I have a Navien NR-240A tankless water heater. My reason for going tankless had to do with the location of my HVAC equipment closet, which is on my SECOND floor. I purchased a tankless water heater in the hope of avoiding the kind of major water damage which a conventional, tank-type water heater can cause when it has a leak. The biggest downside to my tankless has been having no hot water during power outages -- e.g. the 11-day outage caused by Hurricane Sandy. NOT fun. Recently I came across information online about a new battery backup system for tankless water heaters. It's called HUGO http://hugo-powersupply.com Unfortunately, the only information I can find on this product is provided by the guy who's trying to sell it. Though the info and videos he provides seem credible. If you have any knowledge of this HUGO product, or others like it which can provide MULTI-DAY BATTERY BACKUP for a tankless water heater, I'd really appreciate hearing about it!