Just before saying anything, I just want to warn you that I don't know much about batteries and energy storage, consider that you are talking with a kid of 8 years and a half. Maybe this question is also more suitable for electrical engineering department. I have heard that one of the challenges about energy, is storage(batteries). Maybe I am wrong about this statement.
More specifically, how can we store energy efficiently that is originating from solar/wind source.
Let consider this example : Our Sun produces some X amount of energy that goes through the earth's atmosphere, and when it gets to our solar energy farms, it will all be concentrated and let's say that 100 Joules leaves our solar parks.
It will go through the wires, to our battery packs (e.g Tesla Powerpacks).
Some of that energy will be lost through the wires(AC current), so we will have 98 Joules getting to the powerpacks.
Once the 98 Joules gets to the batteries, it is being converted to DC current and while being stored in the battery, a lot of energy will lost, and the batteries will be heating which causes even bigger losses. Lets say that we end up storing out of 98 Joules, only 70 Joules per hour.
Note that I have chosen Joules just to simplify the problem.
The energy that is being stored in the battery will also be lost over time.
Also, when will want to use that energy from the battery, it will go through the same process and once it reaches our homes, some more energy will be lost.
Now, can someone guide if this all makes some sense, and confirm if the greatest challenges with energy storage is when the energy gets to the battery and is being stored for later re-use.