How do ects grades compare to American grades?
They don't compare at all.

With ECTS, grades correspond to relative position of the student for a given course. For example, A is for the top 10%, B for the next 25%, and so on. These percentages are out of passing students – even E is a passing grade, by definition. Students who fail get F or FX depending on the severity of failure. This is based on statistical data for that year and previous years, so that if in some year many students perform exceptionally well (or bad), then that should be taken into account, in theory.
(Converting grades is not an easy task. In my understanding the letter grades are not used anymore, rather the administration will actually convert the local grade into a foreign grade. There is an example in Annex B of the ECTS guide from 2015.)
See this page for more information: http://ec.europa.eu/education/ects/users-guide/grade-distribution_en.htm

Grades in the US are completely different. First, the passing grade is often C, not E, therefore the distribution of grades will obviously be different. From what I understand (correct me if I am wrong), anything lower than a B+ is quite bad for undergrad courses.
Moreover grades are not necessarily correlated with the student's standing relative to other students, unless the grading is done on a curve (which an outsider does not know) – it's possible for everyone to get an A, which is by definition impossible with ECTS grading.