I comment 50+ times a day and upvote much less than 20 times a day. I have my own rules for what I upvote and don't feel like I need to justify my habits. but this is what I do what anyone asks me why I did not upvote something (rare, thankfully).
If I comment on a minnow post without upvoting, I will tell them in my comment that I do not have the power available right now. If the same thing happens on a big fish post, I do not mention the non-voting. I know some minnows obsesively scan their votes recieved, so I try to cut them off at the pass with my original comment.
But if someone asks me why I did not vote for any post or comment, I will tell them something like this:
"I had low power when I was on your post and could not vote. I make a lot of comments each day and I cannot upvote everything I run into. According to the reports I get from penguinpablo, the average steemit post gets less than three comments and a good portion of those are bot comments. Comments and engagement are low here unfortunately, so I spend my steemit time on commenting for the most part. As you know - upvotes on a minnow post are hard to come by, so I do much more commenting than posting."
I find I do not usually need to make this explanation, but I have it ready for when needed :)