Run time depends on battery sizing and use rate.
We won't be running A/C or Flat Screens in emergency, and have 2 wood burning fireplaces for winter.
We were offered 30 year 2.5% financing for our solar system, but declined it when upon taking a closer look at the terms. First, they required that you pay over to them any tax credit. In other words, you agree to provide them your tax return and pay to them in one lump sum the amount you would have paid in tax but for the credit. Then, the finance company extracted a mult-thousand dollar fee from the installer, which was added to your principal, in addition to the three or four points you agreed to pay on the loan origination. The whole thing was cleverly designed to make the monthly payment equal almost exactly what your savings on you monthly electric bill was expected to be.