Yea, negleable. This is why no country has done it and why most countries are ploughing large amounts into grid storage research. as well as the cost of subsidies to build new plants as private wouldn't when there's no guarantee there product will be purchased. As your storage would be a pathetic amount and you would still have to force plants offline.
It's gigawatts actually using currently available technology.
Look at the price of batteries and you can easily calculate how much it would cost for x amount of power.
So does this massive array actually exist or are you just speculating that it could exist?
They are used as backup power solutions for large facilities like hospitals etc