Assuming nothing exists except what you already know about gets you reliably wrong answers.
In fact there are a lot of hills. Few places are very far away from any, even in Nebraska. Any hill may serve for pumped hydro, although those over 2000 ft (600m) and have a natural depression are best. A simple earthen levee suffices, otherwise, with a penstock extending down.
But we also can compress air, for "CAES". And we can liquify air. Both of these are extremely mature technology. Compressed air is cheaply and safely kept deep underground or underwater, and released through a turbine to recover power put in. Liquified air is vaporized with heat from ambient air.
We can electrolyse water and keep the resulting hydrogen in underground cavities, under increasing pressure; both the hydrogen and its pressure store energy. We can synthesize ammonia, to tank and transport, and burn it for fuel in ships and existing combined-cycle gas turbines.
Ammonia synthesized in the tropics or where wind is reliable may be shipped anywhere in the world to fill in for shortfalls of local storage. So, nobody needs much local storage.
Aviation will soon transition to liquid hydrogen because the value proposition is so great. Anybody lagging will find themselves wholly unable to compete. LH2 will be made and banked on-site at major airports, from power delivered via transmission line.
Another simple storage method uses buoyancy, drawing floats down toward the sea floor. Another, raising and lowering a heavy weight in a disused mineshaft. The mineshaft may also contain compressed hydrogen.
Your off-grid domestic batteries are the most expensive way to store energy. But they probably will last you longer than 10 years, just as your refrigerator will probably last longer than 3. In the near future you might come to bank hydrogen instead, and put excess in your car. Your electrolyser will last more than 10 years. The compressor might wear out in time, but is cheap.