This article doesn't really say, but gas power plants are frequently used for peak demand because you can easily throttle them to meet fluctuating demand (and generated power that is not used is wasted). So, even though we're comparing energy production (gas) with energy storage (batteries) it does make some sense because the batteries can probably replace the need for peaking plants eventually. If we can smooth out the demand cycle with storage for power generated during off-peak hours it would make the power grid much easier to manage.

Can someone explain to me how this is economical? (The article is pretty light on facts, and the few facts that it has are suspect anyway due to the article's technical mistakes, like measuring capacity in "megawatts".)

The maximum price of electricity (that I could find) in California is $0.66/kWh . That means, if you charge at night, or at some theoretical time when electricity is free, and then sell at that maximum price every day, your round-trip profit is $0.66 for each kWh of battery capacity. Lithium-ion batteries, if I'm being generous, last up to 2000 charge cycles. Let's say they don't lose any capacity during that time, either. That means your profit $1320 per kWh, for the whole life of the battery.

The cheapest grid-tie batteries I can find are about $3000 per kWh, so about twice as much as the total lifetime profit.

Is there something I'm missing?