Sorry I didn't notice the lack of object name earlier, as it the first time I have seen this particular setup in Tasmota.
You need to set the json prefix (object name) in the meter definition script. eg METER like this -
+1,3,o,0,300,METER,1,30,2F3F210D0A,063030300D0A
Then the property would be msg.payload.METER.Total_in
Try this flow and see if it works for you -
[{"id":"f2ebace250bb8c68","type":"mqtt in","z":"ccc06d33.2e8e08","name":"","topic":"tele/tasmota_7E4C70/SENSOR","qos":"2","datatype":"auto","broker":"","nl":false,"rap":false,"inputs":0,"x":265,"y":3750,"wires":[["5efc0d3414122b4a","64a94d8f913a7c29"]]},{"id":"42e8a6e9e98930aa","type":"mqtt out","z":"ccc06d33.2e8e08","name":"","topic":"solar/116183066761/cmd/limit_persistent_relative","qos":"1","retain":"","respTopic":"","contentType":"","userProps":"","correl":"","expiry":"","broker":"","x":965,"y":3750,"wires":[]},{"id":"f0330ce30ce0d831","type":"debug","z":"ccc06d33.2e8e08","name":"payload","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload","targetType":"msg","statusVal":"limit_persistent_relative","statusType":"auto","x":835,"y":3675,"wires":[]},{"id":"5efc0d3414122b4a","type":"change","z":"ccc06d33.2e8e08","name":"","rules":[{"t":"set","p":"payload","pt":"msg","to":"payload.METER.Total_in","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":515,"y":3750,"wires":[["f0330ce30ce0d831","42e8a6e9e98930aa"]]},{"id":"64a94d8f913a7c29","type":"debug","z":"ccc06d33.2e8e08","name":"payload.METER.Total_in","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload.METER.Total_in","targetType":"msg","statusVal":"limit_persistent_relative","statusType":"auto","x":550,"y":3675,"wires":[]}]
If not it may be you need to send to the shorter topic with "limit_persistent_relative" as the command
In which case try this flow -
[{"id":"81fb36033f46e144","type":"mqtt in","z":"ccc06d33.2e8e08","name":"","topic":"tele/tasmota_7E4C70/SENSOR","qos":"2","datatype":"auto","broker":"","nl":false,"rap":false,"inputs":0,"x":265,"y":3945,"wires":[["41b5d45db0cfa3c3","dda63fc947bc4591"]]},{"id":"bf8bfbce35454587","type":"mqtt out","z":"ccc06d33.2e8e08","name":"","topic":"solar/116183066761/cmd","qos":"1","retain":"","respTopic":"","contentType":"","userProps":"","correl":"","expiry":"","broker":"","x":885,"y":3945,"wires":[]},{"id":"ba61fd81b975e590","type":"debug","z":"ccc06d33.2e8e08","name":"limit_persistent_relative","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"limit_persistent_relative","targetType":"msg","statusVal":"limit_persistent_relative","statusType":"auto","x":885,"y":3870,"wires":[]},{"id":"41b5d45db0cfa3c3","type":"change","z":"ccc06d33.2e8e08","name":"","rules":[{"t":"set","p":"limit_persistent_relative","pt":"msg","to":"payload.METER.Total_in","tot":"msg"}],"action":"","property":"","from":"","to":"","reg":false,"x":565,"y":3945,"wires":[["ba61fd81b975e590","bf8bfbce35454587"]]},{"id":"dda63fc947bc4591","type":"debug","z":"ccc06d33.2e8e08","name":"payload.METER.Total_in","active":true,"tosidebar":true,"console":false,"tostatus":true,"complete":"payload.METER.Total_in","targetType":"msg","statusVal":"limit_persistent_relative","statusType":"auto","x":550,"y":3870,"wires":[]}]
Let me know if this works for you, also as I mentioned earlier you could send this directly from Tasmota without using node-red if that's better for you.