will change once they have availability on the newer unit.

One thing not mentioned: Regulations are very strict about how you set up for this, to keep you from killing or maiming a line worker.

Yeah it'd have to have an automatic transfer switch to isolate from the grid. The solar inverter also have this safety feature so it would somehow need to be told to allow power to go through the inverter when the grid is down (however it is configured now will not allow this). I am sure there is or will be a solution to do all of this safely to prevent backfeeding to the grid when the grid is down.

If you just want backup and can't use your existing inverter, then I'd look at some AGM batteries and an off-grid inverter/charger/transfer switch. Or just a generator plus transfer switch. Wire your grid tie inverter external to the backup system.

I believe the term you are looking for is "anti-islanding" which prevents the locally generated power from back-feeding the grid. In reality, the chances of hurting a line worker are infinitesimal as: 1. They do not ever touch live circuits. Unless it is in a fall, the only time they really ever get hurt is when they break protocol(usually the case in falls also)... 2. Your paltry few kilowatt piece of power generation equipment is going to promptly stand on it's ear(pop breakers and fuses or burn up circuitry) if it even tries to power a few houses in your neighborhood, let alone the electrical black hole of even a very small part of your local grid... But rules are rules, and it is just plain and simple a good idea...

In the case of an outage I'd want that power going to the batteries not the grid anyway. I'm sure there is a way to have it all work together. Grid fails and grid isolation occurs, tells inverter grid is isolated so it can allow power through.

You can look at hybrid inverters if you want one that can do both grid-tie and off-grid.

The #1 question in hot humid climates concerning off-grid AC is, can it be done, right now, with any halfway reasonable payback time and cost?

Reams have been written about it, especially now, since we seem to be getting close, really close. The experts still say no, not without stooping to using a generator.

If anyone here knows how to do this in North Texas, RIGHT NOW, there are a lot of experts who want to know exactly how.

I am still waiting to hear from Tesla.

Green Mtn Power has a deal where they rent out the 15 kwh powerwall for $1500 (installed) for 10 years which seems like a pretty good deal considering the tech will most likely be obsolete in 10 years.

Whatever new Powerwall is supposed to be compatible with grid tie inverters.

That would only supply 10 amps (of 120VAC) for 12 hours. I don't know of any house that could run that low of draw.

When they say it is 15 kwh is that usable battery storage? Because you can't drain the battery all the way. Like cars have 15kwh batteries but you can only use like 10kwh's of it.

Posted By newbostonconst on 13 Oct 2017 09:25 AM
That would only supply 10 amps (of 120VAC) for 12 hours. I don't know of any house that could run that low of draw.

When they say it is 15 kwh is that usable battery storage? Because you can't drain the battery all the way. Like cars have 15kwh batteries but you can only use like 10kwh's of it.

I know of MANY homes that could run that low or lower during a power outage. Ten amps of 120V is 1200 watts. My home's average use is about 20kwh/day (less than 1000 watts average) but in a power-out situation that could EASILY be cut by at least half. (Don't need to run the dryer, dehumidifier or the central AC until the grid comes back up.) I'm pretty sure I could get at least 48 hours of critical-load use out of a 15kwh battery, but I wouldn't be running the electric clothes dryer, energy pig appliances or binge watching K-dramas all night on the big screen. Most electricity use is dicretionary, and with the grid power down most of us could use a LOT less. YMMV

Tesla's PowerWall ratings are for their useful battery storage capacity, with a guaranteed number of charge/discharge cycles to that (internally software controlled) capacity level. The absolute capacity of those batteries are significantly higher.

They do a similar thing with their car batteries. When Irma was ravaging Florida they sent a temporary software patch that allowed for a one-time deeper discharge for Florida residents to make it easier for them to get out of harm's way. Some less-informed media reporters took that to mean that they were "artificially" limiting the capacity of their smaller-battery cars, openly speculating that it was shipped with the same battery as their bigger battery cars, charging a premium for the not-really-bigger battery. But that's simply not the case. Tesla inhibits deep discharge or overcharging the batteries by design because they have generous warranty terms to uphold, even though they could extend the range quite a bit if they didn't care about the number of cycles the battery would last. Doing it on a one time 48 hour window basis for a limited number of cars isn't a big warranty liability risk to them, but running those cars all the time in that mode would break the bank.

Our usage without really trying to be conservative at all (not including heat pumps) is about 10kwh a day (In a power outage we'd use the wood stove, not heat pumps).

So one 15kwh powerwall would last at least 24 hours maybe we could make it stretch to 48 hours, by being a little conservative, and this is not considering any recharging from solar panels, which may be 0, but may also keep the batteries 100% topped off for 8 hours a day depending on time of year and weather.

In our remote and rural area, many of the off-grid folks are using these batteries:

Edison Nickel Iron Battery

Some even DIY fabricate their own batteries. The Edison nickel iron battery is certainly old school, but they are more durable and longer lasting than other batteries and they are also greener too. However, if you can get 15 kwh battery backup for 10 years for only $1500 total, that seems like a great deal.

Off-grid is a distinctly different animal that grid tied battery backup. With off-grid systems you're counting on it to keep up with your normal daily power use 365 days/year under across the full range of weather conditions. Most off-grid systems have fossil-fired generation as backup to manage the extreme events. Grid tied battery backup is there to provide at least a minimum of electricity for critical functions and some conveniences in the event of a power outage, not to provide 100% of the normal daily power use of an on-grid house. The number of deep cycles of a grid tied battery backup are orders of magnitude fewer than a typical off grid system.

In most of the US power outages of more than a few hours is pretty rare, and more than a few DAYS exceedingly rare, the current conditions in Puerto Rico notwithstanding. While extreme weather events such as hurricane Maria or Super Storm Sandy took large numbers of people off grid for several days, managing the residential outages would have been a lot easier for most people with 15 kwh in storage to work with. Having at least some power available makes communication and tracking news a lot easier, which simplifies and enables good decision making under tough circumstances. The vast majority of residential power outages in the US are resolved in under a day. Of the ~8.5 million customers (metered accounts, not people) out of power during Sandy's outage-peak, fully half had power restored within three days, 90% within a week. (Puerto Rico should be so lucky!)

Agreed, off-grid is indeed very different than grid tie battery backup as you indicated. Edison nickel batteries also like to be deep cycled often and they don’t like not getting often used. Most of the off-grid folks in our remote and rural area use a combination of micro hydro power and solar photovoltaic power to charge their batteries. Some also additionally use diesel engines like the Lister or Listeroids that will run on waste motor oil or vegetable oil which can be readily obtained when gas or diesel isn’t available.

I would also say that many of these off-grid folks are also preppers who tightly clutch bibles and guns, are thrilled to have our current President, and are eagerly looking forward to a US “fire and fury” event or a global warming event to permanently eliminate the grid and allow them to gain an equal or greater parity with the general population and government. So if ones goal is to retain long-term electrical power, I don’t know if just planning for grid tie will be an entirely adequate solution. And to be 100% clear, I am not in favor or against this prepper mentality...it is just the current state of mind of many folks given the low confidence many folks feel about our government and state of the world. And as I previously said, if one can get 15 kwh battery backup for 10 years for only $1500 total, that seems like a great deal.