Adding remaining time on battery widget
There is simply no way to get an accurate number for the remaining time (it's not possible without predicting the future ;-). As soon as you switch e.g. from working on document to watching a video or compiling some heavy piece of software, the battery of your device will get used much faster and thus the "remaining time" of your battery decreases in a much faster rate than "predicted".
"Smart" batteries (e.g. those with support for SBS) are generally only found in laptops (so devices are out) and do a very good job of knowing the current percentage, but do not solve the future-use-prediction issue.
There were already several discussions about adding the remaining time. See e.g. 
In this commit for Plasma 5 the remaining time was re-introduced by default. The rationale behind this can be read in the linked commit message. (The current Plasma 4.x situation won't change)