Realtime Clock in Arduino

Internal

Arduino has an internal clock. Is shut down if Arduino loses power. Can use 3.7 battery for power. Library is RTC.h.

https://www.arduino.cc/en/Reference/RTC

Power draw for Arduino on standby on 3.7V is about 20ma. So uses about 1.5Whr every 24 hours.

A solid state relay will draw .1W on the driving side, but the load side loses significant power (1V drop). 20W loss when running a 20A load - significant through the day.

May want a mechanical relay. Sainsmart relay is 15-20mA at 5v to energize. And no voltage drop on the power side, as it's not going through a transistor like in a solid state relay.

External

Module is cheap - [1]