This will revolutionize the grid. I was reading that lithium ion batteries are around $500/kWh right now wholesale (and I've seen some you can buy from China that make me believe that's roughly true). Then there's a projected cost as low as $180/kWh in about 5 years after Tesla's factory ramps up (and no doubt others start to come online).
Right now (in Ontario) I can buy peak electricity at about 13 cents per kWh and maybe 7 or 8 cents per kWh at night. Imagine a system of batteries where I buy power at night, store it, and then use that during the day. I worked the rough numbers and at today's battery prices I'd be hard pressed to get a return on my investment in 20 years, and that's only considering battery cost. However, if you use $180/kWh, suddenly you might see the payback period on a system like that drop below 10 years, and if I can do it at that price, what can a utility do with its economy of scale?
The addition of economical grid-level storage will radically change the way the utilities run their business. You won't need so much idle generating capacity such as natural gas or coal sitting around to service peak loads because you can charge up your battery banks at night using nuclear and during the day with solar and consume them during the peak periods.