What are you planning now? I'm intrigued...

Anyway, not that I have any experience of this, but I'd be wary of relying on battery power. If I were projecting something in my garden, I'd run an extension from a socket in the house. If you are looking for something more mobile, then powering a laptop & projector from the 12V power socket in a car (with the engine running) might be a solution. Keep in mind that DC to DC convertors are more efficient than DC/AC inverters.

Your question reminds me of this stunt back in 1996...