Re: ...advising customers to keep their phone batteries charged...
"Most phones can work for over a week when turned off."
They'll hold charge much longer than that if kept within their recommended temperature range... i.e. don't leave it on a cold window ledge (or snow drift), the battery won't like it.
Just turning off data will increase the life significantly, although I suspect most won't know how to switch their phone to "phone only" mode, but as you say, the ultimate solution is to have the phone charged, and off, or at least airline mode... Turn the cell radios back on when you need to use it for an emergency.
If you're worried about relatives and friends, you could always arrange set times to check in, and leave the phone in low power mode for a majority of the time.
Also, charge the laptop... They're a useful source of USB power to recharge your phone. As are most modern cars, or even old ones with lighter socket adapters.
Although having said all that, personally I'd want to prioritise that I have a way of preparing hot food/drink if the electrics went out more than the phone!
- Yup, I own a couple of paraffin (kerosene) primus stoves :-D
Stay safe people.