If like me your not bothered counting your pints, and you want to find out how much beer is left in your keg , simply leave the door open for 5 - 10 min then you Will see a condensation ring up to the level of your beer. Nice one its half full.
Good tip!
Off top of my head...I think they're ~4.5kg empty. I use a travel scales thing for weighing your suitcase to check mine. They only cost a couple of euros.