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You can do so, but it takes market knowledge to find such a plan. There are several plans that offer 100% renewable energy, usually certified in some way (example: http://www.tuev-nord.de/cps/rde/xbcr/SID-7251F8DB-F18E650D/tng_de/kriterienkatalog-oekostrom.pdf). The certified electricity is usually from hydroelectric power plants which are already 100% depreciated and don't get any surplus from the renewable energy levy.
However, there are some plans (for example http://www.gruenerstromlabel.de/english/) that come at a higher price, but also pay for installing new renewable power.
To answer the OP question: I'm not an EA-specialist, but I think that depends on your goals. If you agree with the version of utilitarianism that most EAs hold, buying the cheapest mix and donating the difference to GiveWell is best. If the goal of promoting renewable energy in your country is of similar importance as improving living conditions in developing countries, you should find out if in your country, solutions which actually cause investment in green energy exist and then buy those (the most effective solution would probably be to buy 100% renewable energy from depreciated hydroelectric plants and directly invest the difference in cost to higher-quality labels in green energy projects, but I assume this option isn't available to you).
I don't think those hydroelectric power plants would shut down when nobody would pay a premium for green energy.