Hello @djh
I posted an example yesterday here which may be useful:
https://community.openenergymonitor.org/t/uploading-data-to-emoncms-org/9863/4
This uses the input/bulk api, you need to create and link your feed to the input before uploading data and upload starting with the oldest data first, as the first datapoint determines the earliest start time of the feed - you cant add data further back in time.