This is alternating current (AC), not direct current (DC). Talking about positive and negative is not useful. Both lines are both positive and negative sixty times per second, at opposite times. Generally, at any instant, one line is positive when the other one is negative.
How you tell whether something is providing power or consuming it is by comparing the phase of the current sine wave with the phase of the voltage sine wave.
If, when the voltage is positive, the current is flowing out of the electrical panel, that means the device at the other end of that wire is receiving energy.
If, when the voltage is positive, the current is flowing into the electrical panel, that means the device at the other end of that wire is producing energy.
And vice versa, when the voltage is negative.
This is why, when the solar current sensors are installed the wrong way, with the label towards the panel instead of towards the energy source, the Sense monitor gets the phases backwards.
When the L1 voltage is positive and the L1 solar current is flowing into the electrical panel, if the L1 solar current sensor is backwards, then that looks like current flowing out of the electrical panel, which doesn’t make any sense, since we know solar panels are supposed to produce power, not consume it.
But when the L1 voltage is positive the L2 voltage is negative, which means that the L2 solar current is supposed to be flowing out of the electrical panel. So the Sense monitor concludes that the backwards current sensor on L1 is actually on L2, and gets the phases switched.
What shocks me about this is that the whole idea behind Sense is that it can deduce the subtlest nuances of your home electrical setup through Machine Learning and Artificial Intelligence. And yet somehow it hasn’t spotted that they’re telling everyone to install their solar current sensors backwards.