Mex, - the opposite to big fridge is a small fridge, - not necessarily old and/or inefficient one. Inverter fridge is the way to go.
There seems to be a confusion there. CFE regional tariff zones are based on average bimonthly consumption, and the worst/lowest residential limit is 500 kwh - for 2 months. 400kwh bimonthly is something new. Anyway, most areas enjoy the limit 800kwh bimonthly or higher. Your bi-monthly average is calculated by dividing 12 months consumption by 6. Your bi-monthly average is not revised every 2 months. If your average exceeds the limit, you lose subsidy and jump into higher rate FOR A YEAR. On top of that, there are "first 100 kwh a month" that cost less, then the next 200 cost more, and the rest - more yet.
It's complicated, indeed. From official horse mouth:
CFE tariff zones - scroll down to the table.
Q. to Moisheh: How much BTU in summer you found to be sufficient with your mini-split? I presume there is a tin roof "sombra" over your bus in Kino.