I got my CyberQ yesterday and gave it a 6 hour test last night. At 250* it held with +/- 4* tollarance. More than acceptable. I do believe how much it varies will depend on how dense the lump you used is and  where the grate probe is located compared to the fire. (that will hold true regardless of controller)    5v would be slightly more convenient. With that said I did purchase a relatively small car jump starter for when the CyberQ goes on vacation. I'm quite certain with a good bit