"National Grid have received a price of $1,160 per kilowatt, which is a good price - slight premium to re-investment cost," said analysts at UBS.
Ravenswood is valuable as it supplies over 20 percent of the electricity for New York City, where it is difficult to find land on which to build new capacity and demand for power is always high.
The deal, which is expected to complete this summer, will reduce TransCanada earnings for the first two years but boost profit thereafter.
The sale will generate a gain for National Grid, which said KeySpan had recorded a carrying value of $1.2 billion for the plant.
Analysts at Credit Suisse estimated National Grid would gain $2 billion in proceeds after capital gains tax and accounting for fuel stocks and lease prepayment, which would give 7 pence a share of upside to their 880 pence target price.
"We estimate the sale converts into a gross $1,170 per kilowatt valuation, slightly above our estimated newbuild cost of circa $900-$1100 per kilowatt for U.S. Combined Cycle Gas Turbines and for other recent transactions in the U.S.," they added.