The emonpi is not waterproof, I would not recommend using it externally. Another option would be the use an emonpi (located inside) then use an emonTH outside with a optical senior connected to it.
The emonth is battery powered and will transmitt data back to the emonpi via RF:
https://guide.openenergymonitor.org/setup/emonth/
The emonth is not naturally waterproof but it can easily be placed in a waterproof enclosure: