From the politifact article:
On Block Island, it’s the Block Island Power Company, whose on-island generators run on diesel fuel, which must be shipped to the island by boat.
A 2010 Providence Journal story on the island’s power system noted that diesel fuel regularly costs $1 more per gallon on the island than on the mainland.
In fiscal 2011, according to a report by the town’s Electric Utility Task Group on the fiscal costs and benefits of the wind-farm project, the average cost of electricity on the island was 47 cents per kilowatt hour. In the rest of Rhode Island it was 14.8 cents.
Once the cable is laid and the wind farm project is on line, in 2014 or 2015, Block Island Power will be able to purchase electricity from the New England power network at much lower costs.
The task group estimated that electric rates on the island -- based on a 20-year agreement between Deepwater Wind and National Grid -- would fall to 30.7 cents per kilowatt hour, a 35.4-percent decrease from 2011 rates.
(The island’s rates would still be substantially higher than those on the mainland because its customers would be paying for a portion of the costs for installing the cable and for maintenance of the island’s power system.)
The task group’s analysis noted that current power costs on Block Island have risen to 54 cents per kilowatt hour because of the increasing diesel costs. Based on that figure, the decrease would be a 42-percent drop -- about what Deepwater said in its Tweet.
The other article doesn't mention anything about how much power and at what price the wind farm will be generating it. It sounds like the public relations department is doing all the talking.