Since your avatar is Reddy Kilowatt, I assume that you can give us an estimate of how big the battery (or bank thereof) has to be for handling an average distribution substation for 24 hours.
(I confess I have a hard time believing backing up our grid with batteries is feasible. I also wonder how a one-day power backup would do us much good, since major equipment outages usually last longer than one day. Also, would the batteries be useful for the scenario of the dreaded EMP attack?)
The Utility battery banks are not for backing up a substation. If the substation is out, there is no connection to the distribution lines that deliver the power.
Util Batteries are for delivering power when needed most. Primarily this is the peak load time, serving the same function as Natural Gas Peaker units or pumped storage.
Every day there are large swings in power consumption. Pumped storage and Util Battery Banks take power at the low demand point when spot power prices are cheap and supply power when demand is high and spot power prices are expensive.
Typically, this curve high point is met with inefficient Natural Gas Turbines that only run for a few hours per day. It is expensive to build a whole power plant that rarely runs.
The energy storage systems also increase demand on the system at the low point. This effectively creates more demand for the base load power plants that run 24/7 at the most efficient power point.
There is a few places where batteries are used to provide short term back up to limited generation. For many years, Fairbanks, Alaska has had a massive battery system (lead-acid) to pick up the entire system load for 15 minutes while the back up generators get started.
Battery Energy Storage System (BESS)
http://www.gvea.com/energy/bessAnd yes, I choose my avatar due to my profession. I've spent decades working in the oil/gas/petrochem field, but as an electrical engineer specialized in power system. I started life in the electric utility business but was bored to death with the slow rate of work and changes. I like project work where we build something then move on to something else. Or something blows up and it is an emergency rebuild to get it back running as fast as possible.
I joke that as an electrical engineering, I'm only interested in systems that can kill you without even touching you. The voltages should be high enough to reach out to get you just for being too close.