Background
I'm working on a seismic alert network and seismograph station at my school in Puerto Rico. We're building a solar system for the system to continue running during a potential seismic event. The system only consumes about 10 Watts, but with the satellite internet equipment it might go up to about 40 Watts. Based on my calculations, the needed battery bank will be about 200AH (12V System with AGM batteries for safety). This goes a bit over our budget, so I thought of redesigning the system so it would be more of a "backup" instead of 24/7 solar power.

Question

Could I use a relay to change the load to battery power if the mains electricity power goes out? This way, the satellite internet equipment will always be powered by the school's mains electricity, but once it goes out, it automatically changes to battery power from the solar system.

What I had in mind

I thought of using a relay to control the change as follows:

Common: Load Positive (AC)

Normally Open: Mains Electricity Positive (AC)

Normally Closed: Solar System Battery Power Positive (AC)

The relay will be activated with a 5V Power Supply connected to the school's mains electricity, so if the power goes out, it will transfer the load (satellite internet equipment) to the battery backup power.

What I want to accomplish with this

So what I'm thinking is that by maintaining the satellite internet equipment connected the majority of the time to mains electricity, I can downsize a bit the battery capacity needed for the solar system, since it will only consume the 10 Watts from the seismograph. Then in the event of a power outage, it will provide some hours of power to the internet system, even if it goes out before the sun goes up again.

EDIT

Is this an automatic transfer switch I'm referring to?


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Feel free to give suggestions about the system, if it could work, if it would be dangerous, or if there's a better solution to it! I appreciate your feedback!

 

It is a simple solution that should work well as long as you select a relay with contacts suitable for switching the load. There is one potential problem. There will be a short interruption in the supply to the internet equipment as the relay switches over. Will the internet equipment recover from that? If not, it will be a problem that needs to be addressed.
Regards,
Keith

 

It is a simple solution that should work well as long as you select a relay with contacts suitable for switching the load. There is one potential problem. There will be a short interruption in the supply to the internet equipment as the relay switches over. Will the internet equipment recover from that? If not, it will be a problem that needs to be addressed.
Regards,
Keith

Thank you for the feedback! I don’t think the short interruption will be a problem, but will sure have to do tests with the equipment. We don’t mind some minutes of downtime, if it does go back online afterwards.
I appreciate your response!