Paul I agree with what Leeboy is saying.
Gas generators are noisy and outdoors only.
Some keyboards can accept 9V or 12V DC directly from a battery. Most (i.e., recent Yamahas) require more than 12V, which means you'll need to plug them into a battery-powered "inverter" or UPS that simulates a wall socket. I'll guess that a big, heavy ($200) UPS will run a keyboard for 45 mins to an hour on that setup. Edit: I think the device in your link is a good choice, and a good value for the price. But I personally wouldn't bother with the solar generator. Of course you'll need your battery-powered amplifiers too.

No battery-powered keyboard will compare to a pro arranger. The Casio CT-X3000 has a pretty complete feature set. But navigation is awful with its tiny LCD display, and the keybed is junky. On any of them, the speakers won't be very loud. Which takes us back to the inverter setup.

It can be done but you would probably need a cart, because you'll be hauling some heavy gear. My $.02.