Nearly all public utilities are traded on the open markets. In the normal course of things, the minimum investment is either one dollar (if your broker supports fractional share trades) or one share (generally less than $200: there are few companies that like to follow Berkshire Hathaway's practice).

None of them, to the best of my knowledge, are currently planning on being 100% solar. It's unlikely, although a 100% renewable company is pretty likely to either now exist or happen shortly. Hydro-Quebec is, courtesy Wikipedia:

hydroelectricity (96.78%)

wind (2.16%)

biomass, biogas and waste (0.75%)

nuclear (0.19%)

thermal (0.12%)