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# Power

Power values occur in many USEF messages.
The sign of a power value is used to distinguish between production and consumption, from the perspective of the prosumer.
A positive value indicates that power flows towards the prosumer (consumption), a negative value indicates flow towards the grid (production).
The following statements are true for ISP elements in all USEF messages:

- Each ISP in a period may only be addressed once.
- ISP elements may not cross period boundaries.
