What is the number of generators where the power capicity is 78.7?
{'header': ['Power Source', 'Number of Generators', 'Power Capacity (GW)', '% of total Capacity', 'Capacity factor', 'Annual Energy (billion kWh)', '% of annual production'], 'page_title': 'Renewable energy in the United States', 'section_title': 'Statistics', 'caption': 'Electricity Production by Renewables in 2011', 'rows': [['Hydro', '4048', '78.7', '7.5', '0.463', '319.4', '7.7'], ['Wind', '781', '46.0', '4.4', '0.298', '120.2', '2.9'], ['Wood', '345', '8.0', '0.76', '0.535', '37.5', '0.9'], ['Biomass', '1660', '5.2', '0.5', '0.421', '19.2', '0.50'], ['GeoThermal', '226', '3.5', '0.33', '0..499', '15.3', '0.37'], ['Solar', '326', '1.6', '0.16', '0.128', '1.8', '0.04']]}