Ken, I am thinking your planned porch addition/ enclosure will be surely more cost effective to heat.
Since my greenhouse is connected to the oil heating system of the house and share the same duct work, even on those bright and sunny cold Winter days with temps not climbing above 18 degr F. At noon the greenhouse temperature climbs into the seventies. The warm air circulates through the whole house, keeping the oil consumption down at least during day time.
