On Etherscan the daily GasUsed chart shows current usage of 10-15 bill. How do I convert this correctly to ether (ETH)?.
There's no such thing as '1 gas'. You can't convert gas to ether because those are two different things. Gas is a measure of the computational power needed to execute the transaction. You're probably asking about gas cost which is
gas * gas price and is usually measured in Gwei. 1 eth = 1e9 Gwei.
The gas price is dynamic. It is determined by the miner of a block and can therefore change from block to block. You can find interesting insights without having to download the entire chain on http://ethgasstation.info. Many miners are currently accepting transactions with gas prices of 4GWei and below. You can also see that most users do not want to wait for one of those nights accepting lower gas prices and actually send their transaction at 20 GWei.