Space heaters are limited to 1500 watts, which runs on average .20/hr. If they are run constantly while the employee is present (8 hours/day, but not likely) then it would cost the employer $32/mo. Granted, a non-zero number of employers are stingy enough to make this an issue, but realistically, it's not a big deal. Also they have safety systems in place which make them not particularly hazardous. Most have thermal fuses that burn out/disconnect power at temperatures significantly below what is needed to start a fire.