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Looking for a solar-battery-net metering sanity check
#11
(06-22-2019, 06:42 PM)Korishan Wrote: You would want a Hybrid Inverter. It charges from the grid and/or solar. You can usually adjust the preferences for each. And if you loads are higher than what the battery/solar can supply, it'll auto pull from the grid the remaining power needed.

You never pull 100% from the battery unless there is no charging happening at all, or you are pulling more than what solar and battery can supply (so at night, you always pull 100% if not grid connected)

Longevity of cells (the battery is full assembly) varies depending on how much current you pull from them on a regular basis and what the voltage range you run at is. You can get 1000's of cycles if you only use the cells between 3.2V - 4.1V. But going from 2.9V - 4.2V you drop down to only 100's of cycles.

Please the read the FAQ, it is located on the main page. Since you are new to this, there are probably a lot of questions that are already answered there.


Deleted your other thread as it was a duplicate and this one is in the right location.

Thanks, Korishan. I'll look at hybrid inverters. Are there any brand names that jump out at you? 

Based on your response, my gut tells me that to reduce my demand charge, I would need to have some piece of electrical gear between the meter and the battery that would not limit energy in the direction of the utility (this would be the solar energy left over after charging the battery to 100% that would then be net metered for later use), but that would limit the amps coming from the grid (this would be the grid energy that would be used to charge the battery when no solar--but this is also the energy that I would have banked via net metering, while also being the energy that I need to limit coming back into the system due to the demand charge). Does anyone know what this type of electrical gear might be called? It seems like for me that this is the key piece of gear.

On the longevity of the cells, how would one design a system to maintain voltage between 3.2-4.1V rather than 2.9-4.2V? Sorry if this is a blatant newbie question, but I'm trying to get my head wrapped around this...

Thanks a ton for your time, Korishan. I really appreciate it.

(06-22-2019, 07:28 PM)Sean Wrote: 87kWh a day is rather a lot - do you have lots of either roof or ground space available for panels, and do you have permission to install them, as you'll need a fair number in order to put a dent in that daily usage.

Yep; solar system size is currently planned as 19.6 kW-DC. We have plenty of space here. I own an energy company so I can install them at cost.

(06-22-2019, 09:00 PM)Korishan Wrote: Agreed, 87kWh is a lot!  That's almost 3x what I use. Altho I only run the A/C down to 74F, and only at night. During the day it's set at 78F. I only don't use electric range. My electric water heater only runs twice a day for 2 hours each.

aventeren: What are your loads? Is there a possible way of lowering your load requirements?

I touched on a bit more of my data in an earlier post, but basically the house was set up with electric heating, which we actually REALLY liked prior to Northwestern Energy changing the net metering rules to include a demand charge. We could convert a portion of the heat to natural gas, but then we aren't doing our part for our 2 kids on the climate side--so we're exploring different ways to skin this cat rather than just burn more natural gas and buy less electricity.

We might be able to time shift our energy use around bit, but what we are finding out about this house is that the VAST majority of the electric bill is from the electric heat.

(06-22-2019, 09:12 PM)OffGridInTheCity Wrote: Reposting on this thread per request (forgive me if its wrong one Smile).

AIMS Pure Sine Wave Inverters allow for grid charging of batteries and transfer switch to run off the batteries. The AIMS has an adjustment to set the amps of charging from max to min. There are many factors here and I'm not pushing AIMS, just pointing out that a charge/transfer-switch/inverter unit might be a way to go. You can still have a MPPT controller to charge batteries from PV array at same time the AIMS does.

Right on, thanks. I'll look into this. Being able to set the amps of charging from a max to min is exactly what I'm looking for--so long as I would be able to effectively limit the grid charging while not limiting the solar charging. That's the technical question I am trying to solve. Grid charging comes with a demand charge, so I'm trying to limit the grid energy coming back to the system--while having no limitation on the solar energy side (both solar energy charging the battery and solar energy flowing into the grid).

(06-22-2019, 10:19 PM)billvon Wrote:
(06-22-2019, 06:11 PM)aventeren Wrote: So I guess here are my questions:

1. Do grid charge controllers exist that would allow us to essentially trickle charge the battery in a manually adjustable fashion that would allow us to minimize our demand charge?

2. Are there any issues with pulling primarily from the battery 100% of the time? Longevity, degradation, practicality, safety, etc

3. If I was to use Jehu’s 18650 Lithium Ion battery-based powerwall (likely in a 48V configuration), what is the likely life of these batteries? The batteries are where the cost will be. How long will they likely last in the sort of operational configuration?

I’m sure I will have more questions as I continue to plod along here, but I’d like to start with getting some feedback on the above questions to help me build a foundation. 

Thanks to all for your time and thought and experience and willingness to pitch in and help out. I REALLY appreciate it.

Here is some more info:

Average Monthly kWh use: 2,655 kWh/mo
Average Daily kWh use: 87.3 kWh/24-hr day
Average Daily Wh use: 87,287.7 Wh/24-hr day
Average Daily Ah use: 727.4/24-hr day

I'll post up what I'm thinking on system size after I've done some high level calcs.

1) No, charge controllers don't work like that.  They charge the battery until it's full then taper charge.  They don't care about demand charge (nor should they.)

2) Sure, you can pull 100% from the battery - just shut down the AC input and the solar charge controller.  But why on earth would you do that?

3) Using a standard 18650 you'll get about 500 cycles.  That's about a year and a half at full depth cycling.

Here's what you probably want to do:

1) Get a big hybrid inverter like a Radian.  Add storage (48V) and a solar charge controller.

2) Set the inverter to "mini grid" mode - this will use AC from the utility only when needed to charge the battery.

Be aware that you are looking at something like $100K for this system.  (You are going to need something like 20kw of solar, and battery backed solar runs about $4-5/watt without batteries.)  Also be aware that if you are building the pack yourself you are going to need around 25,000 cells.

Thanks; so when you say that the 18650 cells have a life of 500 cycles, where did you get that number--and how did you determine that it would be 1-1.5 yrs of full depth cycling? What's your high level math there? Having to replace 25,000 18650 cells every 1-1.5 yrs obviously is a deal killer.

I'll look into the hybrid inverters--and I'll check out the Radians. Thanks

I have the solar piece covered. I'm just looking to better understand the battery side at this point. I'm first trying to understand the design elements--which will then inform cost and payback.

(06-23-2019, 01:35 AM)OffGridInTheCity Wrote:
(06-22-2019, 10:19 PM)aventeren Wrote: Here is some more info:

Average Monthly kWh use: 2,655 kWh/mo
Average Daily kWh use: 87.3 kWh/24-hr day
Average Daily Wh use: 87,287.7 Wh/24-hr day
Average Daily Ah use: 727.4/24-hr day

I'll post up what I'm thinking on system size after I've done some high level calcs.
I have an operating system that produces 10,000 kwh / year off-grid.  That's 833/mon which is *1/3 of your goal*.   
To produce 10,000kwh/year I have
- 24 panels that total 6,885kwh
- 2 x Midnite Solar Charge Controllers
- 40kwh 18650 Battery Bank (5,340 cells)
- AIMS 12,000 watt Inverter - e.g. 240v@50amps
- Several automatic transfer switches and a couple of APC UPSs to feed it all into the house
Cost - US Tax Credit brings the bill to $26K.   My electricity is .12 = $1,200 savings/year = 22yr ROI  and that doesn't include replacing equipment/batteries over 22yrs.

SO - I agree that with previous post that you're talking over $100K outlay.  

Its important to note that the size your talking about - e.g. 72panels + 36,000watt Inverter + x, y, z - is quite large in todays home solar.  You might have non-technical issues such as neighbors / city that object to 72panels.   Bottom line - its a significant undertaking to do the scale and quite serious level of power you list and you might do well to get some professional consultations.
Great info, thanks.

(06-23-2019, 01:49 AM)ajw22 Wrote: Are you sure it'll apply to you?
"NorthWestern spokeswoman Jo Dee Black said the new charge would ensure net-metering customers pay their fair share of service costs. Customers with solar panels already in place would be exempted."
https://www.apnews.com/f5e7fbca9e0c4bd1b4cc6b0236655585

Your 87kWh daily use is rather high.  A review of what's using power _AND WHEN_ will be crucial to plan a battery system.  
Shifting your loads (eg. hot water system) to run only when you have solar power will significantly reduce the required battery size.  
And most conservation measures will likely be much cheaper than a battery system.  
From my experience, replacing small electric space/oil heaters with heat pump systems made a huge difference.
Converting night time lighting to LED had the biggest bang for bucks&effort.

Yeah, I've been involved with the rate case.

Given that we use electric heat in the winter--and electric heat is where the vast majority of our electrical use is--I don't think we'll be able to time shift that energy use. We could convert it to natural gas, but we have 2 kids that we are trying to do good by.
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#12
There's a lot there, so I'll just skim answer:

Yes, AIMS is a good brand. There's a few others too. Just do a search on the forum for hybrid inverters, and you'll get lots of hits.

500 cycles is based on full cycle per day, soo that'd be 500 days, or about 1.5 years Wink

The inverter can usually be set to how low it will discharge. By setting the discharge from anything lower than 3.2 to 3.2V, you've changed the bottom end that the inverter will discharge to. The same goes for the charger(-half) by setting the upper limit in the settings to 4.1V instead of 4.2V.
Altho, this is done with a little bit of math involved because you don't set "3.2V" and "4.1V", but instead you'd set it to 44.8V (3.2V * 14s) and 57.4V (4.1V * 14s).

You own a power company? I think you need to enlighten us a little bit on that one Wink in another thread, of course. I'm curious, and I'm sure others are too. Also along with that, your location/country-state?

Solar can directly heat your water and heat your house (if using radiant heating or something similar). So this load could actually potentially be removed from the battery/inverter requirements.
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#13
If you use electric to heat your house, check out the Electrodacus DMMPT450. It uses solar panels to directly heat a thermal mass, completely bypassing batteries.

If you still want to go the battery route, go for LiFePo4, it can last many more cycles than the 18650s can. Most of us use 18650s because we get them for free or dirt cheap, but if you are paying retail, LiFePo4 is probably your best bet.

Also, what kind of electric heat do you have? Heat pump or resistive? How many kW is it rated for? Aims UL inverters only come in 8kW and 12kW versions, but they do make a 30kW and 50kW inverter that runs on 384V DC. Aims is kind of a medium brand, better than the no-name stuff, but not as good as Outback, Xantrex, Schneider, Magnasine, etc. What's your budget?

If you want to do some high-level math, look into commercial peak-shaving. How you do that will depend on if your demand charge is based on Time of Use or other factors.

By energy company, I'm assuming solar installer or solar lessor?

Lastly, and please don't take offense at this, your heart is in the right place, but your kids will grow up in a world of climate change. Your best bet is not to prevent or deny it, but rather prepare your kids to deal with it. If you still think you can stop it, come to my neighborhood and tell all my neighbors not to idle their cars for 30 minutes while picking up their kids from the school bus stop 100 feet from their house and ask them why they all have 3 overflowing 96-gallon garbage cans every week.
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#14
(06-24-2019, 03:09 PM)aventeren Wrote: Thanks; so when you say that the 18650 cells have a life of 500 cycles, where did you get that number--and how did you determine that it would be 1-1.5 yrs of full depth cycling? What's your high level math there? Having to replace 25,000 18650 cells every 1-1.5 yrs obviously is a deal killer.
An NCR18650B (common 18650 cell) is down to 66% capacity after 500 full cycles.  You can get more cycles by keeping cells cool, not charging them fully, charging them more slowly and not discharging them as deeply - but then to get the same capacity you need more cells, so that's not necessarily a win.
Also, since you are using used cells, they have part of their life gone already, so assuming 300 cycles is more reasonable.  (About a year.)
In general it's a mistake to think this is going to save you money.  It's a great hobby, but doesn't make economic sense.  Used batteries save $$$ but take a lot of labor, and your time is worth something.
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#15
(06-26-2019, 06:18 PM)billvon Wrote: In general it's a mistake to think this is going to save you money.  It's a great hobby, but doesn't make economic sense.  Used batteries save $$$ but take a lot of labor, and your time is worth something.

Depends on what your time would of been doing if not shelling cells. If it was sitting in front of a TV being a couch potato, I think the time is irrelevant. If the time is spent "while" watching TV you'd do anyways, again, it's irrelevant. If the time is "keeping" you from doing something else productive, then yes, this time now costs you something.
This is how "I" view the whole "your time is worth something, so it's not as cheap as it really seems" remarks.

Working on your own vehicle also costs you time, but ppl who know how to do it do it willingly because they'd rather not give the hard earned money from work to someone else. Same principle here.
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#16
(06-26-2019, 07:25 PM)Korishan Wrote:
(06-26-2019, 06:18 PM)billvon Wrote: In general it's a mistake to think this is going to save you money.  It's a great hobby, but doesn't make economic sense.  Used batteries save $$$ but take a lot of labor, and your time is worth something.

Depends on what your time would of been doing if not shelling cells. If it was sitting in front of a TV being a couch potato, I think the time is irrelevant. If the time is spent "while" watching TV you'd do anyways, again, it's irrelevant. If the time is "keeping" you from doing something else productive, then yes, this time now costs you something.
This is how "I" view the whole "your time is worth something, so it's not as cheap as it really seems" remarks.

Working on your own vehicle also costs you time, but ppl who know how to do it do it willingly because they'd rather not give the hard earned money from work to someone else. Same principle here.

I'm starting to realize that my 5,000 cell 18650 battery bank is not going to live up to Battery University's 6,000 cycles shown in one of their charts.    I agree that I had plenty of TV time on my hands made this 1st battery cost effective in terms of my personal time but on the other hand I truely intend to produce power for rest of my life..   so in 2 or 5 years, even at 40% DOD, I'm going to have failing packs all over the place.  Yikes.   I have significant $ into it already.   Not sure how to transition to LifePO4 easily.    Maybe hope for LifePO4 prices to drop in next 4 years?
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#17
At some point, sooner or later, folks wanting to build walls of a reasonable capacity will realize that using small format cells of unknown age is rather a futile, time consuming exercise.

Building low capacity systems using questionable and limited longevity as a learning project is sensible,.

Building very large capacity systems, using unknown small format cells, and expecting it to be productive for numerous years is daft.

Folks expecting to build energy storage systems for very little cost, and expecting to see a quick return on investment need to be prepared to be disappointed.
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#18
Economically ideal, the battery size only needs to be large enough to catch around 60% of the solar excess above your base solar output and to fully cycle it at least daily. Economics rule out typical 18650 cells because they do not have enough cycle life if used anywhere near the rated capacity. Larger the battery, less it is actually used. Depends on latitude as to solar profile as well. Sizing a battery to catch every kWh on a sunny day mid summer is not always economically practical.

If your demand capping then the battery specification also needs to take into account the maximum demand out of hours (i.e. maximum inverter draw).

Potentially if the demand is relatively flat and you can apply heat storage (water or rock) you may be able to avoid a battery altogether and is much more reliable and cheaper.

Separately, if the demand is high (kw's) but low in duration (minutes) then a small high cycle battery pack may work better to cap out demand.

Charging, separate flat rate chargers may work better than an integrated inverter charger because the integrated chargers can be less efficient due to design compromises or shortcuts (multiple Flatpack2 units may prove more viable in the long term).
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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