It says it's ideal for unpredictable load or infrequent demand. My reading is that billing is done per minute only while the database is being read / written to

It says it's ideal for unpredictable load or infrequent demand. My reading is that billing is done per minute only while the database is being read / written to (active), but what I'm not sure about is if the storage is completely free whilst it's inactive. If that's the case it would seem like a great solution for BI and analytics type applications where you want to collect data over time (and are able to batch writes) and produce reports relatively infrequently.