Our standard offering is unlikely to fit your requirements exactly, but with a few small modifications I think we could solve your problem.
I would propose an emonTx V3.4, with a wired serial connection to a Raspberry Pi.
The emonTx with the standard software is most likely not capable of averaging over a 30 minute period - our normal averaging period is 10 s, but our standard single-phase sketch already sends the accumulated Wh values every 10 s, and with a small addition to the code, the 3-phase sketch can do likewise. The 3-phase sketch can be set up for 3-wire (treating the supply as 2 phases & neutral) or 4-wire systems.
The Raspberry Pi would run our logging and display software emonCMS, and record the data to its SD card (and, if an Internet connection is available, to a base server somewhere in the world also running emonCMS). emonCMS is already capable of recording data at ½-hourly intervals, so you would have on record the accumulated value every half hour. emonCMS is available from the shop on an SD card, or you can download an image and flash your own SD cards, as you prefer.
So for each installation I think you need:
1-off a.c. adapter to suit the local voltage - line-neutral for single phase and 4-wire 3-phase, or line-line for 3-wire 3-phase,
1-off, 2-off or 3-off current transformer as appropriate,
1-off Raspberry Pi & case
5 V Raspberry Pi power supply
1-off emonCMS on a SD Card
You will need to make up a 3-wire serial connection to connect the emonTx to the Raspberry Pi.
To load and configure the emonTx’s and set up emonCMS
A portable computer with the Arduino IDE loaded,
1-off Programmer & lead.
Depending on the quality of the electricity supply you are monitoring, you might like to consider a battery backup/UPS for the Raspberry Pi. I’m afraid I have no experience of those, so I cannot advise you, but I think one of my colleagues here might be able to help in that regard.
As Internet time won’t be available, you might also want a real-time clock on the Raspberry Pi too.