I would suggest and strongly recommend using an emonTH instead.
The emonTH software is highly optimised for battery operation, and (especially if you disable the humidity & temperature sensors) should give a much greater life than you could expect from an emonTx - unless of course you ported all those optimisations into the emonTx.
For the emonTH, battery life of course depends on the type (and brand) of battery, I think you are talking in the order of 6 months to a year with a good high capacity, low self discharge, battery.
That’s the ‘best’ use of the pulse count, because deriving power from it is problematical - a c.t. is always better for that. Bear in mind of course the pulse has a serious limitation if you export energy - it doesn’t work when exporting because the meter LED can’t do a negative flash.
( - If the LED went from flashing on when importing, to flashing off when exporting, and the on and off periods never came close to being equal, then it would be possible in software by finding the light/dark ratio to determine the direction of energy flow. But I know of no meter that does that, most put the LED on continuously when exporting.)