Methods of increasing voltage

Hello folks first post on this site. Im currently at college doing a project for my HND (Higher National Diploma) and I have to make a solar paneled air conditioning unit. The main idea behind the project was so when in the event of a mains failure, the solar panel kicks in and continues running the air conditioning for a further two hours. Basically so you dont fry when you wait on the mains getting fixed 😀.

We were told to come up with three solutions of how this can be done, the only thing I could change would be how the voltage is increased from 12Vdc to 230Vac. The main solution I am using is an inverter, the second solution a step up transformer and...I can't think of a third solution. I know its probably pretty obvious once someone tells me but please help as its annoying trying to find something 😀

Many thanks.