Having an uninterruptible power supply is critical with an aeroponics setup where roots could dry out within an hour during a power outage. Due to cost reasons, I decided to power only the DC devices like the Arduino automation modules, pumps, and solenoids during an outage. My goal was to keep the aeroponics system operating for up to 24 hours without power. Without frequent misting, the plants would be dead within a few hours, while losing the lights and the ventilation is not crucial. Technically you could run the entire grow tent off a large UPS that can provide 800W/h for hours, but the price of the batteries would be huge. Another option would be to detect the AC power outage and automatically dim the lights and slow down the internal and exhaust fans.
The idea of using an old UPS with a car battery came from:
https://www.instructables.com/Make-your-computer-UPS-last-for-hours-instead-of-m/
https://www.instructables.com/UPS-Battery-Upgrade-15-Hours-Backup-Time/
First, we need to know the power consumption of the devices that should be kept on during a power outage. For two Hempy buckets + one aeroponics tote, this is 130W/h. Since the DC power supply is only around 80% efficient the actual power consumption is around 160W/h (worst case). In a general rule of thumb, you should look for a UPS that is rated for twice the actual power consumption. I have selected a 400VA (Volt-amps) rated UPS that can provide around 320W/h (Assuming a pretty low 0.8 power factor). Make sure the UPS uses a single 12V battery (or two 6V batteries connected in series) if you plan to use a single car battery. Some UPS-es use 24V batteries where you would need to connect two car batteries in series.