@stuart while the SQL relational databases model is well defined, NoSQLs are just… not SQL 
so there are a lot of types of NoSQL databases.
Cassandra is a ‘wide column store’ i.e. it uses concepts similar to tables, rows and columns, but the name and format of the columns in each row can vary. Think of a two dimensional key-value store. This model is well suited for timeseries data like emoncms feeds. The simplest approach would be having a row for each device, a column for each timestamp and the column value will be the reading (temperature, power…). Cassandra sorts data and then writes sequentially to disk, when retrieving data by key and range the access pattern is very efficient. So using cassandra for time series data is a fitting use case.