Black Hole Sun
To trick the solar sensor, you can use a black light in order to recharge your power.
Obtain Dark Gun Parts
Beat the game once to get the Dark Lens. Beat the same game twice to get the Dark Frame. Beat the same game again on hard mode to get the Dark Battery. You cannot mix the Dark parts with the normal parts. The Dark Gun can only function when all the Dark Gun parts are combined.
Sensitive Sensor
In the beginning of the game it tells you to cover your sensor, do this in an extremely dark place and then you will get more light.
Infinite Solar bank energy
Go to the solar bank and deposit all your Sol there, then save and turn off the power, turn it back on and go to options, set the date to a few months/days (possibly years) forward, then load, go to the solar bank and your account will have grown to about 999, withdraw the sol and then you have near enough infinite sol in the solar station.