Since some questions still havnt been answered, i'll be the Technonerd for today and give my input.

For a standard 12"
The inverter nominally requires 12VDC.  You can provide as much as 14VDC without harming the inverter or lamp and as little as 6VDC before the lamp quits glowing due to insufficient power.  The power consumption is between 350 and 400ma at 12VDC.  That between 4.2 and 4.8 watts.  The inverter generates as much as 1000 VAC to power the lamp tube itself, so be careful!