the biggest brownout I experienced, I guess one could call that
already a black out, lastet 2 weeks and was unusual.

These days they can last anywhere from 0.0001 minutes to 10 hours.
The nice thing about the car battery approach is that if things take
longer
you can use the battery out of your car or truck, you could even drive
around the block until a weak battery is recharged.
==================================
Uwe , With a normal AC system and long duration outages you mentioned an
appropriately sized UPS with external batteries would do the trick
(depending on the power you need)
For full domestic power requirements a 2-4 kWatt petrol or diesel engine
( portable) set would probably be the best.
Many of these sets have both AC and nominal 12v DC (for charging batteries)
Check the Honda range of petrol engine/generator sets ,often advertised and
mentioned by RACES and ARES emergency communicators and by those involved in
Amateur Radio Field Day operation.

Frank GM0CSZ / KN6WH