i agree that using less energy would be bad; we sure as hell aren't going to achieve atmospheric carbon capture that way, much less terraforming mars. but i think using less energy is not what's being advocated; rather, presuming that consumption patterns will remain unchanged in the face of a new incentive structure will result in an unrealistically pessimistic assessment of the required battery storage. having energy that's literally free during the day is likely to result in using
more energy, not less
commercial building hvac systems have been responding to these incentives for decades: freeze water with chillers at night when electricity was cheap (or free, or actually negative cost), then circulate coolant through the ice during the day to get cold coolant to cool air through heat exchangers and thus air-condition your office. you could imagine freezers and refrigerators that worked the same way, storing energy when the sun is up to keep your food cold when the sun goes down
the popular evacuated-tube solar hot-water heater of course only heats the water during the day, storing the hot water for nighttime in an insulated tank, usually supplemented with an electric heating element for the rare occasion that the stored heat is insufficient. and there are already places where electric hot-water heaters respond to commands to preheat water at off-peak hours. these so-called 'sensible heat storage' devices are much bulkier and leakier than the phase-change type from the previous paragraph, but they can be very simple indeed
going beyond phase-change energy storage, tces energy storage uses phenomena like the enthalpy of hydration of the muriate of lime. this provides thermal energy storage that's another order of magnitude more compact; it can be used for heat and dehumidification as well as cooling, and some storage media such as lye can even get hot enough to be used for cooking
this stuff is straight from the 22nd century