Yeah there are a number of ways of going about it depending on what you wish to achieve. Assuming you have a UPS or something similar to continue to supply power to your tank there are 3 main options, all require you to use a small power supply up to 10V hooked into a 0-10V I/O input or a 5v power supply (like a USB charger) to an analogue sensor port that is connected to the mains but not through your power backup service. Then you just need to setup an input in hydros to read the signal from your power supply so you have an event trigger when the power supply voltage goes missing.
Option 1 (if you have no buttons or I/O inputs): The most generic way from here is to then use the input as a requirement for things to turn on and off depending if the 5v power supply is on or off, this will give you the most direct control over devices. This means you can use a spare sense port if you don't have a controller with an I/O input and use either a "depends on" drop down or directly as an input to trigger an action for an output (turn on or off).
Option 2 (recommended): Use the input to turn on a low power mode you setup in the "Modes" menu, this allows you to simply tick a box on any output/schedule to run in "low power mode". It also gives you the option to have additional schedules on pump outputs etc, assuming you have either direct drive or 0-10V pump controls, so you can say turn pumps down to a minimum/alternative flow when in low power mode. This can be a little trickier to setup if you are using a button box to trigger other modes via I/O Input, but easy enough if you just to use the input triggers voltage also as the power supply for the other buttons and set the low power mode trigger for 0-1 volts. Keep in mind you lose a mode/button to this as well as lose any other button inputs when power goes out (if you have some knowledge in electronics you can work around the "no buttons when low voltage" limitations).
Option 3 (not recommended): You can manipulate the BUS voltage down to 12V to trigger the Hydros low power mode. To be honest this not a great way of going about it and I am not even sure this works if you have hydros controllers in the collective that are mains AC connected to a UPS during a power outage (as they will continue to supply 24V to the BUS) and these devices (like the Launch, X8) don't work without AC power so not exactly a well though out feature by Hydros The only reason to use this option is if you have a Kraken and no available I/O or sense ports available to use. I sent my Kraken back as the logic of the device was not suitable for my needs, most devices are simply not happy receiving 1/2 their voltage and a 12V battery is not a great voltage for any sort of battery backup/storage. It's a bit of an odd one as a simple Ideal diode and dual voltage charger would have allowed them to use a 12 or 24V battery as a backup and make this feature (and the kraken) actually worth using.
If you are crazy like me and keep endanger animals for restocking programs you must have serious redundancy for power (and everything else I might add) and it needs to be able to run a system for a couple of days on its own power due to me being away on the regular. So I have the hydros stage through a couple of low power modes dependant on additional triggers in order to maximise the outage time it can remain running for while ensuring the least impact on the system (just because the power went out for 10mins doesn't mean you want to go into low power mode if you have plenty of power backup). As I have triple redundant DC power rails and dual redundant AC inverters with redundant battery banks I initially don't have anything happen until the first inverters battery bank hits 20%, then it triggers the initial low power mode that turns the the wave pumps and skimmer down, turns off ozone and non-essential media reactors to extend the AC power production. The next stage is triggered when the first Inverter turns off due to low battery power, this then forces everything except the UV's, chillers/heaters and hydros AC powered controllers over to their own dedicated battery back-up power (through dc-dc regulators) leaving the 2nd inverter and its fresh battery bank to just look after the devices that have to run on AC (everything else is DC powered). When the power on the 2nd inverter battery bank drops to 50%, then the UV's get turned off so as to maximise the chillers run time, and most of the pumps also get turned off with the return pump being dropped to minimum speed. After the 2nd inverter dies due to batteries depleted it turns everything off except the return pump, 1 wave maker and the NO3 reactor which runs until their dedicated battery is depleted.
The dual inverter setup also allows me to plug the system into 2x mains AC power circuits (in case of a circuit trip) and also has a dedicated generator AC power input so someone can be sent to turn plug in a generator if required. The Victron inverters also allows the system to drop the mains AC during the expensive parts of the day electricity wise, ensuring the system only draws AC when the power price is 1/5th the average (sometimes this can be hundreds of times cheaper in late afternoons).
