Common folklore is that the Ells family first coined the Sunset Lake name to attract people from the Boston suburbs to rent or buy their "Sunset Lake Cottages". It does have a better ring to it than "Wash Pond Cottages", doesn't it? Supporting this story, the first known property deed reference to Sunset Lake was by Ells in June 1928, while all of their previous deeds from 1926-1928 referenced Wash Pond. However, this 1902 article belies that folklore; this is the only known reference to Sunset Lake prior to the June 1928 Ells deed.
First known reference to Sunset Lake: Derry News, 8/8/1902