Most electric heaters produce the same amount of heat per kilowatt-hr, about 3400 btu/ hr. The difference in heaters relates to options like thermostats, fans, etc. Oil-filled may not cycle as much since the oil stores some of the heat, but they produce the same btu per kwh as any other electric type. Radiant heaters in a greenhouse would be used to direct the heat in a more narrow location, but also have the same efficiency rating. Since most greenhouses are humid, if not wet sometimes, a heater rated for this use should be used.

Assuming you have outside lows of 15 deg, want to keep the GH at 50 deg, you would need about 13500 btu/hr.

Here is a link that might be useful: greenhouse heating calculator

I thought most people used water heated pipes in green houses, like a steam heat, I am not sure. You want humid conditions for some things, but not for succulents. The want a dry heat or they will rot.