SELECT MIN("Watts") FROM table_6143 WHERE "Class" = 'a' AND "Frequency" = '91.5 fm' AND "City of license" = 'elkhart, kansas'
How many Watts has a Class of a, and a Frequency of 91.5 fm, and a City of license of elkhart, kansas?
CREATE TABLE table_6143 (
    "Call sign" text,
    "Frequency" text,
    "City of license" text,
    "Watts" real,
    "Class" text
)