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tbleakne said:
He did imply that the PW would automatically cut in if the grid goes down. This by itself is a big step for the utilities to approve, because it means they are relying on the software and not a manual disconnect to keep your generation off the grid when the grid is down. If you folks have different information, please post.

Haven't they already relied on software in grid-tied inverters that shuts down production when the grid goes down? Same kind of idea.

Anyone knows how these batteries are installed? It would seem there must be some sort of disconnect between the MSP and the grid for backup operation.
 
In the spirit of "Know thy Enemy" I am posting this link to an interview from Fortune with the NV regulator, Chairman Thomsen, who had a lot to do with the alarming decision by the public utility commission in NV killing Net Metering in NV in Dec 2015.
The original decision phased out grandfathering which no other state had tried, but I believe grandfathering of Net Metering was re-instated later.

http://fortune.com/2016/04/12/solar-firestorm-nevada/

At one point deep in the interview he states:

I’ll tell you, we sat through 20 hours of public comment and the most moving parts of that were the seniors who said, 'I've taken out a second mortgage on my home to put on solar that starts at the retail rate and escalated 3% annually.' I was gobsmacked to hear that. That to me is a reprehensible sales tactic. I wish the commission had the ability to prevent that from happening.

I seriously doubt this. Most likely he was conflating two different sales deals:

Either buy system outright (and possibly do that with 2nd mortgage),
Or pay nothing upfront, and buy the electricity from the array owner at a rate that goes up 3% annually.

If it was both, I agree it would be a fraudulent sales proposal. I also agree the second deal by itself is not good, but some people get pressured into it.

You say what’s the difference between solar power coming off a rooftop that the utilities have to pay 11 cents for, versus the 4.5 cents they're paying for solar energy from large-scale solar panel projects? The delta has gotten so big. That’s the debate.
Compared to SCE prices, 11 cents looks awfully cheap to me. He seems to attribute very little extra value above wholesale to fully distributed power from our rooftops.
 
tbleakne said:
I was told by a Powerwall installer guy at a Tesla OC meetup last month that PW was currently only licensed for Backup, not load leveling. I believe this means you can't draw from the battery unless it thinks the grid is down, but perhaps I am wrong. If you are doing load leveling, you need different software that watches you house usage and solar generation simultaneously.
I'm still not totally clear on how all of this works, and am currently seeking answers through my contact person at Pick My Solar. What I do know is that, to be eligible for SGIP rebates, battery systems must be used for more than just backup purposes; the battery must be cycled at least 52 times during its first year of operation (partial cycles count), from a renewable source. Tesla/SolarCity has been assisting Powerwall customers in applying for SGIP. That implies that Tesla has obtained, or expects to shortly obtain, whatever licensing may be needed to support load leveling.

tbleakne said:
He did imply that the PW would automatically cut in if the grid goes down. This by itself is a big step for the utilities to approve, because it means they are relying on the software and not a manual disconnect to keep your generation off the grid when the grid is down. If you folks have different information, please post.
In the "Powerwall 2 waiting list" thread on TMC, the user jeeps1979 (in Crestline, by the way) posted a system diagram that seems to show a "gateway" between the main panel and the service meter for this purpose. But there is some question as to how that could work, particularly with main panels that integrate the service meter. Also, if the grid is down, the battery is fully charged, and there is surplus PV generation, I presume that the PV would need to be automatically disconnected. I hope to learn more.
 
Following is a query that I sent to Pick My Solar over the weekend. They just responded that the answer is "yes" to all of the below questions:
Thanks for getting the contract sent out. I’ve reviewed it, and would like to ask for written confirmation of a few things before I sign. I’d like to confirm that the contract includes the following, which would reflect our phone conversations, I believe. If my understanding isn’t correct in some area below, then please let me know!

1. All circuits in our home will be backed up, meaning that our entire home would be powered by the battery in the event of a grid outage. I understand that our usage during an outage would be limited by the 5 kW maximum continuous power of the Powerwall 2 (when new) as well as by the available battery charge.

2. During a grid outage, subject to available sunlight, our solar panels will continue to power the circuits in our home, with any surplus going into the battery.

3. During a grid outage, our home’s electrical system will essentially function as an isolated microgrid.

4. During normal operation (no grid outage), we will be able to choose to charge the battery from our surplus solar power (above whatever our house is using) during certain hours, namely during the TOU “super off peak” and “off peak” hours.

5. During TOU peak hours, we will have the ability to zero out our use of grid electricity by drawing from the battery instead, subject to available battery power/charge.

6. During TOU peak hours, we will also be able to feed any surplus solar power (above whatever our house is using) into the grid rather than using it to charge the battery.
 
Make sure this becomes a part of the contract as it may well be an ignorant sales rep just trying to make you sign it, just an e-mail may not be enough of a proof in court.
 
In olden times battery based grid tied inverters like my old Xantrex/Trace SW4048 would do all of the above.. albeit with PV strings in a relatively low voltage (48-55V) tied to batteries with no ability to do peak powerpoint tracking.

If the grid goes down, the house isolates itself and can run off the battery. If the sun is shining the battery is charged from the sun simultaneously. If the sun is providing more than the house is demanding the batteries could overcharge. I was too cheap to get a charge controller for my original lead acid pack 16 years ago so I setup a circuit to disconnect the PV by sending a small current through one leg of the GFCI thus throwing the breaker, disconnecting the PV and preventing the overcharge of the batteries. If I came home at night and saw the GFCI tripped I'd just flip it back on... I know, crude..

Now with the 10kWh lithium battery I never go over about 85% SOC or below 35% SOC so I have extra margin in both directions if the grid goes down for a few hours. Worst case would be the grid going down at 2pm right when the batteries are full on a sunny day.

Modern micro inverters on PV systems all have the ability to disconnect automatically in the case of a downed grid to prevent islanding.

Presumably the PW2 also can disconnect from the grid and power the house like my old inverter.

The question of when to charge the batteries is a good one though:

Should I charge them at super off peak rates ($0.13/kWh) and then let the solar power generated in normal off peak ($0.27/kWh) flow right out to the grid and get credit at that rate?

Or should I not charge overnight, put the grid in silent mode all night and during most of the day while the sun directly charges the battery. When the battery is full then turn around and sell full blast until 8pm at the end of the super on peak? That might be healthiest for the grid but there's no incentive in TOU-D-A for me to do that.

Charging the batteries from the sun is more efficient.. But in the end I think I'd rather get credit for the solar at $0.27/kWh and buy extra energy at $0.13/kWh to charge the batteries.

Like I'd mentioned before, this cycling of about 6kWh/day is netting me about $400 annually on the SCE TOU-D-A plan.

The actual battery cost (used EV batteries) is about $120/kWh. The cost for someone without one of these old 48V inverters would be the cost of adding another inverter.. potentially $1000-$1300. Battery backup might still be possible with one of these inverters but I'm looking into even cheaper options ($400-$500) that might be able to do this but not provide the switch to provide backup power.. it depends what your priority with batteries is.. arbitrage or backup.
 
GregH said:
Like I'd mentioned before, this cycling of about 6kWh/day is netting me about $400 annually on the SCE TOU-D-A plan.
Is this actually permitted under TOU-D-A? While I haven't closely studied the rules, I've gotten the impression that you're not supposed to feed into the grid unless it's directly from your renewable energy generation. Otherwise, there'd be nothing stopping someone from buying lots of batteries (as in, two Powerwall 2 units totaling 27 kWh of usable capacity) and fully cycling them daily for arbitrage. That'd net about $1800 annually in credits, on top of a possible 30% income tax credit.

Under the terms of the SGIP rebate, you have to cycle the battery 52 times per year, and those charges have to come from your renewable energy source.
 
abasile said:
GregH said:
Like I'd mentioned before, this cycling of about 6kWh/day is netting me about $400 annually on the SCE TOU-D-A plan.
Is this actually permitted under TOU-D-A? While I haven't closely studied the rules, I've gotten the impression that you're not supposed to feed into the grid unless it's directly from your renewable energy generation. Otherwise, there'd be nothing stopping someone from buying lots of batteries (as in, two Powerwall 2 units totaling 27 kWh of usable capacity) and fully cycling them daily for arbitrage. That'd net about $1800 annually in credits, on top of a possible 30% income tax credit.

Under the terms of the SGIP rebate, you have to cycle the battery 52 times per year, and those charges have to come from your renewable energy source.

Good question.. If you're willing to shell out the cash for Powerwall 2s with installation I don't know what would stop you from doing this.

There's nothing wrong with charging batteries from the grid at whatever time you're willing to do so. The question is when you feed it back to the grid in the evening (when the grid REALLY DOES NEED IT), does it matter where those electrons came from? I really don't know.

Yes the addition of SGIP $$ would seem to tip the scales. My inverter was 16 years old and the batteries were cheap.

I'm cycling the battery 365 days a year but I haven't received any SGIP money.
 
GregH said:
There's nothing wrong with charging batteries from the grid at whatever time you're willing to do so.
Yes, agreed.

GregH said:
The question is when you feed it back to the grid in the evening (when the grid REALLY DOES NEED IT), does it matter where those electrons came from? I really don't know.
I agree that feeding into the grid during the evening is good for the grid.

Whether the utility is okay with crediting you at peak retail rates is another matter. The CPUC (under NEM 1.0) forces them to credit you at retail rates whenever your solar panels are producing a surplus. But does the utility have to buy whatever comes out of your battery? I don't know, and I'd be happy to be proven wrong.
 
I don't know either.. but if they want me to charge my batteries from solar power they need to make the daytime rate when the sun is shining cheaper than the night time rate.

If they made the rate from 10am to 4pm $0.10 or less I would gladly switch to daytime charging.
 
Looking into the arbitrage stuff a little more it looks like SCE is one of the most advantageous with regard to differentials between on and off peak. SDGE looks terrible and PG&E only slightly better. Many northern utilities (BCHydro, Liberty Utilities) have a small differential but also a smaller price over all.
The one exception seems to be SMUD with $0.07 super off peak for EV drivers and a summer on-peak of $0.31 (4-7pm).

Going through the web sites its somewhat confusing though.. some (like SCE, PG&E and SDG&E) have baseline credits to encourage lower consumption without the old tiered pricing systems. It's also clear a lot is changing over the next 9-18 months and details of upcoming TOU plans are hazy. Many are shifting on-peak to start as late as 4pm or 5pm.
 
Wow, thanks to Abasile and GregH for all this info on battery systems and arbitrage. It would seem policy is rapidly changing, but perhaps not all the silos are equally on board or even aware of SGIP for example. IIRC SCE has made statements in tariff hearings and/or actual tariff documents that they have concerns that this arbitrage with batteries, solar, and TOU could get out of hand (my words, not theirs). Of course they have the protection that net "funny money" credits are zeroed out at the annual True-up.

I know that the utility in HI is so worried about excess solar production on their grid that they are actively encouraging the addition of batteries by disallowing export credits to be rolled over from one month to the next. Within the current billing cycle, export is credited at about .5 retail price.

Of course this is an extreme solution that would hit us in SCE-land hard. Batteries for day-to-night use are clearly practical, but seasonal storage notsomuch. I estimate that enough batteries to fully replace net metering for my system in my climate, with big credits generated in the Spring to be used for heat-pump cooling in Aug and Sept, and now in Dec and Jan for heat-pump heating, I would need roughly a battery capacity of 10 Tesla MS.

I just completed my 3rd month on my current NM plan with my new solar. Solar production is rising fast. Still net positive cumulative usage of 300 kWh left over from heat-pump usage in 1st month, but monthly credit of -$181 and cumulative credit of -$238.
 
I just got my latest bill. Month 10 of the cycle.
-$43.59 (mostly thanks to the $31 CA climate rebate). Down to -$271.44 net with 2856 kWh consumed from the grid.

Also noted in one the CPUC filings (in the bill) that SCE is seeking a $70 million rate increase to cover two Tesla (Ontario) and two GE Energy (Norwalk) batteries they've already acquired. A quick net search says the Ontario/Mira Loma Tesla station is 80MWh. One report said the Norwalk hybrid gas plant used a 4.3MWh battery.. does that sound right?
 
tbleakne said:
Batteries for day-to-night use are clearly practical, but seasonal storage notsomuch. I estimate that enough batteries to fully replace net metering for my system in my climate, with big credits generated in the Spring to be used for heat-pump cooling in Aug and Sept, and now in Dec and Jan for heat-pump heating, I would need roughly a battery capacity of 10 Tesla MS.
10 Model S batteries would roughly equal 60 Powerwall 2 units! (810 kWh usable / 13.5 kWh usable) Clearly not practical for the foreseeable future. Particularly at non-tropical latitudes, we ideally want large grids with diverse sources of renewable energy.

That said, I'm of the opinion that the best way to mitigate seasonal heating and cooling requirements is to design better homes and buildings. Retrofitting existing homes with additional insulation is also quite helpful. While our mountain home is far from perfect, we've made efforts to improve its insulation and upgrade the windows, and our winter heating costs are quite modest compared to similar-sized houses in our area.

I'm particularly sold on the benefits of high-R-value foam board insulation (see https://www.rmax.com/thermasheath-3/). We used a bunch of this between the roof joists in a converted attic. It's amazing how stable the temperatures are; the room has stays relatively cool in summer and warm in winter. There's a dramatic difference compared to the conventional fiberglass insulation that we were using before.
 
abasile said:
I'm particularly sold on the benefits of high-R-value foam board insulation (see https://www.rmax.com/thermasheath-3/). We used a bunch of this between the roof joists in a converted attic. It's amazing how stable the temperatures are; the room has stays relatively cool in summer and warm in winter. There's a dramatic difference compared to the conventional fiberglass insulation that we were using before.
Yes I like the rigid foam board as well. I have only 1" of this foam under my composition roof. I wanted more, but that is the most thickness the roofer could do from the outside. Half of my house has no attic, just high ceilings, so I went with that. 1" gives my a R-value of about 4.5 as I recall. I am sure your inside foam is much thicker.

I believe one reason the foam seems to perform better than fiberglass is the foam is much lighter in lbs/sq-ft. This means it has less heat capacity. Most of us who have studied electronics or physics know that RC (resistance * capacitance) has the dimensions of time. A capacitor C charged up and then allowed to discharge through a resistance R will lose have its charge decay to 1/e in time RC.

Thermal insulation has a similar time constant and the R-rating performs the same role. The fiberglass of the same R-value will have a much longer decay time, so heat accumulated during the day will still be radiating into the evening. There are other situations where you might want more heat capacity. It is also possible that over time the fiberglass becomes compacted, with less air, and its R-value declines.
 
tbleakne said:
1" gives my a R-value of about 4.5 as I recall. I am sure your inside foam is much thicker.
Interesting thoughts about the effects of the foam board insulation itself having a lower heat capacity than fiberglass! Yes, I had been thinking this might be helpful, but hadn't thought of the problem in terms of electronics! :D

That Thermasheath foam has a thermal R value of 6.0 for one-inch-thick boards. We sliced, packed, and stacked several 1.5 inch boards between each pair of 2x12 roof joists. So I think we achieved a pretty high overall R value!

Aside from improving the R value, we replaced the fiberglass insulation in that attic to get rid of a condensation issue. During the winter, warm, relatively moist air from the house was slipping past the fiberglass insulation and cooling when it got just below the roof deck. This caused moisture to condense and get trapped in the fiberglass insulation, which happens to be capable of holding a lot of moisture. Then, with intense sunlight hitting the roof in early Spring, the moisture heated up and created high vapor pressure which led to water dripping from the ceiling area. With the foam boards, this problem seems to have completely disappeared.
 
abasile said:
In the "Powerwall 2 waiting list" thread on TMC, the user jeeps1979 (in Crestline, by the way) posted a system diagram that seems to show a "gateway" between the main panel and the service meter for this purpose.

But there is some question as to how that could work, particularly with main panels that integrate the service meter. Also, if the grid is down, the battery is fully charged, and there is surplus PV generation, I presume that the PV would need to be automatically disconnected. I hope to learn more.
No the PV would not need to be disconnected, at least from a technical standpoint. It is a common misconception that all the solar power generated by the PV panels and associated inverter(s) "must go somewhere."

In the normal situation with an active grid, the grid acts as an infinite load that will absorb any excess power, so the inverter has the simple goal of maximizing output by operating the panels feeding it at their MPPT (Maximum Power Point Tracking) point on their I vs V curve. However, the inverter is free to operate the panels anywhere on their I-V curve from Short Circuit (maximum current, zero voltage) to Open Circuit (zero current, maximum voltage). At both of these extremes, SC and OC, the power generated is zero. OC is equivalent to no inverter being connected at all. The illuminated panels are still generating their full photocurrent, but this current is exactly canceled by the forward bias current in the opposite direction.

Even when the grid is up, the inverter has to handle the common case of more DC power coming in from the panels than the inverter is rated to output. The inverter simply moves off the MPPT point far enough to bring the output back to its rated maximum value. For example, if the inverter reduces the current draw by 20%, the panels will be underloaded and their voltage may rise about 10%, for a net 10% loss in power. I have sized my new system so that this situation of "power plateau" occurs for an hour or so on the best solar days of the year. I can see the voltage rise above its MPPT point and the current drop below its MPPT point. Buying the next larger size inverter was not cost effective.

So in the case you consider, the surplus PV generation could be reduced to zero. The inverter would have to track the dynamic load of the house and adjust its output accordingly, rather than just optimize the output of the solar array.
 
Thank you for this succinct explanation! I have to admit that I now can't recall ever having studied anything about "forward bias current", though. :cry:

tbleakne said:
Even when the grid is up, the inverter has to handle the common case of more DC power coming in from the panels than the inverter is rated to output. The inverter simply moves off the MPPT point far enough to bring the output back to its rated maximum value. For example, if the inverter reduces the current draw by 20%, the panels will be underloaded and their voltage may rise about 10%, for a net 10% loss in power. I have sized my new system so that this situation of "power plateau" occurs for an hour or so on the best solar days of the year. I can see the voltage rise above its MPPT point and the current drop below its MPPT point. Buying the next larger size inverter was not cost effective.
Interesting. I have 360W (DC) panels with micro-inverters that max out at 320W (AC). I've noticed that some of the panels are now reaching the maximum 320W by roughly 10:30am, well before solar noon (~ 1pm). Even in mid March, I was seeing panels reach the max of 320W. This makes me wonder if perhaps the micro-inverters are a bit under-sized. I have a first year guarantee on energy production, and I would not be surprised if my installer ends up having to add a panel or two to attain the guaranteed production.

tbleakne said:
So in the case you consider, the surplus PV generation could be reduced to zero. The inverter would have to track the dynamic load of the house and adjust its output accordingly, rather than just optimize the output of the solar array.
I would imagine that micro-inverters could do the same thing, but I don't know.
 
You'll almost never see panels outputting their rated power, inverters are often undersized for cost reasons, and generally some healthy clipping isn't going to cause any significant loss in production overall. Over time panels degrade and become dirty, so clipping become even less of an issue. Having said that my inverter never clips as its oversized, but the cost was about the same and my electric panel could take it, so I didn't see a good reason to undersize.
 
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