I figure total cost with my setup would be about 64KW of batts and 13 KW of inverter power (dual inverters sungold power clone LV6548), and 15KW solar panels would be about $45,00.00 assuming I do most of the work.
Assuming the AIO inverters and charge controllers last 10 years, and the solar panels stay above 80% for 20 years, then you're looking at $50k for 20 years of electricity.
In general, for every kW of solar panel installed, texas delivers 4.8kWH per day on average throughout the year. Obviously sunny summer days with panels pointed optimally will deliver double that, and cloudy days with panels pointing suboptimally will deliver less than 10% of that.
If you install 15kW of solar panels, and use all that energy, you would be collecting 72kWH/day. Multiply that by 365.25 days, and 20 years and you'll have collected 526MWH of energy.
Divide that by your $50k cost, and your cost per kWH is $0.095/kWH.
Given your current cost is $0.12/kwh (and I'm assuming you're factoring in all the costs of your current power source - ie, gov't charges, taxes, fees, etc) then over those 20 years you'll save $13,000, assuming the cost of electricity, inflation, etc doesn't change at all over the next 20 years.
Or, in other words, if you're going to consume 526MWH over the next 20 years, then you can pay at least $63k to the utility company, or you can pay $50k now. Get a low interest loan and you might be able to save more if inflation + electrical rate increases is above your loan interest rate.
This, of course, doesn't account for gov't rebates, or waiting for good sales and deals, or buying second hand panels and other equipment. Quite frankly, your system seems expensive for a DIY project, but I'm frugla/cheapskate, so I'll assume your pricing closely aligns with your needs for support/warranty/reliability, and won't delve into that further.
So given all that, I don't understand why you feel this won't pay for itself, unless you've described a system that will generate half a terawatt over its lifetime, but you'll actually only use maybe half that - at most - making the apparent cost of the solar system closer to $0.19/kwh or more.
The system you've specified is for a house that consumes an average of 72kWH/day.
Have you done a full energy audit? Have you already optimized your power consumption - for instance, are your air conditioners 20 SEER already? Are the windows all double or triple pane, argon filled, with appropriate solar coating to reject heat? Is your roof at least R-60, and walls R-30? Is your stove induction? Are your hot water and dryer heat pump types? Have you thrown out your oven, or at least use a smaller convection oven, and that only sparingly?
If not, then for every dollar you put into creating a more energy efficient house, you're going to save $3-$10 on your solar system needs. For instance, if you have a standard R-15 roof, paying for professional spray foam insulation to bring it to R-40 or preferably R-60, will reduce your energy consumption overall enough to reduce your solar system needs by 2-3 times what the insulation cost.
If you're consuming 72kwh/day and there's little to optimize because it's already close to passive house standards, then you must be in a 10k foot house and people are leaving windows open or something, or perhaps you're already saving $$$ by driving an electric car hundreds of miles a day. In which case, yes, that $13k savings on a $50k system means the payback will occur sometimes after 15 years, which isn't the 7-10 year payback people are trying to get. Most of us are still driving gas guzzlers, or hybrids, because spending $20k - $50k on a car isn't feasible, but spending $5k - $15k on a solar system is.
Going off grid, without changing your energy consumption, is going to require a premium, because your energy use isn't constant, and isn't even well aligned with the solar input. In Texas and states with moderate climates it's far cheaper to go off grid because the air conditioning is well aligned with solar input, and you rarely have one or more weeks in a row of cloudy weather, placing a heavier demand on energy storage and energy collection in low light. Doing this in Michigan, where I'm located, is quite a bit more difficult as we have weeks where there's 20% or less of the sun hitting the ground during the day, and these conditions are the rule for 1/3 of the year.
So my payback only occurs because I'm not going fully off grid. I choose a time of day rate - $0.18/kwh 11am-7pm, and $0.16/kWH otherwise. I'm fully off grid during the high rate hours during the summer, and often for several hours after the rate lowers, and in the winter I'm only off grid on particularly sunny days, but my workshop is off grid almost all the time.
What this does for me is reduce my power bill during the months where it's most expensive - summer with the AC on, it offsets my bill the rest of the year, and it provides backup power for outages and storms - which are rare, and usually not long for me, but it brings a significant peace of mind given the apparent fragility of the grid, and heightened tensions in the world. The only reason my power is stable is because I'm on a 12 mile rural loop which has multiple connection points to the grid. Just two miles away from me the power goes out a few times a year for an hour or more, and every year or two it goes out for 1-4 days.
In most sunny, temperate locations, payback occurs within 20 years in a fully off grid system with no rebates/incentives, and assuming inflation and utility rate increase are both zero.
In almost every location, payback occurs in 7-10 years in a grid-tie system with rebates/incentives, assuming inflation and utility rate increases are zero.
In most locations, payback occurs in under 7 years in a grid tie system with rebates, incentives, and taking into account 3% inflation.
If you use time of day rates, and engage in arbitrage - charge your batteries on the lower nightly rate - then payback is even sooner than the above three circumstances suggest.
Your electricity is cheap - well below the national average, so no, it's not clear cut, and unless you don't move for those 20 years it's quite likely that you won't get payback, because this system won't increase the value of your home by the amount it costs.
So taking this from a pure cost perspective, going off grid is far from a sure bet, and I'd tend towards it being unlikely to be cheaper.
But if you're going from a pure cost perspective, going off grid, even if you do break even, is nowhere near as much savings as going grid tie.
Do a grid-tie under your circumstances and it'll pay for itself at least twice over before its end of life, if you don't export. Export and it may reach 3x payback before its end of life, even without net metering.