What the solar generating capacity (whether roof-top or large-scale) actually provides is electrical energy during mid-day hours (say 8am to 4pm), to replace an equivalent amount of electrical energy from thermal power plants during those same hours. This directly reduces (i) the amount of coal burnt in the power plants (whose day-time generation is curtailed), (ii) amount of ash produced by these plants, (iii) CO2 emission, and (iv) the requirements of coal mining, import and transport, and of ash disposal. These are all most beneficial from environmental angle, and are the reasons for the global push for solar (and wind) power generation.