During winter, you can line the inside of your greenhouse with 4-mil polyethylene film. Charley's greenhouse has bubble insulation that doesn't need to be inflated or installed with a gap to separate the layers. Keep greenhouse cool at night - don't heat above 50 degrees. Grow plants with low temperature requirements or short growing season. They are 1600 watts which is expensive to operate.
Gas is much more economical than electric.
We light our heater in the evening and usually turn it off in the morning and keep the greenhouse just warm enough to prevent the plants from freezing. If you want to start seeds, you'll probably want to build a covered propagation bench with a heater coil under the soil. Btu's where 'A' is total wall and roof surface area, 'D' is the difference between coldest outdoor winter temperature and the night temperature desired in your greenhouse. Btu's is the heat requirement. Charley's has a few gas and electric heaters available with a range of prices.