> Even when we collectively manage to reduce our energy usage they'll likely divert the extra energy to another power substation for storage and redundancy.

That's not how it works. On the electric grid, generation must always be precisely matched with consumption, otherwise things can literally burn up. Any excess of generation will cause an increase in voltage and frequency, any lack of generation will cause a drop in voltage and frequency. That's why the generation always follow the consumption; all generators sense changes in consumption (by measuring the system frequency), and adjust their input to match (for instance, by adjusting the fuel intake). When they don't (usually because the change was too fast, like when large blocks of generators or consumers drop suddenly from the grid), protective systems will disconnect the generators and/or consumers until the grid is balanced again. Storage (which is still uncommon) only does a temporal shift of the consumption and/or generation, it doesn't change the amount (other than the inevitable losses).

That is, if you use 200W less, the generators will adjust to generate 200W less power (actually a bit more than 200W, because of losses). If millions of people use 200W less, the generators will adjust to generate hundreds of megawatts less power.

In that case -- great! I happily stand corrected.