Satellite Writer delivers multiple bleeding-edge AI models, including text, image, and audio skills. You can use your credits to generate with any skill/AI model. So billing by words would make no sense.
For quick reference: if you used the Article Writer, one credit would be 500-700 words per output, and if you're on a premium plan, you will get triple outputs. So for the Article Writer, each credit would be worth anywhere from 1500-2100 words. (Note: it might be more expensive for other fine-tuned AI skills depending on the use case.)