POWER OUTAGE

Your tankless water heater will not operate during an electrical power outage because it needs power to operate the internal computer and pump. 

A simple computer backup or UPS will work. This is how to use it. 

Plug the tankless unit's power supply strip cord into the battery backup or UPS. 

DO NOT keep the tankless unit plugin into the battery backup or UPS all the time only when you need hot water. If you keep the tankless unit plugged into battery backup or UPS all the time the battery will go dead within an hour. By using the battery backup or UPS only when needed it should last for several hours or days before it needs to be charged again. 

CyberPower EC850LCD Ecologic Battery Backup & Surge Protector UPS System, 850VA/510W, 12 Outlets, ECO Mode, Compact, Uninterruptible Power Supply UPS

You can find this type on Amazon or any office supply store.

Keep your battery backup plugged in inside your home and not in the garage or attic. The heat and cold will reduce it's life.