Neodymium magnets have an energy capacity of 512 kJ/m3 or 0,512 MJ / cubic liters. That's equivalent to 0.0085 kWh or 8.5 Wh each 1 dm^3. You could recharge an mp3 player for 34 hours, and then you'd have to throw away your magnet. That's still 468 times less than the capacity of a Lead-acid battery. Do you think a Neodymium magnet is a good energy storage? Even though a lead acid battery is 468 times more powerful than a neodymium magnet, doesn't mean it can deliver 3978 W (power a whole house) for an hour at the size of your milk carton. That's probably too much current drawn from a battery, which would lead to an explosion. Trying to draw that much energy from a magnet is probably even harder.