Welcome, @Whitehse46 to the OEM forum.
Let’s deal with the Efergy kit first. I don’t know which particular model you have, but most of the commercial monitors fall down in several areas. If the c.t. is linked by radio, then it almost certainly reads apparent power, and if the base unit/display is battery-powered, it almost certainly uses a nominal or your ‘best guess’ at line voltage. So it could - and some makes do - have both those failings, and a ‘best guess at v.a.’ is touted as power.
As regards replacing it with OEM kit, we’ve just received a first shipment of the new emonTx V4, how many are spoken for and how many are sold I don’t know. As far as I know, the old emonTx V3 is out of stock. I see the combined base unit/monitor, the emonPi, is also out of stock at present.
So what would you want if The Shop could supply it? Is it possible to run a ‘microphone’-type cable (or Ethernet Cat-anything UTP or FTP) between your C.U. and the garage? If it is, and the distance isn’t too great, you could have an emonPi by the main house C.U. and a c.t. on the end of the cable measuring the P.V. infeed.
If your P.V. generation meter has a pulse LED, you might consider an optical pulse sensor on the meter, feeding a pulse count down the cable to the emonPi by the C.U., the emonPi having one c.t. on the mains incomer. The problem with a pulse count is it’s historical, you only know consumption (total energy) at the time of the last pulse. But it’s 100% accurate for energy. It’s not good at all for instantaneous power.
If you can’t run a cable, you must look to radio. If a pulse count is good enough, you can use the optical pulse sensor feeding an emonTH (also out of stock), which sends the count via 433 MHz ISM band radio to the emonPi. If you want instantaneous power as well as energy, then you’ll need an emonTx V4 with the emonVS voltage monitor/power supply (or if one comes up for sale, an emonTx V3 with a.c. adapter and 5 V d.c. PSU) and one c.t. also feeding the data on the same radio channel to the emonPi.
And if you have WiFi in the garage, it’s possible to send the data from an emonTx that way too, by adding a Pi ZeroW or ESP8266 to the emonTx.
In each case, the emonPi needs a 5 V. d.c. PSU and an a.c. adapter, as well as the things I mentioned.
Hopefully an obvious point - you can’t use the LED on your main meter - it doesn’t flash for export, or doesn’t distinguish from import if it does.
If you’re totally confused, I understand completely, so don’t hesitate to come back with more questions or if you need clarification.