GNU Units has builtins for some standard energy equivalent units, specifically "tonoil" and "barreloil". "litreoil" doesn't seem to be included by default (though it can be added via a local configuration file).

    literoil = 1 barreloil * liter/barrel

And the calculation gives me ...10.69 kWh/L

The barreloil value is for crude, diesel may have a slightly higher net energy density. But we're good to 3 sig figs.

I could have bypassed the tonoil step and calculated thermal energy in barreloil directly though saving another conversion.

Otherwise I'd have to remember energy density/L of diesel fuel.

I've added the definitions for litre, pound, and kg oil and can now run:

  $ units --terse '40 MWh/30%' literoil
  12470.969

The fun part though, at least for me, is showing how to do conversions with GNU units, and the equivalences which become apparent doing so.

I looked around a bit in "definitions.units" and there are actually energy densities in there for various fuel sources, including diesel.

  You have: 1 diesel
  You want: Wh/L
   1 diesel = 10103.465 Wh/L

Slightly different value than in Wikipedia, but close enough.

*blink*

TIL, there are!

Thanks.

(For the curious, the definitions file is ~7500 lines, there's a lot in there. GNU version. For those on MacOS, the stock BSD units offers far less.)