Germany currently uses about 530 billion kWh of electrical energy a year. If we assume that is evenly spread across the year (it isn’t), and we’d only need a single day to cover worst case needs (really you’d want a week probably), that is a billion kWh of storage, or $600 billion at $600 kWh. Hard to say real costs, I see numbers from $200 to $800 depending on what would be included.

If you convert natural gas heating to electric (using efficient heat pumps), that adds roughly 9x the energy needs. So $5.4 trillion. And that is just to cover average ‘overnight’, with a little buffer. If there is a problematic couple days during winter? People would freeze to death.

Energy needs would likely be more ‘peaky’ during winter, so these would need to go up a bit to deal with the spikes, but it’s probably order of magnitudeish correct.

The current German population is about 83 million, so that would only be about $65,000 for every man woman and child in Germany for the minimal best case storage needs.

For realistic ‘no mass casualty events due to lack of available heat energy every other year’, it would probably need to be 3-4x higher at least.

To protect against a 100 year storm or the like, 7-10x is probably a better idea. But I doubt anyone would ever pay that.