New "off-grid" installation in urban (grid-tied) setting...

We had solar at one of our previous homes and loved it, back then you could get huge utility rebates off the installed cost and so we went with an installer and did a 5kW system (limit of rebate), it cost us about $12K out of pocket (after tax credits and rebates, etc) and was installed for us with a SMA 7kW inverter in case we want to expand it later a bit... 

Well, fast forward 10 years and our present house is quite a bit larger and energy bills are killing us!  (over $500/mo) We decided to strive for net-zero energy independence and being both on a budget and a do-it-yourself type of guy I'm taking on this project all on my own!

We have about 1200 sqft of flat roof (pitched 6 degrees South) where we are currently installing everything.  We found 27 used Sharp 170W panels and bought them as well as 16 230W "Jetion" panels (unused).  We also acquired a couple Xantrex XW inverters and an XW PDP as well as 2 Xantrex 60A MPPT charge controllers and 2 MidNite Classic 250V charge controllers.  I am a battery guy and love the Lithium Iron Phosphate technology, light-weight (easy to work with), energy dense (not too much space), clean (no corrosion/acid) and long-lasting.  To me all those pro's are worth the premium price (and when you factor the price in terms of cost per kWh stored and retrieved from the battery over its entire life, it is actually not a premium price but is cheaper!); so I went with Ohmmu Group 31 Lithium batteries, we have the Gen. 1 set now which are 12.8V and 100Ah (we want to upgrade to the Gen. 2 when they come out which are 150Ah and same size/voltage!)

I decided to do the install in phases:

1st phase: install all of the AC-side stuff; New sub-panels (2 of them), PDP (with dual-inverter breaker and maintenance bypass breakers) and XW inverters and Xantrex SCP.  Had to buy a lot of 4 AWG wire to finish this but got it all done in a day.
2nd phase: Add a battery bank; I found the best batteries possible since this will be so critical to our system.  We installed Ohmmu Group 31 batteries and I built a wooden rack to hold 24 of them, currently we have 20 installed which gives us 25kWh of usable energy (Lithium Iron Phosphate batteries so we can discharge them down to 20% or so and still get many thousands of cycles out of them!)

After these two phases were complete we were able to start doing load-shifting.  We are set up to charge our battery bank up from 11pm-5am when we get EV-super-offpeak rate from SRP of around $0.045/kWh.  Then at 2-8pm we do "load shedding" mode which uses the battery bank to power our house (rather than pay the "peak rate" of $0.25/kWh).  This process allows us to off-set around 20kWh of peak energy cost with the offpeak cost, at about $0.20/kWh ($0.25 - $0.045) that means we save $4.00 per day utilizing this method! (weekdays only) $20/week and ~$1000 per year.  The savings is off-set by the cost of the batteries though, it was about $14K in batteries to get 25kWh, so that is a 14 years pay-back.  HOWEVER, the bigger benefit is we now have a higher level of comfort throughout the day, we no longer turn our AC's off during peak-time and so the house doesn't climb too high in temperature.  So although we won't get our money out of it for 14 years (and if we replace the pack at that time we will never really get our money out of it) we will get benefits from it starting immediately!  And, when we disconnect from the grid a battery pack will be absolutely necessary to make it through the night, so there is that benefit too...

3rd phase: reroof (needed it after inspecting it, the asphalt was all worn away)
4th phase: install feet (roofing tar underneath during lag-bolt install and again another layer of roofing tar over the top and around the foot after mounting)
5th phase: install racking and panels for 4 strings which will feed Xantrex Solar Charge Controllers (16 Jetion panels, 4 strings of 4, 2 strings in parallel to each Xantrex)

At this point we will have 3.7kW of solar feeding the battery bank (and house), we expect to gain about 18.5-22.2kWh of energy per day from this solar bank.  Now is when the savings starts to build, if we capture 20kWh of energy per day at an average rate of about $0.10 (accounting for peak-load shifting happening with battery bank already) that is another $2.00 per day (365 days per year), which is $730 per year in cost savings.  Add that to the $1000 and we are up to $1,730 per year.

6th phase: install feet, racking and panels for 5 strings of Sharp panels (2 strings of 6 feeding one of the MidNite Classics and 3 strings of 5 feeding the other MidNite Classic);

At this point, this adds another 4.6kW of solar feeding the battery bank (and house).  We expect to gain 23.0-27.6kWh per day of energy from this bank of solar panels.  Again at an average rate of about $0.10 and 25kWh per day that gets us $2.50 per day more savings.  Annually this means $912.50, add that to the $1,730 and we are up to $2,642.50 in total energy savings (if this is all we did).  Our total system cost for all this equipment is around $8,000 plus the $14,000 for batteries for a total of $22,000.  If we deduct fed tax credit from that cost we get to $15,400.  If we save $2,642.50 per year we are at a 5.8 years payback on the system, after which we will be ahead $2,642.50 every year that we don't have to spend more money on repairs/batteries/etc.

This is a lot of information and thank you for anyone who was interested enough to read it all, I'd love to hear feedback from others going through this same process and what they foresee as the costs/savings/ROI/etc to this entire project...  I assume most "off-grid" style installations are done in a truly off-grid location where it is the best option for energy, certainly my situation is unique in that we live in the city and are connected to the grid...  We want to do this to protect ourselves from increases in cost and also allow ourselves the ultimate freedom of potentially one day cutting the cord from the power company and being energy independent within the city!  Also we have had a couple brown-outs in our area over the last 2 years and we are now immune from those!  

I'd also love to hear folks thoughts on doing an "off-grid" system where we won't be selling any power back to the grid vs a traditional net-metering installation.