To me not all greenhouses are solar.
Some,,,, well a lot of the greenhouses on the market are a "one type fits all" type greenhouse.
The south facing glass angle is very important for collecting winter solar heat. If you have the incorrect angle you'll reflect more of the winters solar heat than you collect.
The sun angle is lower in the horizon and the sun is closer to the earth in the winter months so we have to adjust are south facing glass to collect the maximum solar heat.
For me in Northeast Ohio, the south facing glass angle of around 66.5° (+/- 20°) works best for collecting winter solar heat. Now someone in the southern U.S. might be better off with the south facing glass around 20 to 40°.
The Quonset style cold frame/ greenhouse take some of the guesswork out since they are half round and collect solar heat no matter the sun angle. The downside with this type of greenhouse is that they get very hot in the summer months.
Keep in mind in order to be a good solar greenhouse the greenhouse should collect that maximum amount of winter solar heat possible, should have a solid north facing roof and wall (no windows), the east, west walls should have minimal window openings and all exterior surfaces should have a maximum amount of insulation in them to help retain heat.