Making a backup of my wallet

Question

Let's suppose I make a backup of my wallet when it has 0.001 BTC, but then I transfer to it 0.001 BTC more (so at the end it has 0.002 BTC) and then my PC dies...

If I restore my wallet it will have 0.001 or 0.002 BTC?

Supposedly all transactions are recorded on the network (blocks), so what I understand by backup wouldn't be a backup, but rather a way to re-make my wallet on another device and my final balance will always be rescued no matter if I did a restore with a backup made one day or 5 years ago...

or do I have to do a backup once in a while?

Answer 1

If you made the second transaction to same Bitcoin address so yes you will not miss that second transaction even if you made the transaction after the backup.

if you made the transaction after the backup and you made the transaction to a different address you will also get any future transactions for two reasons:

1- Bitcoin Core using HD wallet so you have one master private key and they derive all your future addresses from that one seed.

2- if you are using old bitcoin wallet or HD wallet set to 'OFF' you will still have future transactions but not forever and that because of the limit of keypool future.

Keypool New public and private keys are pre-generated and stored in a queue before use.

This pooling feature was added so backups of the wallet would have a certain number of keys that would be used in the future. By default, the number of entries in the queue is 100[1].

Answer 2

Your backup contains in the wallet.dat file the keys (amongst others). With these keys the wallet would lookup the amount of the funds on the blockchain. Whatever is recorded on the blockchain, will be displayed by your wallet software. (The wallet does not keep the amount, it is not the ledger - blockchain is).

If I restore my wallet it will have 0.001 or 0.002 BTC?

You'll have 0.002 BTC.

You are ok with your backup, and if you use bitcoin regularly, a general rule of thumb is a monthly backup. Backup is always a good protection mechanism against data loss - that shouldn't happen to your keys. If you have a five year old backup, how do you assure data is still readable? (this is media dependent, for sure). Also, in 5 years, wallet software may have changed data structures. Better increase backup cycles!