I'll also highly recommend TeslaFi for recording this.
I didn't sign up for it on my S until I was at about 14K miles, the data it's collected in the 20K miles since is awesome. The charge summary report breaks down charging sessions to AC and DC. The bottom of my SuC report shows:
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I also have FUSC, but this gives me an estimate of how much I would have actually spent (I figure about $800 now).
The other report I really like is the lifetime map. This misses the early trip we did to VA (before I was using TeslaFi), but it's still impressive.
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My TMC username is a
referral code for TeslaFi that will give you a 1 month trial.