How is the word-usage calculated? What happens to my unused words?

You only pay for the generated, visible words that you select in your document. In the Basic and Power Plan we credit your words for the next month, so that you never lose unused words. In the Free Plan you really have 2,000 words for free per month. Even if you wrote less in the previous month, it will reset to 2000 words at the beginning of the next month.