To elaborate on this: assume the sine waves of voltage difference and current are fully in phase when the house is net-consuming. Multiplying the voltage and cu

To elaborate on this: assume the sine waves of voltage difference and current are fully in phase when the house is net-consuming. Multiplying the voltage and current functions and integrating over time gives you the total energy consumption. When the house is net-producing due to solar panels, the current will be 180° out of phase with voltage, i.e. "negative" AC current. Multiplying and integrating yields negative energy consumption. Note that current is directional, so "negative" current isn't anything special - if you reverse the meter, the sign will be inverted.