I really wouldnt worry about 1 400 watter. Isnt enough to even worry about.
If you were running a few 1000 watters then maybe but 1 single 400 watter. I wouldn't worry. Even if it is metered by the minute.
If it worries you that much. Hell veg 24/0. or 23/1. wouldn't be noticeable at all maybe your running a space heater 24/7 or and a/c unit for cooling a small room.
Either way 400 watts is nothing to worry about. You vaccum, fridge, heater, ac units, some computers, use more then this.
Really man no worries.