Not sure why this is so hard for me to figure out lol, but my quick analysis is as follows:
Assume 90k gets you 25 hours of fuel, which is ~85k roubles per day of fuel cost. I estimated it costs about 3.5M roubles to build the solar panel. Maybe others can get a more exact number on that one.
So based on those two assumptions, you break even on construction of the solar panel after 41 days. (3.5 million divided by the 85k). But since it's only saving you 50% of costs and not 100%, it really takes you 82 days to breakeven.
Does this make sense? It's definitely not a slam dunk deal depending on when in the wipe you are. And you always need to have a discount factor of what you can do with that 3.5 million in the present versus what the inflated value of your money is in ~3 months time. AKA if you use your 3.5 million for some slick and meta guns, which lets you survive labs raids which nets you millions, then you need to consider that as well.