If your greenhouse is not larger than about 10 X 14 ft., then a small 1500 watt electric "milk house" heater should keep it from freezing inside.

Agreed with that advice.
If you are keeping seedlings and small plants warm, another possibility is heat mats below the plants, with plastic dome covers on top of them, and then a couple of blankets on top of that. Usually that is plenty warm in a "normal" March, here in Montana. This method uses less energy, but only works if your plants are fairly small.