For years surfers have always assumed Indonesia has an 8 month swell window between March & Oct. Although the South West swells that bombard the Indian Ocean during this time are notably more consistent during this time, there is a solid case for the 'off season' score if you are willing to get skunked.
The beauty about travelling to Indonesia during this 'off season' is that its usually uncrowded and the only other crew in the water are the few who know more than you do or are taking that 'risk' we mentioned above.
Ryan Williams, a super talented filmer ventures to North Sumatra every year and captures the proof we all needed to see. North Sumatra has a 12 month swell window people...you only need to watch this 4min clip to see we arent telling porkies!