How to rig an audio alarm for a tankless water heater

I have this Rinnai propane-fueled tankless water heater. It is installed in an area of my basement that is insulated but not heated. Other plumbing is installed in that area as well.

I live in a cold climate, and that part of my basement gets colder than I thought it would - sometimes it gets colder than 32 degrees F in there. The water heater could be damaged if it is allowed to freeze, and that is why it has an "anti-frost-protection" feature that is essentially a small electric heating device that turns on when the temp inside the unit approaches freezing temp.

I would like to rig some kind of device that would emit a sound when the anti-frost-protection feature kicks in. According to the specs for the water heater, the heater pulls 2 watts when it is in Standby mode (i.e. when it's just waiting to be asked to do something), 76 watts when it is heating water (the 76 watts are needed to power the exhaust fan), and 120 watts when the Anti-frost-protection feature kicks in. So my idea is to rig up a device that will emit a sound when the water heater is pulling, say, more than 90 watts.

I don't want to modify the heater itself; rather, I'd want to have the alarm device outside the heater, in line with the power-supply cord.

Currently I'm not using the water heater because I'm concerned it will freeze if it has water inside it. I've purchased a small propane-fueled radiant heater that will hang on a wall in that area, and it will probably be installed within a couple weeks. That heater will probably keep the area warm enough so that nothing will freeze - but I'd like to have the audible alarm in place just in case the heater fails, or just in case the area gets too cold in spite of the heater.

Any thoughts on this?

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How to rig an audio alarm for a tankless water heater