24, it will take up to 28 before it is possibly damaged.
The problem with 24V for backup is keeping the batteries charged gets complicated and expensive. We generally just need enough flow to keep things alive and 12V will suffice, my comments about battery charge were mainly because the regular DC streams can run on 10 and the smallest nano streams 8, the 6150 won't run below 12 so battery choice becomes an issue, I wouldn't use for example a lantern battery.