please critique my solar ideas

[mike_belben]

Midnight solar and fieldlines were excellent forums a decade ago as well.  Lot of pioneers on fieldlines. 

[peakbagger]

If you want education the pretty much universal recommendation is Solar Power your home for Dummies book. There is an older version floating around on the web or you can pick up new one from Amazon or any other bookseller.

[maple flats]

I ran a dual system for 9 years at my sugarhouse. My solar array was actually 2 systems, 1480 watts on battery back-up and 4840 watts grid tie only. When the battery bank was full, that portion of the battery back up system sent excess power to the grid (net metered), and the whole of the other system sent the power to the grid. My inverter is a Xantrex 6048 (6000 watts max constant, 48V battery bank).
In the 10th year my battery bank needed replacing (it was rated for 7 years). I changed to LiFePo4 batteries but I made a huge mistake. Those Lithium Iron Phosphate batteries could not be charged if frozen. I read how to address that and added a 65 watt heater mat and made an insulated cover. My mistake was how I wired the mat. I ran it directly off the BMS (battery management system). That BMS said it shut down at 35 degrees F, it did not, thus the mat ran the battery bank down too low and ruined $2500 worth of batteries. They could not be saved. That happened 3 years ago, since then I've only used the 4840 watt system. This summer I plan to go back to the battery backup by getting 4x12V lead acid batteries and once they are fully charged I'll again send excess power to the grid. I now have no loads automatically on the back up and I'll leave it that way. Just adding those LA batteries to energize 2 outlets as needed.
Back when I had my original dual system I never got a bill, since removing that system I've had a bill of up to $10/mo for one or two months each winter. The reason I'll add the new batteries is so I can send more to the grid, and my home bill gets the credit, and in about 18 months I'll have off set the battery cost and start to realize a gain again in credits to my home bill.
The 6048 inverter is not designed to run without a battery bank, my other 840 system has a Fronius 5000 inverter, which is gride tie only. I of course can use the power with net metering.

[maple flats]

Had I used a BMS that actually shut off at freezing temperatures, the LiFePo4 batteries would have just sat there, until the temperatures again went above freezing and could again be charged. That was truly an expensive mistake. The BMS that failed me was made in China and they would not stand behind it. I should have tested it, rather than believing the spec sheet and I would have gotten a different BMS.

[Joe Hillmann]

Had I used a BMS that actually shut off at freezing temperatures, the LiFePo4 batteries would have just sat there, until the temperatures again went above freezing and could again be charged. That was truly an expensive mistake. The BMS that failed me was made in China and they would not stand behind it. I should have tested it, rather than believing the spec sheet and I would have gotten a different BMS.

I am building an off grid house.  My original plan was to buy individual 1kwh LiFePo4 cells and build them into a 16 kwh battery.  But then I read all the horror stories of bms's failing in cold weather.  So now I am planing to go with lead acid and hope by the time they fail there is a better option available.
I like the idea of in the future being able to shutter the house and leave for the winter if we feel like it so cold sensitive batteries would be a issue.
I considered digging a "basement" under the front porch and insulating it heavily to store the batteries with the hope that the ground would keep them above freezing, kind of like a root cellar.
The other issue I had with lithium was figuring out a way that if the bms disconnected the batteries that the controllers would be protected from current coming in from the panels but not being able to go into the batteries.  Otherwise if the BMS sensed a problem(such as cells to far out of balance or too cold or overheating) and stopped accepting a charge it could destroy my charge controllers.  The problem is not too difficult to overcome but it adds complication and cost, both things I want to avoid.