The Powerlet system is a good one, albeit a bit pricey when bought from them. Still, much less expensive than buying an electrified tank bag from Bags Connection.
Their system is essentially an
SAE 2-pin bulkhead connector and the appropriate SAE cables. SAE connections have the advantage of being highly weather resistant. They are the same pins as used on 4-wire trailer lighting connectors and you know the kind of slop that those are subjected to (think boat trailers). It's also the type of connector that is used on battery tenders and 12V air pumps, so the battery power connections installed on the bike can have multiple uses.
You can find the SAE 2-pin connectors (for very little money) all over the place. Here's a prior thread that discusses a few options:
clicky