The Shi are really their own faction and are not a part of the NCR. Whether or not they help the NCR is up to them and is not guaranteed.
In New Vegas it's mentioned that they do want to route power from the Mojave to not only New Vegas and other areas but to Shady Sands as well. Not only that but they wish to optimize the power flow as well and it's mentioned that Helios One is only outputting at 1% of its potential.
Here's some excerpts from the NPC Thomas Hildern that kind of paint the need for power and also hint that NCR's food production and consumption are not balanced.
[Do you control the Dam from here?]
>Directly? No. But we attempt to maximize its output. You have no idea how difficult it is to provide power to an entire nation.
>So far, they've only managed to achieve one percent power output. I'd send help, if I could spare the personnel.
[NCR has a food shortage?]
>Not yet. But our government understands the value of proactive thought. Our studies project an imbalance between production and consumption.
And here's an excerpt from Angela Williams that shows they send power back to Shady Sands
[Are you a scientist?]
>But I know every inch of the power grid from Hoover Dam to Shady Sands. That's what we do here - try to optimize the power output from the Dam.
>Some of the others are working on an agricultural project, but... truth be told, they aren't making much progress. Not yet, anyway.
Short of it is NCR has BIG problems during the events of New Vegas and the first battle of the Hoover Dam and New Vegas wasn't really written with Shady Sands being nuked in mind. With it being nuked, it really both changes some story elements and also enhances others. In particular, NCR's food problems and potential power problems.