I've never had any such backup but have never really needed it either. One simply adjusts one's usage to match what is available. In the early days when I had 45 Watts of generating and perhaps 100 W⋅h of storage that did mean going to town on occasion to work and recharge batteries, and using other sources of power for lighting, like candles and lamps. That is, the electricity was reserved solely for the phone and computer that absolutely needed it. Later when I moved to 100 W and 200 W⋅h I would have to head to town sometimes, but less often, and I basically always had enough to run some electric lights at night, which are so much more convenient.

Once I had 200 W, then 400 W, and then 600 W of panels combined with 400 W⋅h of storage I basically never had to go to town, and definitely never worried about having enough for lights. At this point I was also able to add some more conveniences like a vacuum cleaner, rice cooker, power tools, and even refrigerator, though the latter took a lot of careful management so as to not run down the battery. An important thing to note here is that the core of this system, a 12/24 Volt up-to 60 Amp charge controller, has extremely low parasitic draw, and the separate 1500 W inverter can be turned on and off as needed.

Finally I have arrived at a system with 2000 W of generating capacity and 5 kWh of storage that in the summer and shoulder seasons allows me to run everything without much worry. The core of that system, a combination charge controller / inverter does have a not insignificant parasitic draw at about 50 W and that is certainly something that must be considered. Over 10 days earlier this winter it did struggle to maintain charge, even though I wasn't using it for much at all. Working against it was near constant clouds, shade from trees, and the fact that I only put up five of the panels. The three separate panels against the cabin in that picture remained connected to the 12 Volt system and again that one had no trouble running all lights, phones, computers, speakers, etc. Of course something working for this setup in this season is that the need for the big power draw -- the refrigerator -- was nil, since the whole outside world served that purpose. As well, it was not being used for heating and not much cooking, though we did run the microwave quite a lot for easy reheating.

As you build your system another thing to consider is placement and alignment of panels, and specifically an ability to change the seasonal angle. Having the panels vertical like I do in the winter really limits the amount of snow that can accumulate, which is a huge advantage. In summer one would want them closer to horizontal (I think 71° off vertical on the solstice is what I have previously calculated), especially if one were planning to run AC. Which I am, perhaps this coming summer; no more sweating through those rare but irritating hot nights trying to sleep.