Review on 🔌 SunFounder 2 Channel DC 5V Relay Module with Optocoupler for Arduino R3 MEGA 2560 1280 DSP ARM PIC AVR STM32 Raspberry Pi: Expand and Trigger with Low Level Control by Kenny Marozas

I think this has one major disadvantage that most will find similar. You would think that if you send 5V to a relay it would turn the relay on, but the opposite is true. Applying 5V to the relay opens it, otherwise it closes. So if you want to turn something on for the hour/day, you have to turn on the relay for 23 hours if you don't want to turn it on. Using this with a solar application means you are draining the battery at night when there is no sun as you have to send 5V to keep the relay open. The dangerous thing is that every time your Arduino powers up/boots, all the relays turn on for 2-3 seconds until the boot completes, and you can power the ports to turn them off. There is another 4 channel red board relay at the same price that announces the fact that the relays are disabled with no power and are turned on by energizing and that's better. In contrast, when the Arduino is first powered on, none of the relays turn on for 2-3 seconds, and energizing the relay turns on the load, which makes more sense. I recommend the one with the red sign.