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Inverters require power to function, even in standby mode. When there is no production (for example, during the night or when panels are covered with snow) this power is drawn from the utility. Thus, inverters will show negative generation (or consumption) during periods where there is not enough production to offset inverter standby usage. If an eGauge is properly configured, this overnight consumption will be reflected in the total site usage and does not need to be added back in manually.
Example of "negative production" (inverter standby consumption)