Redstone Flux provides the RF API as a standalone JAR for Forge on Minecraft 1.12–1.12.2. Its interfaces offer mod developers a performant, reliable coding template and can be cast to capabilities. The author warns that repackaging this API or the old cofh.api.energy inside mods causes crashes.
For mods that need the RF API, rent your own Minecraft server and install Redstone Flux with one click from the server panel.