External batteries are the best to have, but in a pinch, you have other options for recharging a device when the power is out.
You can charge off a laptop if you have one — a good reason to charge your computers, even if you don’t plan on using them. If there’s an emergency hand-crank radio nearby, check to see if it has a USB port you can connect your phone to. (Be prepared to crank for a long time.)
If there’s a car available, you can connect through a USB port, whether it’s built in or through a lighter-port attachment. You can charge in many cars by turning the key one notch to fire up the battery but not the engine. Do not attempt to start the car if it’s in deep flooding, buried in snow, or in a closed space like a garage. If you’re going to need the vehicle to evacuate, be careful not to drain its battery on a phone.