Good old pencil and paper + carrier pigeons. It's just a puzzle. Minimal electricity means minimal computer mining, which means the difficulty would decrease dramatically on next update. Besides, even if some catastrophic solar event were to take down all the satellites and knock out some sections of the grid, it wouldn't adversely affect hydroelectric, geothermal, solar, or wind power. Absolute worst case is the network would go dormant.

Now, let me turn that question around: How can you survive without electricity? If the world loses electricity, you wouldn't be worried about bitcoin, you'd be worried about survival.