Transformers in inverters are a must for off-grid systems because the DC side (generation and batteries) must be isolated from the AC side. Low frequency transformers are heavier but they're thought to be more efficient than high frequency ones but the situation is somewhat more complicated that that. It's widely assumed that low frequency transformers tend to be more efficient than high frequency ones. But the situation is more subtle than that. While low frequency transformers do have higher efficiency at lower power levels which makes them suitable for low DC voltage systems, they are actually less efficient than their higher frequncy counterparts at high power levels and high voltages. The second link gives a good description of the efficiency of high and low frequency inverters.

http://www.allaboutcircuits.com/vol_2/chpt_9/8.html http://www.homepower.com/articles/how-inverters-work