Secure storage of Data in external EEPROM
There's a couple of things going on here:


You have write operations that might be interrupted by a power loss.
You have 2s of time before a loss of external power becomes a problem.
you need to write a couple of bytes


That's basically not a problem – the time to write data should be within the micro- to lower milliseconds. State-of-the-art encryption for this would also not take seconds on a DSP microcontroller, so if this is an issue, you chose the wrong encryption algorithm, or your implementation is slow. (Hint: you need to sign your electricity counter data, not encrypt it it. That's often easier, since you then only need to encrypt a short, fixed length hash).

The question is whether storing confidential or sensitive data externally is a good choice - I don't know the dspic series well enough, but I'd assume there's variants of the same controller with integrated EEPROM that you can write to.

Also, if whatever you use to encrypt is worth bare money, never assume that anything isn't readable by an end user. That applies to your firmware too, so if you safe the user's private in external eeprom, any user can potentially use their own key to en- or decrypt anything.