I think you would have to ask your local utility company whether they are buying from out of state. Once you get them talking, ask about all the other electricity sellers in your state.

 

The only time you will know where 100 percent of your energy comes from is when you take your home off the grid and start generating your own.

 

I think you would have to ask your local utility company whether they are buying from out of state. Once you get them talking, ask about all the other electricity sellers in your state.

There are two separate issues at play here. If you pay extra for "green electricity" it means that your utility pays for as much green electricity as the amount of electricity you use, creating a market for the green electricity which is the point. But this has no effect whatsoever on where your energy comes from, it only affects the flow of money.

The actual energy flow is rather more difficult. It would perhaps be reasonable to assume that the majority of energy you consume comes from the most local generation, after all that is probably the lowest impedance route. At any particular moment though the energy flow will depend on the power and phase of all the generation and loads on the grid and you can't relly determine where it is coming from.

If there is publically accessible information about energy flow across state boundries, a bit like this page for the UK: http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Demand/demand24.htm then you can say "my state is currently importing X GW from unknown sources, and generating the rest which is largely from sources Y and Z" then maybe you could get a reasonable estimate.

In the UK enough information is published to allow this kind of calculation to be made: http://www.owningelectriccar.com/national-grid-electric.html I'm not sure how they get their CO2 figures for the imported electricicty, but I suspect they just use the national averages for those countries.