Just to give everyone a heads up of how long this should take.
I managed to get 10% of the way to the achievement and I had driven 280 miles or 450 kilometres.
So by my calculations 'Approximately' you'll need to drive 2800 miles in total or 4500 kilometres. This isn't a prefect calculation as some tracks are better for generating energy (Hard/Long corners).
*Figured out an easy way to grind this*
Go to unranked races, pick 25%, 50% or 100% race (Preferably longer races) When the race starts, just hit the menu button. You'll get credit for generating energy as the CPU drives for you. This will take longer than manually doing this yourself but with the free time you can watch a film, work or browse this lovely website! (I'd also make sure your ERS level to 5 to make sure the bar is always able to fill up). A 100% race on Brazil is about 10% of the achievement done and that takes around 1 hour 45 mins.
Luckily for this achievement you can use the achievement tracker which is pretty accurate. Online and Career races all help towards this so it should be one of the last achievements you go for.