I don’t know about your cousin in HI but you have to size your system for the number of KW you plan to use every month. There are a lot of variables with batteries. It depends on what kind of battery you use and how low you bleed them down before you recharge. Usually you should not let your batteries get below 50%. If you have good batteries and a sufficiently sized bank for your needs they should last at least 5 years minimum.. That’s in constant use. The battery bank I’m planning buy is going to run about $2,500 . So 5 years would be $500 per year so less than $50 per month. Not such a bad electric bill.
Its also a good idea to put most of your appliances on propane and only use the solar for lights, TV, well pump, washing machine, computers, vacuum cleaner etc. That’s what we are going to do.
Well, they are empty nesters (except for company) and live modestly. I think maybe he got rooked on the battery portion of his system. Hawaii electricity (they’re actually on Kauai) is expensive (diesel fuel) so they thought it would be a fair bargain, but I don’t think they will live long enough to see break-even (both in their 70s). Make sure you get a reputable vendor for your batteries. Those things can be dangerous. Put them in a well-ventilated area. You don’t want hydrogen gas building up.