Facets in search page URL

Note: This is similar to this question, but I'm asking specifically about the use of facets.

I am working on re-implementing search functionality on a site for which search is quite important. There are tens of thousands of products, so faceting is a big part of how users search. There are dozens of properties that don't all apply at once. I do not expect users to bookmark the search, type it in directly, or interact with it as an API. My question is regarding the best practices when generating the URL, only insofar as they may see the URL in the address bar.

Consider that a user may choose to facet down to widgets that are red or green, supply 10A or 15A of power at 120V, are currently in stock, and contain "foobar". Currently, the URL looks like this:

https://www.example.com/search.aspx?q=foobar&num=30&start=0&fc=amps%3A10%7Camps%3A15&fc=color%3Ared%7Ccolor%3A&fc=volts%3A120&fc=instock%3Atrue

I could leave the URL structure as is, or do something like this:

https://www.example.com/search.aspx?q=foobar&num=30&start=0&color=red&color=green&s=10&s=15&voltage=120&instock=true

Alternatively, I could even do something like this, and disregard the user entirely:

https://www.example.com/search.aspx?q=foobar&num=30&start=0&f=Y29sb3I9cmVkJmNvbG9yPWdyZWVuJmFtcHM9MTAmYW1wcz0xNSZ2b2x0YWdlPTEyMCZpbnN0b2NrPXRydWU%3D

Are there any best practices or suggestions on how to format the facets in the URL, or thoughts on if the user actually cares about it in the first place?