It would help to know where you are or what zone.

western Pennsylvania....pittsburgh area

And what material you GH glazing is (glass, twin-wall, triple-wall...)

I'm in eastern PA, all glass ,larger and home attached and a small ceramic heater set on low may keep a very small area near the heater above 40, but on nights in the teens (again depending on your glazing) it would likely dip MUCH lower inside the GH. We wrap our GH in a solar pool cover and heat it with a small electric AND a 30,000 BTU propane heater, but we're maintaining temps in the 50's.

I can tell you we struggled to maintain 50F last year with both heaters and no solar pool cover.