With my emonPi Solar PV bundle up and running, I need to get my head round where the data actually lives. I guess the SD card has limited capacity. I would like in any case to accumulate an historical record of all data on my PC, possibly by monthly downloads/ copies, in a similar way to logs from my PV inverter.
Hello @ChrisJD, there’s a guide on exporting CSV data here Exporting CSV - Guide | OpenEnergyMonitor
The data itself is stored on the SD card in the following directories: /home/pi/data/phpfina and /home/pi/data/phptimeseries. The data format is a custom binary format for compactness and efficiency of access by the emoncms application, there is info on exactly how its stored here: Learn | OpenEnergyMonitor
Thanks, looks like I have some homework to do. 
Is there a process to expire old data on the SD card as it fills? Will it warn me before it gets critical?
That depends on the size of your SD card. AFAICR, the size of the micro-SD card currently shipped with an emonPi is 8 GB. The data is stored on a partion separate from the OS and IIRC, the size of the data partition is ~0.8 GB.
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One way is to download the data via the backup module, but it would be a complete log each time. Some folk have also recorded the data to a different database (influx?) and graphed it with grafana (search on those terms).
What size card? It fills very slowly so, unless you are recording a very large number of feeds, it is unlikely to fill up (talking years).
You can see how full the data partition is from the admin screen.
If you want to monitor the data partition, a package called monit can be installed via apt and configured appropriately.
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