On Sunday, October 29, 2017 at 2:28:30 PM UTC-4, sms wrote:
> My wife and daughter are going on an Inca Trail trip to
> Machu Pichu next year.

Nice!  FYI, do get into good physical shape, particularly
if you're taking the classical trail (including altitude
acclimation work if possible).
 
> What are some ways to manage the lack of any way to
> charge devices along the way?

I looked into this very same question when we did the 
(easier) "Sacred Valley Trail" a decade ago.  

We ended up having 4 days/3 nights on the trail without
power.  Since one does spend quite a bit of the day 
just hiking, I found that much of the time spent taking
photos was after we hit camp each afternoon.

For how many pics to expend, my data's limited because
it was in the days of film/digital "carrying both", so
much of my stuff was on film ... nevertheless, I'd probably
today gage my expected consumption rate to be ~200 images
per day of hiking, and then potentially as many as 400
images on the day of arrival into Machu Pichu (since the
common itinerary doesn't get you back into a hotel until
after your first day at the site).  

IIRC, the Inca Trail hikers had traditionally spent their
last day on the trail at the campground next to the building
(Youth Hostel?  Pub?) that's above "Winay Wayna"; waking up 
predawn to then be at the Gate of the Sun for sunrise.  I
would have expected for there to probably some power at this 
building, but with supply-vs-demand of all of the other hikers, 
I'd not assume being able to get an outlet...and from a quick
web search, it sounds like it may be a moot point now, as it
appears that this business has since been shut down:

"This is why we can't have nice things on the Incan Trail."

<https://www.atlasobscura.com/places/the-former-winay-wayna-pub>


> I can [various options] ...

Yes, you've pretty much covered all of the options. The only
thing that's really changed from when I did it is that 
you're not also carrying film and that digital cameras have
become much more power-efficient.


> Has anyone here come up with a good solution for trips of
> several days where they will not have any access to electricity?

Yes, as you've concluded, a pile of spare batteries.

FYI, the one thing that you've possibly missed is to make sure
to have **redundant** battery chargers for your batteries.  Not
only does this allow parallelism (time compression) for when 
you do have access to power, but it eliminates a single point of
failure risk.

(BTW, also carry spare lens caps too).

> The other suggestion I made is that we resurrect one of our 
> many old P&S film cameras and that they take a bunch of film.

I'd limit that option today to if you're going to be off the
grid for more than a week, and if you expect the quantity of
photos to be taken is manageably low.  Even so, today's digital
camera battery "storage" life is quite good too, such that we've
been able to become lazy and not terribly concerned about 
constantly keeping batteries topped up, etc.

----------

FWIW, I had a similar battery life concern & planning on
some of my African safari trips as well ... what would happen
if we hit a camp with zero power?  My basic plan was to have
adequate batteries to be able to go through 3-4 days of normal 
operation, which at ~400 pics/day works out to be a requirement
to have roughly 3 batteries per camera.  FYI, having two 
cameras which use the same battery can help out here too, 
especially from the perspective of reducing the number of
(redundant) chargers you need to have along.


-hh