It is indeed an energy issue, but for the orbital change, not system operations. Every kilogram you have to spend on fuel is a kilogram you can't use for cargo instead. You can use porkchop plots[1] to determine ideal launch windows (least ∆v = least fuel needed). Late summer of 2022, for example, is a good window[2].
For manned missions, you're more interested in the least '∆t' (time spent traveling) since crew sanity/health is more important than fuel. (Yes, astronauts are exceptional and willing to endure a lot of discomfort, but the less time they have to spend in a small tin can, the better.)
[1] https://en.wikipedia.org/wiki/Porkchop_plot
[2] http://www.amssolarempire.com/Programs/porkchop_plot.png
