As someone who recently had a huge power outage in their city (200,000 with out and still had 53,000 still with out power 4 days later)and having plants in 76 hours of darkness myself. Also having multiple online friends in the same boat. Lighting is the least of your problems. The outages in my city ranged 4 hours to 12 days from growers I know. No one lost plant's due to lack of light. All lost plants to other reason other than lack of light.
I would say the number one problem was not watering their plants. Plants still drink in the darkness.
Also not being able to water their plant either.
I have multiple Battery-Operated Pumps that uses D batteries. It made it much easier to water with no power.
Google "Battery-Operated Pump"
I would never bring my plants outside and then bring them back in. That is just asking for all sorts of potential problems. If they really needed light then use a window. If the must go outside, they should never ever come back in.
Plants can go 5-6 days with no light. It's the other things like watering your plants, having air flow (batterie or rechargeable fans), controlling the humidity and heat.
I had previously purchased a rechargeable fan from vivison I would rate it garbage as a regular grow tent fan but in a power outage it does it's job only for around 6 hours on low before having to charge it. It charges by usb so a car can do it but I would do it at my work.
Heat could be a big problem too.
Also remember when 20% of your city goes with out power, batteries become harder to find.
Cheers
CCG