Comment by cornholio

You wouldn't have such a capacity mothballed and forgotten for years, it would have to be maintained and tested regularly. There's a concept in energy market design that helps finance such standby operation called a "capacity market". The plant would sell emergency capacity and get paid for not emitting, just staying available. Failure to respond to an emergency cycle would presumably carry hefty contractual penalties, erasing all the previous revenue.

So this makes the battery and the gas plant compete in the market place, each with it's own economic strengths. The gas plant won't handle daily cycles since the emissions cost would kill it, but it can provide emergency power at a rate and for a duration that would make batteries monstrously capital expensive.

By slowly sliding up the emissions pricing, you will tradeoff the long term emissions versus the energy cost, and let the market efficiently allocate the resources until net zero, or near zero, becomes economically attainable.