You can only access electrical usage on a charge by charge basis. The OnStar MyVolt page is suppose to have that info on it but it has not been recording all the data downloads from the car due to internal OnStar problems. The closest we have is VoltStats.net for accuracy. It has stored more data than OnStar has but even then some Volt owners have noticed discrepancies from what is reported there to what they have been recording themselves from the car. VoltStats does afterall get it's data from the OnStar network so there must be a reporting glitch in there somewhere. Most who have commented on these errors have said it is reporting lower mileage numbers by 5-10mpg than what they are recording themselves.
Though if you think my numbers are exaggerated I'd say go look at the numbers at
Volt Stats! Tracking real world usage of Chevy Volts in the wild.... I am way down on the list of who is getting the best mileage.