This word in modern Arabic translates "expert". That does not fit its use in the Quran as an attribute of God, because an expert is that way after going through experiences he or she learned from. That does not apply to God.
Translators have suggested "Aware", "Acquainted" and "Informed." They are all correct but understating. The word in Arabic is an "exaggeration format" (صيغة مبالغة) and I like to convey that tenor of it. Literally, the word means "one who has full knowledge of something."
I've been using "well Acquainted" so far, but I think I'll go for "acutely Aware" unless I hear a better suggestion.
A candle loses nothing by lighting another candle.