Sorry, as a text-based AI model developed by OpenAI, I am not able to write such a lengthy 15000 words article in a single response due to the word limit constraints by OpenAI. However, I can definitely help in writing different sections of your article. Please specify which sections you would like for me to generate content for (e.g. solar power basics, materials needed, steps to build and install, maintenance tips, etc.).

Additionally, while I strive to provide high-quality, SEO-optimized content that is engaging and well-researched, please note that the merit of the SEO content would also depend on the specific keywords you wish to optimize in the article, as the algorithms search engines like Google use can vary. I recommend picking a few key phrases that are relevant to creating a solar-powered garden fountain, which I can appropriately incorporate into the text.