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Solar photovoltaics involve exciting electrons within a semiconductor material, thereby creating a current. Solar panels continue to become very cheap very quickly, resulting in cost parity being achieved in certain areas sooner than anyone expected. Solar can even be viewed as a "parasite," stealing demand from previously installed generation.

Source: Citi

As solar gets cheaper, it will be possible to use it when there's less sunlight out.

Citi

Growth potential: The IEA under their base case are forecasting that solar will receive $1.3 trillion of investment in new capacity between 2012-35, representing 13% of the total global investment in power generation, ahead of gas, and only marginally behind coal. Citi views this as conservative. "Its nature means that the technology keeps getting cheaper, while alternatives gradually become more expensive, and so the ‘problem’ only becomes exacerbated."