I am thinking the portable jump start packs might be the most simple and economic solution.
Murphy's Law says that you will have a power failure when you can least afford it.
Having a "spare" battery seems illogical as it will need to be wired in like the other batteries and cycled with load and charging like the rest of them.
The jump start pack will need that too but if used regularly for other purposes (can't think of any at the moment) it would be a handy thing.
A power failure is usually caused by one of two things, they being poor management of battery loads or an electrical fault. The main thing is to be able to get the engine started. The jump pack should be able to do that.