the battery charges/discharges every day. If it did so in full, for 10000 cycles, that would be 1c/kwh lifetime electricity premium from battery. In terms of time value of money consideration, it is a fully tax free savings/return, and inflation proof. I gave China as an example. They have very low "risk free" interest rates, but it is easiest to adjust long term government bonds to tax free return as an approximation of inflation savings, to every country.