Does Low Power Mode turn off all wifi, cellular, and bluetooth? Those are big energy draws. Dim the screen as much as possible, of course.
Out of the box – how about a hand crank electricity generator? I have no experience with this, but it might suffice to give a power top-up to your phone, with (a quick web search reveals) quite a bit of physical effort.
I have quite a powerful signal here after a new tower came up some years ago just across the valley. It is perfectly situated to cover the mini side valley I live in. Before that, it was no coverage here. For a project, I even tried to find a spot without coverage here with no luck. Interesting solution you had there, but not for me, I think. Low Power Mode maybe turns of background refresh? Have to look into that. Location services are only on in apps when I use them and need it.
No, Low Power Mode does not turn off any of those. I can not turn off cellular. Wi-Fi and bluetooth I use in local communication between my devices, but I could turn them off and on as needed. The hand crank idea I will have in mind.
It turns out it is not a solution, even for charging an iPhone, but I did some calculations for using gravity to store energy - gravitricity:
Floating Solar (ignore the title shown here)
Here are my calcs:
http://www.vdrsyd.com/mp/ev_home_power.pdf
This also explains why manual cranking devices are not really a solution.
Adam has just posted an article about low-power mode for the Apple Watch. There might be relevant tips for the iPhone:
I read his article, and that is why I deliberately omitted to ask for advice regarding my Apple watch 
I have a little stream running through my property. It is never dry. I have not looked into it yet, but a mini power plant would be great. I have a feeling it is difficult to get a permit though.
There was this thread a little while back. (I posted the Kite Power link… I just looked again at their site. Intriguing. They’re moving forward)
Yes, it is an intriguing solution, but not here in the forest. I have to look at the top of the pine and spruce trees 30 meters above to check whether the wind blows.
Thats interesting!
I looked at mini-hydro for a friend’s off-grid cabin. Again the energy calcs were not good. His idea was to use solar to pump water into a tank that was uphill and use it to generate electricity. The hope was that it would store “excess” solar energy when the battery was full.
I concluded that gravity is a poor way to store energy unless the vertical drop is very large (the Gravitricty website that I link to above intends to use a 2 kilometre vertical mine shaft!).
I expect that harnessing energy from a flowing stream would have similar limitations.
So … I can understand your wish to reduce energy consumption of your devices.
Incidentally, how do you stay warm in Norway? Here is a wood-burning stove that generates electricity:
(I have no idea if this is credible - UPDATE: the link to InStove on that webpage links to a gambling website. This seems more genuine: Thermoelectric Generators for Wood Stoves - TEGmart but it is expensive [$717 for 100W] and they have nil stock on hand!)
Old fashioned water-mills used Venturi effect to accelerate the water. Worked ok for a few thousand years. 
OK - the venturi effect increases the speed of the water (which might improve turbine efficiency) but the energy potentially generated depends on the flow (litres/sec) and head (metres). This site has the calcs:
https://www.renewablesfirst.co.uk/hydropower/hydropower-learning-centre/how-much-power-could-i-generate-from-a-hydro-turbine/
Settings → Privacy and Security → Location services
Turn off everything you do not need location for. That will minimize gps use, which is a known battery hog.
Then you may also reduce notifications or restrict them to the bare minimum as they consume power when communicating in the background.
Reduce screen brightness.
Preferences → Battery check usage by app and try to minimize use or turn off apps that show up in the list though you do not actually use them. They probably run in the background and eat up your battery.
I wonder if Focus could be used for this. It seems like a big job, when I would want them back in 2-3 weeks’ time when I am back on the grid.
That seems like something I could really use. I keep warm with a wood stove and have to fire it up more and more now in the fall (-3 C/26.6 F this morning) when the sun based energy is producing less because the sun is behind clouds and days are shorter. In middle of June I have 19 hours days here, now it is 10 hours.
I wonder how many watts is needed for charging my iPhone…
Not sure if Focus also reduces the background communications or only the on-screen notifications. Keep in mind that the background work, even if it doesn’t show on screen, still consumes power.
You are probably right. I will remove the less important notifications.
How about a stationary bike that generates electricity?
Thanks for trying to keep me fit.
But I have other use for that amount of money.
Off-grid living? Yagi antennas? This thread is definitely my jam.
You don’t have an iPhone electric usage problem, you have a power generation problem. The good news is an iPhone doesn’t draw a lot of power, so it’s a fairly easy one to solve. Solutions I would explore:
Using an indoor wood stove to generate power is an intriguing concept. I’ll have to look into that.