Here’s how I designed my home alarm.
You can see on this picture 2 detector using IR sensors, sending information via radio signal to a Wifi managed control center. The detection are then sent to my iPhone using Firebase API. Regarding the power issue, I chose to use a 7.4V charged battery per detector, downed to 5V and made myself the connectors between the Arduino Nano and the power cell.
However, the main control center (right part of the picture) needs to be plugged to regular current.
Here are the links to articles dealing with each part of my system: