What is the most efficient way to handle occasional high loads

Peter_V

Brief background, I have a grid tied solar array that produces more power annually than I use.  However, my power coop is introducing a new rate schedule for 'solar' customers that will make my array basically pointless.  I'll end up saving about $5 a month over not having any array.

So I plan on taking my house off-grid.  I have a power monitoring system so I know that most of the time my total household draw is less than 2,000 watts.  However, I have three high draw appliances.  The electric oven is used maybe once or twice a month.  We also have an electric dryer, but it is modified to draw hot air from the attic, so except for perhaps 3-4 months a year it's power draw is minimal.
The last high draw 'appliance' is our Chevy Volt.  Sometimes it's charged daily, other times it might go 3-4 days without charging.  I'm building my on EVSE for it that will allow me to control the charger current and limit it (if I want) to the surplus power being generated by my array.

I can arrange it so that there will never be more than 1 high power appliance running at any given time.  This limits my maximum potential power draw to about 7kw, and even that is only for short periods of time.

So, as I see it, my options are:
A.) Buy one huge 7-8kw inverter 
B.) Buy two smaller (4kw?) inverters and stack them, have both running constantly
C.) Buy one small, 2kw, inverter that runs constantly, and one large 5-6kw inverter that all the high draw appliances connect to and leave it off except when needed.

Considering Tare loses (or whatever you want to call them), which option do you folks think offers the best efficiency?  Or is there another option I haven't considered.

Propane is not a viable option since the cost of plumbing the house for propane, buying a propane tank, and switching to propane dryer and stove is close to the cost of buying a dedicated inverter and then I'd have the reoccurring cost for propane.
Besides it wouldn't help with charging the Volt.

Estragon

Going off-grid means you lose your unlimited capacity "battery" (aka the grid), so you'll have to replace it with actual batteries. Figuring the cost of buying and periodic replacement of batteries, saving only $5/mo may not look so bad.

Anyway, to answer your question, stacking inverters in parallel (provided they're designed to do so) allows a single master inverter to power smaller loads while slave inverter(s) sleep in a low power mode until woken up to power an increased load.

BB.

Do a little research and math and figure out the costs for off grid solar vs staying on grid... Batteries are a major recurring cost too (as well as replacing electronics like charge controllers and inverters every 10+ years or so).

What you are seeing (utilities changing rate plans to make solar power almost useless for Grid Tied Systems--Usually a "high" monthly minimum connection fee of $20-$96 per month and a low kWH charge (roughly $0.03 to $0.07 per kWH).

What is the new rate plan (fixed charges, per kWH charges and solar credits, any tiered rates)? What is your monthly kWH usage?

Unless your electric bill works out to >$0.50 per kHW (actual bill/kWH used), it is very difficult to "save money" by going off grid.

You can can purchase some larger/cheap AC inverters and only run them when the oven/drier is going...

Depending on where you live--You can run into secondary issues with leaving the grid:

• In some areas, you are charged to leave the grid (it is in our plans, not sure it is has ever been charged)
• some places will "condemn" / red tag a property that does not have electrical service
• if you are "at the end" of a road--It is possible that the utility will pull the poles and distribution wiring--Which can mean very high costs for somebody that buys your place later--And reduce your sale price/value of the property.
• Getting "old" on property and having to maintain your own off grid power system (spouse, family, renters, etc.).
  

-Bill

Peter_V

For the sake of argument, let's just pretend that I'm not a complete idiot and that I've already done the math and decided that in the long run going off-grid is more cost effective for me.

The question at hand is about the selecting the best invetrter or combination of inverters for proposed situation, not the merits of going off grid.

MrM1

Peter_V said:

For the sake of argument, let's just pretend that I'm not a complete idiot and that I've already done the math and decided that in the long run going off-grid is more cost effective for me.

The question at hand is about the selecting the best invetrter or combination of inverters for proposed situation, not the merits of going off grid.

Then I dont think your choices are many.   Schneider SW or XW series,  Outback (long warranty), or something in the Magnum Family.  Might be one or 2 others.  Also ... Still Hush Hush on release dates, the fabled  Midnite Solar B-17 series looks really cool with a rack that will allow you to plug and play Inverters and Charge Controllers, hot swap and stack, etc. I also read they are working on an inverter that will exceed 2kW for the plug in inverters and a plug-in expandable Lithium battery pack system to go with the B17.

Whatever you get,  to replace the grid will probably take at least 2 inverters or more.  So keep that in mind when purchasing.

mcgivor

If serious about going off grid to resemble the grid, the equipment should be professional grade, an integrated system where everything communicates with one another, it makes for a more  manageable situation, with less guesswork. Such equipment allows higher surge capacity, stackable inverters, split phase, AC coupling, DC coupling, the list goes on. To add to the aforementioned suggestions, SMA is another manufacturer worth looking at,  if available in your area.

When considering off grid seriously the tare losses are just a fact of life and part of the costs. Naturally we all want the most efficient equipment but most are within a few percentage points of one another, service availability is an important factor, global companies tend to have readily available replacement parts, so much to ponder  

Estragon

There is really no single "best inverter or combination" for every application. If serious about going off grid, the first step is a fairly thorough calculation of your loads in terms of total power (watt-hours per day), and instant (peak wattage).

DanS26

Probably more important than your inverter decisions will be your backup generator decisions.

Photowhit

I'm curious where this is, I would hope you would be given a time period to make an adjustment.

Sorry for the hard change. Already having some of the elements of an off grid system changes the end cost. I was recently forced to move and if I didn't have some of the elements it would have been more cost efficient for me to just buy from the grid. I suspect you are in a very sunny state, I've heard of these hard changes being made in Nevada and Arizona, so you have more sun light available so more productive solar array.

Peter_V

FWIW  I live in southern AZ where we get lots of sun.  My array is on trackers, because at the time that made sense (they don't now because of cost).
I don't need to 'calculate' my power consumption because I have an Brultech ECM-1240 that measures it for me.  So I know that typically my minimum load is around 120watts.  Without the high draw appliances listed above my power consumption rarely goes over 1500 watts and never goes over 3000, except when the Oven, Dryer or Volt charger is running.

Anyway, I was just reading up on AC coupling, Something like the Magnum MS4448PAE and my existing Enphase inverters sounds like the perfect solution to me.

Dave Angelini

Pretending, ok Schneider Electric followed closely by Outback. I use them both in over 150 offgrid homes.
If I was not pretending I would say you should look into self consumption that both of the companies offer, and pay close attention to what Bill wrote at the end of his post.

BB.

What is your kWH per day you are looking to support? Different loads between winter and summer (A/C, heating, water pumping)?

-Bill

Peter_V

Typically around 20kWh during the summer and around 20-25kwh during the winter.  In the past year I have had 35 days where total consumption was over 25kwh, and 10 days where it was over 30kwh including 3 days where it was over 35kwh, hitting a max of ~36kwh 1 time.

I've written some code that takes the output of the ECM-1240 on a minute by minute basis, and then calculates minimum battery SOC, etc. based on assumed charging efficiency, current production, current consumption, etc.  and allows me to set the pack capacity, array size (to calculate effects of a larger array), SOC where a generator will turn on, etc. and the tell me what the minimum SOC the battery pack hit, how often the generator ran, etc.

So, for example.  If I keep my array the same size it currently is, and go off grid with a 30kWh battery bank, and start up the generator when ever the battery drops to 20% SOC, and the generator runs until it brings the battery up to 70% SOC, I would have had to run the generator 10 times over the past year.  If I add 6 more panels, I would only have had to run it twice.  WIthout adding panels I'd need to use a 65kWh battery bank to get down to only running the generator twice, it the times it did run it would have to run twice as long.

Playing around with the numbers a bit I've determined that, after you get to a certain size battery bank, it's better to add more panels than more batteries.

mike95490

Peter_V said:

For the sake of argument, let's just pretend that I'm not a complete idiot and that I've already done the math and decided that in the long run going off-grid is more cost effective for me.

Then I have to ask, are you using new math ?   The battery replacement / maintenance costs will sink any savings you have assumed.

You have to size your battery capacity to be at least 3, and sometimes 5x your daily consumption.

Lets say 3600wh daily  (3.6kwh)  4x that is 14400wh (14.4kwh)  is the size battery you need.  A good flooded battery would last 5-7 years, maybe more if you are uber careful.

 That's 300ah @ 48V,   Trojan L-16 floor sweeper batteries would cost you about  $1,800  split over 6 years, $300 yr, plus the monthly watering and maintenance

AND the backup generator, unless you NEVER get 2 or 3 cloudy days, and can save power in a crisis to not have to recharge till the sun comes out.  No cooking, bathing, heating..... 

  Not sure how you plan to do in, unless you are a hermit.

DanS26

Peter_V said:

Typically around 20kWh during the summer and around 20-25kwh during the winter.  In the past year I have had 35 days where total consumption was over 25kwh, and 10 days where it was over 30kwh including 3 days where it was over 35kwh, hitting a max of ~36kwh 1 time.

I've written some code that takes the output of the ECM-1240 on a minute by minute basis, and then calculates minimum battery SOC, etc. based on assumed charging efficiency, current production, current consumption, etc.  and allows me to set the pack capacity, array size (to calculate effects of a larger array), SOC where a generator will turn on, etc. and the tell me what the minimum SOC the battery pack hit, how often the generator ran, etc.

So, for example.  If I keep my array the same size it currently is, and go off grid with a 30kWh battery bank, and start up the generator when ever the battery drops to 20% SOC, and the generator runs until it brings the battery up to 70% SOC, I would have had to run the generator 10 times over the past year.  If I add 6 more panels, I would only have had to run it twice.  WIthout adding panels I'd need to use a 65kWh battery bank to get down to only running the generator twice, it the times it did run it would have to run twice as long.

Playing around with the numbers a bit I've determined that, after you get to a certain size battery bank, it's better to add more panels than more batteries.

Mother Nature is always analyzed in averages.....but the reality is always in the extremes.  You have to design the system for the extremes not the averages.

Your backup generator will run longer and more often than your analysis has determined.

Photowhit

Some batteries have a longer life than 5-7 years. Trojan is now claiming 10+ years for their 're' versions.

Forklift and other large traction batteries may last 15-20 years.

mike95490 said:

You have to size your battery capacity to be at least 3, and sometimes 5x your daily consumption.

Southern Arizona, where there is regularly available sun, I would shoot for a much lower. Though I suspect Winter with 20-25Kwh and shorter days will be the most difficult. 

For the record, I use 16+ KWhs in the sunny summer time here in Missouri, but only have @16 KWh battery. forutunatly our heat comes with sun. It's worked out pretty well for summer use. I don't use much in the fall and winter. But most difficult here in the fall, even though I use maybe 2.5 KWh's a day, but can have multiple days of overcast sky's. Guess I'm 7 years in on my forklift battery.

Mangas

My 13 year off grid experience.

Mainstream, "Best Practices" manufacturer using their networked Inverter, CC and AGS components with national supply chain and tech support. I use and like Schneider (my Sig).

I prefer FLA Trojans 6v, L-16 REB for the time being, scaled to 3-4 days reserve for worst case solar charging.

Panels are cheap now.  Less run time. Fixed array.

Built in extra storage reserve and surge capacity. Amortized over 10 years. Additional cost is nominal.  Stacked inverters help deliver some redundancy too.

Water cooled, 1,800 rpm genset backing everything up 24/7. I'm not concerned if ours is asked to run three or four times a year.

We run our two photovoltaic systems 24/7.  Reason for two systems, more redundancy.

DanS26

You off grid guys amaze me.......where money is no object....or where money just has to be spent.

Its a life style (voluntary or not) that has to be admired.  

Your spouses when then they said "for better or worse" really meant it.

Dave Angelini

DanS26 said:

You off grid guys amaze me.......where money is no object....or where money just has to be spent.

Its a life style (voluntary or not) that has to be admired.  

Your spouses when then they said "for better or worse" really meant it.

Actually mine said go off grid. I was the one who had to to figure it out electrically. It is thanks to technology, now very seamless!

And of course money is an object of anything we do!

If the OP here lives in a really special place he should pursue his dreams!  It has never been easier!

We go where  power lines don;t!

Peter_V

Photowhit said:

Some batteries have a longer life than 5-7 years. Trojan is now claiming 10+ years for their 're' versions.

Forklift and other large traction batteries may last 15-20 years.

mike95490 said:

You have to size your battery capacity to be at least 3, and sometimes 5x your daily consumption.

Southern Arizona, where there is regularly available sun, I would shoot for a much lower. Though I suspect Winter with 20-25Kwh and shorter days will be the most difficult. 

For the record, I use 16+ KWhs in the sunny summer time here in Missouri, but only have @16 KWh battery. forutunatly our heat comes with sun. It's worked out pretty well for summer use. I don't use much in the fall and winter. But most difficult here in the fall, even though I use maybe 2.5 KWh's a day, but can have multiple days of overcast sky's. Guess I'm 7 years in on my forklift battery.

Between the trackers and the clear skies my array averages 6 hours of insolation in Feburary,   These days I'd go with more panels instead of trackers.  Panels are cheaper and more panels produce more power during overcast weather.

Folks here are welcome to guess at how big of a battery I'll need.  Personally I find that hard data trumps guesses every time.  I measure how much I use and how much I generate.  This tells me how large a battery I'll need 99% of the time.  The other 1% is why they invented generators.
I have no intention of trying to use a battery large enough that I'd never need a generator, that would end up being be ridiculously large, like 3X to 5x my daily consumption

Dave Angelini

Tracking does one thing for you offgrid. It gives you more hours of usable power. You are on the battery later in the day and off of it earlier in the morning. If you staggered east and west arrays (virtual tracking) you get the same effect at a lower wattage than a moving array.

Dozens of Offgrid homes in the southwest that I know of do not need generators. They do not have "crazy" grid type equipment loads though.

Peter_V

It's a trade off.  If I used a propane stove, dryer and furnace, then I could probably get by with a relatively small pack and no generator.  But then I end up using propane several times a week instead of 2-3 times per year with a propane generator.  Seems to me that the propane generator is the cheaper and more environmentally friendly option.

Plus, so far we haven't been making any attempt to time shift our loads.  At this point I'm trying to collect 'worse case' data. 
By time shifting the large loads we can reduce our consumption potentially down to 5-10kwh per day during inclement weather.

I have also moved our critical loads (refrigerator, freezer, mini-split heat pump) to a separate breaker panel with a separate 'generator' inlet.  So in a pinch we could power those loads off the battery pack in the Volt for a day or two. 

However, I can't guarantee that the Volt's battery will be fully charged when the weather turns, nor can I guarantee that inclement weather will only last 2 days (we went 3 days on one occasion) so I want to have the generator option just in case. 

Anyway, right now I'm trying to refine my code to take more variables into account, which is why I'm trying to get a better handle on expect 'tare' losses.  Looking through the specs on different off-grid inverters I see this ranges from about 0.5kwh to 1kwh per day.
Plus I've found efficiency charts for a couple inverters that indicate they get really poor efficiency at light loads.  
My over-night load tends to be in the 90-400 watt range, which with a 4kw inverter might be at only 50-65% efficiency.
Which is one of the reasons I'm also considering using a smaller ~2kw inverter as the main inverter and having a second larger inverter connected but powered down until it's needed.

And yeah the trackers are nice for spreading out the power production, yesterday I had almost 9.5 hours of production.
However, with panels being so cheap these days, if I was building an array today I would got with two arrays facing east and west.  For the same money I'd get the same extended power output but much more power output on over-cast days.

Estragon

Assuming you're thinking lead acid deep cycle batteries, you may want to reconsider your algorithm which has the generator starting at 20%SOC. If you look at a cycles vs SOC chart for pretty much any such battery, you'll see that cycle life drops off considerably with deeper DOD. We usually consider a 50% minimum state of charge as the most cost effective, but it's just a trade-off to be aware of.

Also, at 20% SOC, voltage sag with load (eg pump and fridge start) on a smallish bank could end up below LBCO on the inverter, and it's lights out.

I do use a smaller inverter (12v 300w PSW) overnight so I can turn the 7kw Outback stack off. Lightly loaded, the Outback will use ~1kwh/day plus loads. If I was making my ideal inverter, it would have a small 48v version of my 12v stacked under the bigger ones, so the whole package worked automagically and at the same voltage. The little Morningstar makes an unholy screech if I forget to switch to 48v before making toast in the morning :-(

BB.

For larger systems, the low wattage loads inefficiency is usually not an issue... If you have a system that can output 10kWH per day (sunny weather), and several days of backup, vs (40 watts * 24 hours=) 960 WH of Tare Losses + 90 Watt * 10 hour loads (=900 WH) vs a 4-8 kWatt capable AC inverter--Those small loads+losses are pretty much swamped when compared with what the system is capable of, and supporting.

I would certainly like to see a small inverter that could supply something like 300-1,200 Watts, efficiently, on a 48 volt bus--Run this inverter for "normal" household loads (lights, refrigerator, electronics, washing machine, etc.) and use a second larger inverter that turns on with heavy loads (well pump, shop loads, ovens, etc.). However, small 48 VDC AC inverters are few and far between--And small inverters do not have many of the options/integration interfaces that larger (more expensive) inverters can have.

If the 960 WH worth of losses (plus or minus) is an issue, then 10% more solar panels can usually make up for these low power losses, if this is an issue.

The other issue with a "mixed load" system... The battery bank for mostly small loads (1,200 watts peak or less, and not a whole lot of hours at max load), gives you a "small" battery bank capacity/requirement. However with very heavy loads, you are looking at a large (flooded cell lead acid) battery bank to support those rare (and few hours) of loads... Roughly, a 5,000 Watt*Hour battery bank per 1,000 Watts of load, is a minimum suggested requirement (basically 100 AH @ 48 volt battery bank per 1 kWatt of AC inverter, and suggested maximum solar array size).

If this "pushes you" to a "too large" ($$$) flooded cell lead acid battery bank--Then looking at a different chemistry battery bank (AGM, LiFePO4, etc.) battery bank. Smaller because you do not need the AH/WH for storage, and smaller because these types of batteries support higher surge current. Of course, with LiFePO4 or similar Li Ion battery banks, you are a bit on the cutting edge (both supporting electronics

Then, there is always the two system approach--Smaller system for your daily loads with balanced solar array+battery bank. And then a large system AC inverter+specialized battery bank to support those less common loads.

And there are other issues... Flooded Cell Lead Acid batteries take a "long time" to recharge from deep cycles. If, for example, you plan on discharging to 50% state of charge on a daily basis (overnight usage), you will need close to 9-10 hours of charging (5 hours of 10% bulk charge, plus 4-6 hours of absorb charging at declining current levels). The fact that you are more southern and have a tracking array is a big help here.

With a 2 day @ 50% maximum discharge lead acid battery bank, that turns out to work better... The typical 25% discharge per day/night discharge only requires ~4-5 hours of "sun" to recharge. Works OK with a fixed array.

-Bill

Photowhit

BB. said:

And there are other issues... Flooded Cell Lead Acid batteries take a "long time" to recharge from deep cycles. If, for example, you plan on discharging to 50% state of charge on a daily basis (overnight usage), you will need close to 9-10 hours of charging (5 hours of 10% bulk charge, plus 4-6 hours of absorb charging at declining current levels). The fact that you are more southern and have a tracking array is a big help here.

With a 2 day @ 50% maximum discharge lead acid battery bank, that turns out to work better... The typical 25% discharge per day/night discharge only requires ~4-5 hours of "sun" to recharge. Works OK with a fixed array.

Time of day usage is important. While I say I use 16+ KWh's during the summer, most of that is while the sun is out and I discharge only 25-30% over night, if that.

Your winter time use, if you are running a furnace/heater will be more difficult. 

As you load shift, you might consider putting your water heater on a gray box and only run it during the day. 

Peter_V

Estragon said:

Assuming you're thinking lead acid deep cycle batteries, you may want to reconsider your algorithm which has the generator starting at 20%SOC. If you look at a cycles vs SOC chart for pretty much any such battery, you'll see that cycle life drops off considerably with deeper DOD. We usually consider a 50% minimum state of charge as the most cost effective, but it's just a trade-off to be aware of.

Also, at 20% SOC, voltage sag with load (eg pump and fridge start) on a smallish bank could end up below LBCO on the inverter, and it's lights out. `

I do use a smaller inverter (12v 300w PSW) overnight so I can turn the 7kw Outback stack off. Lightly loaded, the Outback will use ~1kwh/day plus loads. If I was making my ideal inverter, it would have a small 48v version of my 12v stacked under the bigger ones, so the whole package worked automagically and at the same voltage. The little Morningstar makes an unholy screech if I forget to switch to 48v before making toast in the morning :-(

I'm actually thinking about getting used EV LIIon batteries for my bank.
However, the 20% point was just an example and even with that, most days the batteries would stay above 50% SOC, in fact the majority of the time they are above 80% SOC.
Again assuming a 30kwh pack and 6 extra panels, there would be 13 times in the past year where the bank would have dropped below 50% and 4 times when it would have dropped below 30%, 3 times it would have triggered the generator.
If I did go with PbA batteries I'd probably set the generator to turn on at 30%,
I'd also time shift large power draws so they didn't happen during low battery conditions.  There was only 1 stretch last year where we had more than 2 days in a row where a 30kwh battery bank would have dropped below 50% SOC.

Oh yeah, I have a heat pump water heater, it only draws around 600 watts when it's running, and I have an Arduino controlling when it runs.  Typically during the winter time it only runs during the hottest part of the day and is setup so it draw warm air from under the roof.  It's actually one of my smaller loads, averages about 1-1.2kwh a day.

Peter_V

BB. said:

For larger systems, the low wattage loads inefficiency is usually not an issue... If you have a system that can output 10kWH per day (sunny weather), and several days of backup, vs (40 watts * 24 hours=) 960 WH of Tare Losses + 90 Watt * 10 hour loads (=900 WH) vs a 4-8 kWatt capable AC inverter--Those small loads+losses are pretty much swamped when compared with what the system is capable of, and supporting.

I would certainly like to see a small inverter that could supply something like 300-1,200 Watts, efficiently, on a 48 volt bus--Run this inverter for "normal" household loads (lights, refrigerator, electronics, washing machine, etc.) and use a second larger inverter that turns on with heavy loads (well pump, shop loads, ovens, etc.). However, small 48 VDC AC inverters are few and far between--And small inverters do not have many of the options/integration interfaces that larger (more expensive) inverters can have.

If the 960 WH worth of losses (plus or minus) is an issue, then 10% more solar panels can usually make up for these low power losses, if this is an issue.

The other issue with a "mixed load" system... The battery bank for mostly small loads (1,200 watts peak or less, and not a whole lot of hours at max load), gives you a "small" battery bank capacity/requirement. However with very heavy loads, you are looking at a large (flooded cell lead acid) battery bank to support those rare (and few hours) of loads... Roughly, a 5,000 Watt*Hour battery bank per 1,000 Watts of load, is a minimum suggested requirement (basically 100 AH @ 48 volt battery bank per 1 kWatt of AC inverter, and suggested maximum solar array size).

If this "pushes you" to a "too large" ($$$) flooded cell lead acid battery bank--Then looking at a different chemistry battery bank (AGM, LiFePO4, etc.) battery bank. Smaller because you do not need the AH/WH for storage, and smaller because these types of batteries support higher surge current. Of course, with LiFePO4 or similar Li Ion battery banks, you are a bit on the cutting edge (both supporting electronics

Then, there is always the two system approach--Smaller system for your daily loads with balanced solar array+battery bank. And then a large system AC inverter+specialized battery bank to support those less common loads.

And there are other issues... Flooded Cell Lead Acid batteries take a "long time" to recharge from deep cycles. If, for example, you plan on discharging to 50% state of charge on a daily basis (overnight usage), you will need close to 9-10 hours of charging (5 hours of 10% bulk charge, plus 4-6 hours of absorb charging at declining current levels). The fact that you are more southern and have a tracking array is a big help here.

With a 2 day @ 50% maximum discharge lead acid battery bank, that turns out to work better... The typical 25% discharge per day/night discharge only requires ~4-5 hours of "sun" to recharge. Works OK with a fixed array.

-Bill

My current array produces up to 30 kwh in sunny, but cold weather.  This time of year it produces around 25kwh.  When I posted that I use 20-25kwh a day, that includes charging the Volt.  It's rather trivial to set it up so it only charges when the sun is shinning and the battery bank is nearly full (which is what I plan to do).  My actual household load might hit 20kwh during the winter on a really cold day with the heat pump running.  We had 6 days this winter when that happened.

1kwh of losses might not be a big deal most days, but there will be that 10-20 days a year when 1kwh might make a difference.
Back in Feburary there was a 10 day period where we used more than we produced on all but 2 days.  So 1kwh a day would have added up to an additional 10kwh deficit. 

All of my really large loads can be time shifted to only happen when it's sunny.

I'm not planning on going down to 50% daily.  There were only 80 days over the last year a 30kwh pack would have gone below 80% SOC and only 13 times when it would have dropped blow 50% SOC.

Estragon

In my limited understanding of lithium, the cycling is much different than LA, so 20% min SOC is likely fine. Voltage is apparently pretty stable in the 20-90% area. I don't know enough to comment on the wisdom of using used EV battery packs.

I ran the cabin on a pair of GC (~225ah@12v) batteries and the generator for the first few years. There's no law that says you need X amp-hours of battery, or Y amount of pv, just trade-offs.

BB.

I would suggest a separate building or shed for a Li Ion battery bank (really for any battery bank). There are many ways batteries and gensets+fuel supplies can go wrong.

Bill

Photowhit

Peter_V said:

Back in Feburary there was a 10 day period where we used more than we produced on all but 2 days.  So 1kwh a day would have added up to an additional 10kwh deficit. 

Peter, I believe that was with a grid connected system? 

I'm NOT trying to be a jerk, but off grid systems have some additional loses. Lead acid batteries use about 15-20% more energy to recharge than if the energy was used directly, So stored energy incurs, the charge controller losses, similar to your grid tied inverter, as well as the losses charging the battery and then the losses inverting the DC to AC current.

Looks like your planning on adding some additional capacity, so perhaps you had this in mind, but I thought I'd bring it up directly.

Photowhit

Lithium doesn't have as bad a losses, but more unknowns.