Besides what Cheshire Figment mentioned:
How often do you charge your wheelchair normally?
How far do you normally go per charge?
My DD has a TDX power wheelchair with an MK5 joystick/controller. Hers has an odometer that tells how far it has gone. I have not looked at how far hers goes between charges, but if I wanted to, that would be a good way to check. Knowing that the average person walks between 5 and 9 miles per day at WDW, I would have an idea of whether or not she normally goes that far between charges.
Most power wheelchairs will all day without recharging. I looked in the instruction book for the TDX wheelchairs and it does say the batteries will operate on one charge for an average range of 5-9 hours. That would be continuous operation and it's not likely that you would be driving the power wheelchair for 5-9 hours straight without stopping. That is also at the average weight of users. If you are heavier or lighter than average, you will get more or less (For example, my DD is only a bit over 80 pounds, so she gets more miles). The more weight you are carrying on your wheelchair, the less range you will have (so even though it may be convenient for you to carry all your party's stuff, it may lower your range).
There are things that use more power - such as going on an incline (like up hills) or going on rough terain. If you have any power features like tilt or raising, those use power too.
Some things you want to do include turning it off when you are stopped in attractions or for waiting in lines for things like shows where you will be in one spot for a while (that helps conserve energy and also helps prevent the wheelchair from moving if the joystick is hit accidentally).
Keep track of the charge indicator. If it is down toward the 'empty' part of the indicator, you will need to charge. Some people plan to charge while they are eating. If you need to plug in, ask a CM. There are outlets hidden in places you would not think to look, even outside.
I would assume that the
ECV chargers would not fit your wheelchair. You can carry your charger with you, but if it is anything like my DD's, it is prettty big. An alternative would be to store your charger either in a locker at the front of the park or in First Aid. If it runs out of power, CMs can get a manual wheelchair for you or an ECV to use while your wheelchair charges.
After your first day, you should have a good idea of how your power holds out. That will help you to plan for the rest of your trip.
There is a link in post #2 (and also in post #3) of the disABILITIES FAQs thread where people estimated how far they walked each day at WDW. You may want to check that out.