[Feature request] Support "Always Positive" -> Shelly EM 
**Is your feature request related to a problem? Please describe.**
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Hello Alex, 

i got an Shelly EM for my "Balkonkraftwerk" (600W PV) and use the EM for monitoring.
In the shelly app it shows me EM and 3EM and count the whole power.

like 
House: 1000W
PV 600W 

= 400W sum.


**Describe the solution you'd like**
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Home doesnt like negative values. So you add (2020) a support for stop it less then 0.

Please add an option for "always Positive" so my -600w will show as 600w in Home app // Eve App.
