You're not really storing milk but it's energy. in 2 years my cow could stil not produce milk, so I could repay you in something else, like loaves of bread.

And yeah, if society is destroyed, your energy store is drained. Because it's a complicated system and what gives this battery its capacity is not just GDP but also the "metaphysical" belief and social contract that society places on it. That doesn't mean the energy is not stored, it means the battery is not very reliable.

I don't know if you're familiar w/ occult terminology, but it's an egregore: a metaphysical construct created by public. It has power and we charge it with energy. It stores the energy and can give it back. It is fallable and can fail miserably.

Another example: I can spend lots of energy working for a boss and he could underpay me. The amount of money he does pay me does have energy that I can either store in money or conver to callaries, etc. The energy stored in money he doesn't pay me doesn't just disappear. He still has that money that I made for him by working for him. He can use that energy on himself. Even if he burns that cash, that energy doesn't get destroyed but evenly sprad amonst the other dollars that are left in circulation (the opposite of inflation, the supply of cash decreases, thus increasing the price of the currency).