Use the Dictionary of Energy to help you understand unfamiliar terms.
Reference sources are useful for a number of reasons. They can
The sources in this list are reputable and have been vetted by the library and scholarly communities.
The Encyclopedia of Earth is "a free, expert-reviewed collection of content contributed by scholars, professionals, educators, practitioners and other experts who collaborate and review each other's work." See the Energy section of The Encyclopedia of Earth.
Wikipedia can be considered a reference source, but, because the content in Wikipedia can be created by anyone, it has not been entirely accepted by the scholarly community as yet. (Traditionally published reference sources are vetted by scholars, generally.) Read Wikipedia articles with a critical eye in order to detect bias.
Wikipedia articles can be very helpful, however, because they often contain links to external content and additional references listed at the end.