Add device class to sensors

[s00500]

Checklist

• I have filled out the template to the best of my ability.
• This only contains 1 feature request (if you have multiple feature requests, open one feature request for each feature request).
• This issue is not a duplicate feature request of previous feature requests.

Is your feature request related to a problem? Please describe.

Homeassistant complains that the deviceclass is not set when adding these sensors to the energy dashboard

Describe the solution you'd like

device_class needs to be set in sensor.py

value should be 'energy'

Describe alternatives you've considered

Additional context

minor detail, but would be great to have fixed :-) I can also make a pr