I got this casio ctk-100 keyboard at the transfer station a while ago but it was sitting around for a while. It didn't come with an AC power plug and it has space for AA batteries, but it would be much more convenient to run off of DC 12v automotive power plugs or usb (5v DC) which is what most of our off-grid wiring is set up for.
I finally opened it up to install a boost buck converter tuned to 9v which the keyboard expects. I got this boost buck converter in a large pack a long while ago and it was $1 or something. Boost buck converters are really handy: they convert a range of input voltages to a constant output voltage. You can tune the output voltage by turning the tuning screw as you observe the voltage with a multimeter.
I first installed a 12v power port plug and then later (right before heading to a birthday party where people were playing instruments) I rigged the keyboard up to also accept input from USB. Both plugs are wired through the same boost buck converter but if you tried to plug both in at the same time probably something bad would happen.
I also recently got a solar panel at the transfer station that I was somewhat surprised to find worked fine without having to make any repairs. I set up my folding solar panel and the transfer station panel to charge a usb battery with a second larger boost buck converter. The battery can do pass-through so you can play the keyboard while the usb port is plugged into the battery while the battery also charges. Each panel is plenty to charge the battery and run the keyboard at the same time, but not quite enough to run my laptop.