It's never a good idea to guess with measurements, that's what measuring tools are for. No need to look for a power outlet, since it runs on batteries. Just get at it!
From B&Q
Leica Geosystems Leica Lino L2S (Alkaline) Laser Line Level With Carry Case 180 Degree Red Laser - One Size
ITS
Leica Lino L2s Self-Levelling Red Cross Line Laser (25m - 80m) Starter Kit