not really offsetting, (according to this car review site anyway http://www.motortrend.com/roadtests/alternative/1208_2012_te...) at the end it has a little breakdown...
"During our drive, we used 78.2 kW-hrs of electricity (93 percent of the battery's rated capacity). What does that mean? It's the energy equivalent of 2.32 gasoline gallons, or 100.7 mpg-e before charging losses. That BMW 528i following us (powered by a very fuel-efficient, turbocharged, direct-injected 2.0-liter four-cylinder engine) consumed 7.9 gallons of gas for a rate of 30.1 mpg. The Tesla's electrical energy cost for the trip was $10.17 (at California's average electrical rate); the BMW's drive cost $34.55. The 528i emitted 152 lbs of CO2; the Model S, 52 -- from the state's power plants."