a watch requires electrical energy to operate & if something is required then that something must be provided ->  a watch must be provided with electrical energy  a watch must be provided with electrical energy & electrical energy is a kind of energy ->  a watch must be provided with energy  a watch must be provided with energy & a source of something emits / produces / generates / provides that something ->  a watch must be provided with an energy source  if an object converts something into something else then that object is a source of that something else & a cell is a kind of object ->  if a cell converts something into something else then that cell is a source of that something else  if a cell converts something into something else then that cell is a source of that something else & solar cells convert solar energy into electrical energy ->  solar cells are a source of electrical energy  solar cells are a source of electrical energy & electrical energy is a kind of energy ->  solar cells are a source of energy  a watch must be provided with an energy source & solar cells are a source of energy -> solar cells provide an energy source for a watch