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The Rise of the Personal Power Plant

Discussions of conventional and alternative energy production technologies.

Re: The Rise of the Personal Power Plant

Unread postby GHung » Mon 02 Apr 2018, 10:43:02

As I've stated before, IMO, the Powerwall's high voltage greatly limits its versatility. It requires long strings of PV, and upgrading PV output essentially requires another full string of nearly identical panels, as I understand it. I'm free to mix and match my PV as long as I keep my strings below 150 volts Vmp. That's the advantage of keeping charge control separate from DC to AC inversion. Cost a bit more? Yep. Much more versatile and resilient? Absolutely. Only when Tesla markets a Powerwall in the 100 volt range will I even consider such. And I skipped the part where high voltage DC is a very squirrelly thing. A system like mine, with a good hybrid inverter, can be grid-interactive, buying and selling as I choose via fairly simple programming.

Edit: I see the Powerwall2 is available in lower voltages. Hopefully Baha will set me straight. What is the DIY cost of a 13.5 kWh unit these days?

Meanwhile I'm having fun salvaging used 18650 Li-Ion battery cells to upgrade my portable power unit which can supply anything from USB power up to 24 V and 150 watts of AC. Primarily meant to charge USB devices and charge/run 12 volt gizmos. It's a bit astounding how many lithium cells get discarded when they are perfectly fine. If the battery management system in these devices detects just one cell that is questionable, it'll render the entire battery pack inoperable (won't let it charge). Sucks to know those expensive 18 volt power tool battery packs you bought are perfectly fine if one knows how to revive them.

I disassemble the battery packs (mainly from laptops and power tools), test the cells for internal resistance and capacity, and match them into strings for higher voltage to arranged into power banks. My power bank can be charged with PV or AC power. Battery system management systems (for safety and balancing cells) can be had from Ebay ridiculously cheap these days since virtually every lithium-powered device has one. For $5 I got a little unit that can manage, balance, and display data on up to 7 strings of cells. I built my system so that individual cells can be replaced easily, and HD and Lowes provide a constant supply of high quality cells if one wants to dig through the recycle bin. Cells that don't test up to their specs get returned to the bin. Reduce/Reuse/Recycle can be such fun. Eventually I may build my own lithium powerwall.
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Re: The Rise of the Personal Power Plant

Unread postby KaiserJeep » Mon 02 Apr 2018, 15:39:40

George, the Powerwall 2 is essentially an AC backup for a house which runs on 100% AC grid power at 120/240VAC. Your DC systems are by contrast optimal for a DIYer who implemented a piecemeal system over a number of years. They are not especially compatible, even though in the very long term the Powerwall 2 would be cheaper and a whole lot less maintenance.

My off-the-shelf system (installed by the #1 US roofing contractor and solar installer PetersenDean) is by contrast very compatible with Powerwall 2, because I have two strings of nine 24VDC panels and a two-leg "Sunny Boy" inverter that produces 110/220VAC at about 13 amps, feeding my main power panel through a 20 amp 2-pole breaker. However in my case the regulatory environment in California is so very favorable for a grid-attached system that no case can be made for adding a Powerwall 2. I am already using the power grid as a 100% efficient battery with zero maintenance. The state-mandated "net metering" has PG&E buying my excess power at the same peak retail rate they sell it to me, because my power is produced at the same time that California sun maximizes the A/C burden. In fact I just retired my 1hp pool pump when I removed the pool, so my system will produce a lot more excess power this year than last. This means that in 2018 my electrical surplus will more than ever before be reducing my natural gas charges in the same power bill.

Things will be different at my Nantucket residence. The weather in New England is not very much like California, in fact they call the area "New England" for a reason. Electricity is relatively costly, even after the diesel power plants I remember were replaced by a mainland power feed. However, because it is 30 miles from Cape Cod and the land is flat, the island has a place on NOAA's "Top Ten Wind Power Sites". I am still reflecting upon the alternatives, but a Powerwall 2 might be economically justified, depending upon whether or not "Net Metering" is mandated.

The main consideration is the regulatory environment, versus technical design alternatives. In your case, consider using panels with AC microinverters for your future power expansions. This will allow maximum flexibility going forward.
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Re: The Rise of the Personal Power Plant

Unread postby GHung » Mon 02 Apr 2018, 19:11:45

baha wrote:Hi Ghung,
The DC coupled PW 1.0 had a 48vdc battery bank with a DC to DC converter to make 400 volts for the AC inverter. There was indeed a 400vdc interface that had to be hooked up. That makes anybody nervous :) especially inspectors (electricians get real uptight about high-voltage DC). Even though it is dead when you hook it up.

But the AC coupled PW 2.0 does not have a DC interface. All the DC is contained within the sealed box. The only connection is 120/240 split phase AC. This simplifies the permit and inspection. Tesla does not advertise the internal DC specs. That is none of my or your business. It just works...no fire yet :) I will assume it is 48 volts.

Think of the Powerwall as a 240 volt AC battery. Add an AC PV inverter and I can sell my excess to the grid.

Unfortunately there is no DIY price for the Powerwall. Tesla says it must be installed by a trained and certified installer. If someone buys one on-line, it comes to our warehouse and is installed by me...can you say job security :)

I would enjoy making my own battery pack. I just don't have time. Just like everything else...you can have extra time or extra money, but not both :(

KJ - California is the reason PV panels are rated at 72f. Doesn't that get boring? You are in for a culture and climate shock moving to New England :)


I knew a new Powerwall was not an option because the one thing I insist on is a "right to repair and install". I won't buy things I'm prohibited (by contract, agreement, or fine print) from installing and maintaining,,,, period. That's why I've never owned, and never will own, an Apple product. On the other hand, I found a guy here in NC that upgrades Tesla battery packs and sells low mileage Model S packs (24 volt) for a good price. For around $5K I can get four and have a 20 kWh+ battery set that should last a while.

Nothing against you guys and your great work. I just like to over-engineer everything which frustrates the crap out of "professionals". My former building inspector who is now my insurance guy saw how I do things 20 years ago. He stopped by the other day for his 3 year policy check and made fun of me. Said he worries about our place less than any of his customers. Says it'll take an EF-5 to bring this place down.

As for KJ and his "piecemeal" comment, I haven't had a single system failure in over 21 years, and only one planned power outage to swap out batteries. Nuf said about that.

BTW: I pulled one of my first PV panels from the greenhouse this morning for cleaning and testing. Hooked it to a spare MPPT contoller and a well-discharged deep cycle battery. Still producing 77 watts, and it's a 75 watt panel which has been in constant service since October 1994. It's a fairly cool day here and my solar irradiance meter was reading 1020 watts/m2. Right on spec after 23+ years.
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Re: The Rise of the Personal Power Plant

Unread postby KaiserJeep » Mon 02 Apr 2018, 20:41:39

Nobody ever claimed that solar PV wears out in an electrical sense. What usually happens is that the protective films cloud and delaminate, the wire insulation deteriorates in sun and weather, and the aluminum frames corrode.

I implied no criticism of your system, by the way. If you intend to remain off grid forever, then the afore-mentioned "piecemeal" approach and seperate DC components are optimal as long as a knowledgeable person is present to tweek and repair such a system. But if you intend to convert at any future time to a grid-attached system, or even sell your house to a non-solar enthusiast, then integral microinverter panels are a good choice. For example I would choose those today to add capacity to my own DC to AC inverter grid-tied system, which is entirely managed and operated by PG&E at zero cost to me. But I don't ever forsee any Powerwalls here in California for grid-attached systems, unless they change the compensation scheme for grid-attached solar PV.
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Re: The Rise of the Personal Power Plant

Unread postby GHung » Wed 04 Apr 2018, 09:11:13

Yeah, Baha, I've never been a big fan of micro-inverters. Fun to play with, but not for battery-based systems. Where is the sense in inverting DC to AC to be converted back to DC to be inverted to AC for household use. One could make the case for a purely grid-tied system or for very long wire runs, but where's the fun in that?

As I said, I'm also not a big fan of higher voltage DC. My arrays top out at around 90 volts, mainly because my inverters are still 24 volt (48 volts was rare when I originally built the system) , my battery bank is made up of 12 big 2 volt cells (would require 24 cells for 48 volt) and a couple of my arrays are 10 panels (would be around 115-120 volts in two strings of 5). My 5 arrays run from about 36 volts to about 90 Voc, which has been working fine on my MPPT controllers. If it ain't broke ..... and there weren't many panels available over 12 volt nominal for early adopters like me back in the day.

Also, systems over 48 volts were subject to a lot more scrutiny by inspectors in the late 90s. Under 48 volts, they were largely ignored on the DC side. Inspectors didn't really know what they were dealing with so they just asked; "Is it below 48 volts?"....
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Re: The Rise of the Personal Power Plant

Unread postby KaiserJeep » Wed 04 Apr 2018, 13:04:34

Well, for my own on-grid situation, micro-inverter panels make sense because they can be added to the remainder of my roof space and will still produce usable power. I don't have unobstructed roof space to add even one more string of nine 24-volt panels to the two I have, because at least one panel in such a new series string would be shaded by chimney, trees, or nearby houses. The individual microinverter panels would go on and offline as the shade changed throughout the day - and there isn't any space for anything else, even a single tracking array on a pole would intrude in my tiny backyard.

It's an academic consideration anyways. Since I retired the swimming pool pump, I have lots of excess power production, which my neighbors without solar roofs are effectively consuming. The net metering benefits me and it's a wash for PG&E, who are reselling my power at the same retail rate. They do benefit in the sense that as the neighborhood ages and more people add A/C and hot tubs and EVs, they don't have to do an infrastructure upgrade. That would not be cheap in my 35-year-old neighborhood, because all the utilities are underground. Although I was the first to get solar PV, there are now 8 solar homes out of the 38 in our little homeowners association. Additionally, two houses have domestic hot water heating and I know at least one neighbor has a solar pool heater on the back fence of his backyard.

Frankly, with a strong net metering environment like we have in California, Musk won't be selling too many Powerwalls. I am speculating that is the reason he chose Australia as his first major Powerwall and solar roof market - it was a friendly regulatory environment that was still a fairly large sized market.
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Re: The Rise of the Personal Power Plant

Unread postby Outcast_Searcher » Thu 05 Apr 2018, 14:06:54

baha wrote:Enphase used to pay installers for replacing units $110. They stopped doing that because it was out of hand. We don't install them anymore...

This reminds me of the shops which used to build custom PC's locally (good old days, like the 90's). They would show me shelves full of bad motherboards which the Asian makers had refused to honor the warranty on.

It's hard to imagine a company just refusing to do the right thing on parts with a bad reputation, UNLESS its a part of a going-out-of-business (at least in that line of business) plan.
Given the track record of the perma-doomer blogs, I wouldn't bet a fast crash doomer's money on their predictions.
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Re: The Rise of the Personal Power Plant

Unread postby KaiserJeep » Thu 05 Apr 2018, 16:46:39

Outcast_Searcher wrote:
baha wrote:Enphase used to pay installers for replacing units $110. They stopped doing that because it was out of hand. We don't install them anymore...

This reminds me of the shops which used to build custom PC's locally (good old days, like the 90's). They would show me shelves full of bad motherboards which the Asian makers had refused to honor the warranty on.

It's hard to imagine a company just refusing to do the right thing on parts with a bad reputation, UNLESS its a part of a going-out-of-business (at least in that line of business) plan.


You have never had experience with Chinese contract manufacturers, it would seem. When we had a quality issue with one particular manufacturer that they did not seem willing to address, we switched to an alternate and pre-qualified source. Then bad hardware without valid serial numbers showed up at a major customer site, made by the first (now disqualified) manufacturer. They were back-dooring the stuff to a major stock exchange customer. We refused to service machines that contained any parts we had not blessed via our test process, and things got very ugly for a while, the stock exchange held up add-on orders for one quarter and the District Sales Manager blamed Engineering's "unreasonable attitude" for the shortfall.

THAT is the reality of doing business in China. We would LOVE to have had US manufacturers respond to our RFP's, but the only venders that would sign contracts were Chinese. We knew we would have quality and scheduling issues and we did in fact have them. Meanwhile back in the USA the company that had absorbed mine had laid off our entire former manufacturing organization, there were no alternatives.

Then there was a huge language issue as well. TRY to find an interpreter that can adequately explain Engineering level technical details. Try to examine source control documentation written in Chinese. (For all I knew, they had provided an Egg Foo Yung recipe and called it an SCD.)

You can only beat your head against the brick wall for so long, then it begins to hurt, and when you have 38 years on the job and are qualified for early retirement, you take them up on it.
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Re: The Rise of the Personal Power Plant

Unread postby EdwinSm » Sat 02 Jun 2018, 06:56:31

2nd phase of my solar installation has been ordered. The first was a ground heat exchange (basically using stored solar heat in rocks during winter). Now a few pv panels (10) are going on the roof, in a standard grid-tide system.

To go for a battery backup for just solar in a latitude equal to south Alaska, it would need to hold 4-6 months worth of power. The adverts for solar power here warn of 0 kWh production in December and January! Wind would be a good addition if my smallish property was not at the base of a wooded ridge, so the flow of wind over the property is limited. At least we are sheltered from the cold northerly winds.

To go off grid year round in this area the following components seemed to be needed: wood heating (not enough land so would need to buy wood), solar and wind power and a back up generator.

Given current electricity prices (we have some of the lowest in Europe) the economics of adding solar panels is decidedly suspect (ie payback in 28 years!), but this is a sort of hedge against rising prices and the banks here have basically stopped giving interest on savings so the money saved for retirement might be used now to lower costs later.
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Re: The Rise of the Personal Power Plant

Unread postby Newfie » Mon 04 Jun 2018, 21:39:08

We have a 400 watt wind generator. When we lived in a marina at about 42°N it was pretty useless. I considered dumping it. Now that we are living in the trade winds it is doing very well and produces 24x7.

Wind speed drops of sharply when you are on land.
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Re: The Rise of the Personal Power Plant

Unread postby KaiserJeep » Thu 09 Aug 2018, 15:01:50

baha, what would be the installed, full retail price for a Powerwall 2 installed at an existing PV site by your company?
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Re: The Rise of the Personal Power Plant

Unread postby KaiserJeep » Fri 10 Aug 2018, 16:02:07

Your figures are obsolete, from the era where lithium batteries were used in cell phones and laptops and frequently combusted. We don't actually know the lifetime of a current technology lithium battery, but we have some from an earlier design generation that are more than 10 years old and still have better than 80% capacity. Not to mention that premium Tesla Model S BEVs show better than 90% capacity after 150,000 miles and 10 years.

The secret is to have battery management software that monitors each and every cell individually for charge and temperature. Tesla has been tweeking this software for a decade, ever since the Tesla Roadster. The phone in your pocket is a lot more dangerous, some venders think that a single monitor per battery (vs. per cell) is sufficient. They are wrong.
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