The previous solution is definitely a great way to boost the kW, but I found that I had plenty of money from selling off items that were piling up and was able to pay for kW of power from the fisherman on the west side of Old Lestallum once you go down the stairs. I didn't see it mentioned on here anywhere but I can confirm the achievement popped for me after purchasing power from him. 222000 gill trades for 500k kW of power and 45000 gill for 100kW of power. Not sure if this was added in the recent title update but maybe it will help reduce the grinding for some people.
***I would like to add to my solution that this maybe unnecessary for completionists because by the time you played enough to get a level 99 chocobo and all the sigils you likely will have earned enough power anyway but it is an option if you find yourself coming up short***