The short answers to this is yes, you are going to need some form of backup power for the fans / extractors, your concern here is heat and humidity, not light. You have to keep the air moving.
Autoflowers are the best for load shedding, (if you don't have backup power for your lights).
Put your lights on 24/7 and let load shedding be your dark periods. So 2x 2H load sheddings a day = 20 hours of light for the plant, which is spot on perfect. Even at stage 6, you plants will still be happy.
Autoflowers will not care about the broken up light periods either.
Also autos don't need the lights to be as "higher wattage" as photoperiods due to DLI (Daily Light Integral).
So to explain this briefly. a Plant can only take is so much light in a 24 hour period and the rest is just wasted light, this is called the DLI of the plant.
A cannabis plant needs between 30 - 45 mol/m² of light per day. Generally you would aim for abouts 40 DLI a day for cannabis.
When you have a photoperiod plant in flower, it can only have 12 hours of light, so to hit 40 DLI a day, we use higher watt lights like (for example purposes) a 650watt / 1.2m² tent.
When using auto's, your lights are on for 18 - 20 hours, so that 650watt can be dropped down to say 400w to achieve the same 40 DLI in that period. If you have extreme load shedding (stage 16), you can turn up the power of your lights for the available hours you have power to meet the same 40 DLI.
You will also find a lot more mould and mildew resistant autoflower strains than you do photoperiod strains as this resistance has been bred into the strains, this helps with the humidity issues that can occur.
Generally speaking, a small'ish inverter + 2x 100Ah 12v batteries can hold small oscillating fans and extractor fans for a relatively long period of time.