GregH
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Re: Official Southern California Edison thread

Wed Apr 26, 2017 5:47 pm

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.
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GregH
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Re: Official Southern California Edison thread

Thu Apr 27, 2017 8:50 am

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.
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tbleakne
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Re: Official Southern California Edison thread

Fri Apr 28, 2017 2:30 pm

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.
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GregH
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Re: Official Southern California Edison thread

Fri Apr 28, 2017 4:44 pm

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?
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abasile
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Re: Official Southern California Edison thread

Fri Apr 28, 2017 8:48 pm

tbleakne wrote: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.
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tbleakne
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Re: Official Southern California Edison thread

Sat Apr 29, 2017 4:26 pm

abasile wrote: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.
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abasile
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Re: Official Southern California Edison thread

Sat Apr 29, 2017 5:07 pm

tbleakne wrote: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.
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tbleakne
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Re: Official Southern California Edison thread

Sat Apr 29, 2017 10:16 pm

abasile wrote: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.
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Re: Official Southern California Edison thread

Sun Apr 30, 2017 7:14 am

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 wrote: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 wrote: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.
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Valdemar
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Re: Official Southern California Edison thread

Sun Apr 30, 2017 6:44 pm

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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