Just a note to say that my kids have reached adult hood with no problems with bottles sterilised whilst babies on campsites in tents with no electric and just a twin gas hob and grill.
You do not actually "need" a microwave,but as Rob and Lutz have pointed out you should have no problem unless you try to have aircon, travel iron and other electrical goodies running at the same time.
If you have any concerns, just make sure that the microwave is the only thing drawing power whilst you use it.