I'm not sure I would call it "externalizing costs" if the only change was making daytime energy significantly cheaper (and not increasing off-peak energy prices

I'm not sure I would call it "externalizing costs" if the only change was making daytime energy significantly cheaper (and not increasing off-peak energy prices). You could simply not add any batteries and your costs would still go down. You would presumably only decide to add batteries if the cost of doing so was less than the money you'd save by shifting energy usage to cheaper times.