I have a WEM3080T on 1phase PV system. I have seen few times that Yield is -1W.

1W in negative is not a big issue - it could be a rounding problem as well -, but may be there is something else behind. (could other cable's  -outside from sensor- electromagnetic field cause such miss measurement for example?) 

Can you consider and advise PLS?

Thanks Gabor

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There are two possiblitiies.
1: At this time , the inveter is not producing ,but consuming a power about 1 w .
Base on the direction definition, when the inverer yield energy ,the power reading is positive,on the contrary(this will be not a normal mode ,but can be understand) ,it will be negative.
2: it is the measurement error that caused by the resolution .
The minimum resolution is 2w in this meter ,so it is possible to result 1w when the real reading is zero.

Anyway, the data in this graph is not an instant reading 

Please use the reading on this page to calculate the relationship by this formula

Inverter power + grid power = load power

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