Im not sure why, but after starting from scratch the following is now working:
[Service]
EnvironmentFile=/home/pi/emonhub.env
User=emonhub
PIDFile=/var/run/emonhub.pid
ExecStart=/usr/bin/python ${EMONHUB_SRC}/emonhub.py --config-file=/home/pi/data/emonhub.conf --logfile=/var/log/emonhub/emonhub.log