Sentence:  [Rural solar plant] schemes are of little help to industry or other heavy users of electricity. Nor is solar power yet as cheap as the grid. For all that, the rapid arrival of electric light to Indian villages is long overdue. When the national grid suffers its next huge outage, as it did in July 2012 when hundreds of millions were left in the <WSD>dark</WSD>, look for specks of light in the villages. 
Meaning of the word marked with <WSD> tags:  Having an absolute or (more often) relative lack of light.