That's a fairly ambiguous question. It depends on how many watts you need to pull and how much sunlight you receive. Here in AZ I could see using one to assist in charging the batteries. They get pretty pricey when you are talking enough wattage to power more then the lighting, waterpump, etc. When you are talking about charging the batteries all day while using little electricity elsewhere in the trailer I would find that to be a great idea. I've been toying around with it myself. Beats running the generator just to charge the batteries.
Ken....