The easiest and most cost effective back up would be a diesel generator. Kubota makes a nice little 10kw model that sells for under $6k. I have one and it burns about a quart of diesel an hour at 50% load. Much more economical to run than propane or gas, are designed for continuous operation, and are rated for 20,000 hours of use. The only drawback is that you need to have a way to get the diesel fuel delivered, but a 55 gallon drum fuel tank will give you over a week of continuous operation.