Here is what I did:
1. I started with the projections from the EIA to 2035 available here in XLS.
2. I then calculated the share of clean energy in 2011, assuming that natural gas gets a 50% credit for being clean. That share is just under 44% (Nukes 21%, Renewable 13%, Gas 10%).
3. I then calculated how that share could be increased to 80% by 2035.
Here is what I found:
1. Coal pretty much has to go away. Specifically, about 90% or more of coal energy would have to be replaced.
2. I first looked at replacing all the coal with gas, all else equal. That gets the share of clean energy up to about 68%, a ways off of the target.
3. I then fiddled with the numbers to arrive at 80%. One way to get there would be to increase the share of nukes to 43%, gas to 31% and renewables to 22% (Note that the EIA reference scenario -- BAU -- to 2035 has these shares at 17%, 21% and 17% respectively, for a share of 45% just about like today.)
What would this actually mean?
Increasing nuclear power in the EIA reference scenario from a 17% to 43% share of electricity implies, in round numbers, about 300 new nuclear power plants by 2035.*** If you do not like nuclear you can substitute wind turbines or solar thermal plants (or even reductions in electricity consumption) according to the data provided in The Climate Fix, Table 4.4. The magnitude of the task is the same size, just expressed differently.
One nuclear plant worth of carbon-free energy every 30 days between now and 2035. This does not even consider electrification of some fraction of the vehicle fleet -- another of President Obama's goals -- which presumably would add a not-insignificant amount to electricity demand.
Thus, I'd suggest that the President's clean energy goal is much more of the aspirational variety than a actual policy target expected to be hit precisely.
***[Math: (43/17)*898 (billion kilowatthours in 2035)/815 (bkWh in 2011) *109 (nuclear plants in 2011) = 304.16]