Tools

I don't know of anything commercially, but I have an idea of where to go.

I'm not sure exactly what you're after, so let's say I am living with someone who needs medical equipment to live and it needs.mains power and it's for breathing support or whatever. If there was an extended power outage in the middle of the night, bad stuff might happen if the equipment wasn't manually switched over to battery.

I could make something up using customized components bought from a hobby electronics store.

I would buy a low voltage plug pack, say 9 volts and this would plug into the wall and connect to a low voltage relay kit.

Relays have a common point and a normally open point and a normally closed point. Without power you can run an electrical current through common and normally closed. With power, this changes. The electrical pathway between normally closed and common is broken and common is connected to normally open.(or vice versa, been a while)

The shop sells a low voltage relay kit which I would connect to the plug pack and when it's powered, flips the relay.

I'd wire up a secondary circuit, connecting normally open and common to a battery and piezo buzzer. When the power goes off, the relay flips and buzzer makes noise.

I'd also consider wiring a LED up to the battery as a way to confirm it still has energy and check the batteries monthly and replace yearly or so.

I would have to solder some wires and components to do this, but it wouldn't need an electricians license here as it's a low voltage component.

Hope it helps.

Edit: from the website jaycar.com.au, sample parts are:-

Mini piezo buzzer 3-16v dc 12v dc relay card kit ( picture and description doesn't show me if it has wiring I'd need, but I think so) 12V DC 400mA Ultra-Slim Power Supply 7DC Plugs 4AA battery holder.

I'd also ask the staff for advice when Id go to buy the stuff as you probably want a resistor somewhere in both circuits.