I don't understand this. The point of a greenhouse is that you get warmth from the sun without extra heating. Tomatoes and peppers grow in England in greenhouses and polytunnels just fine.
Edit: Separately the Lost Gardens of Heligan in Cornwall are very interesting. They too have greenhouses against brick walls [0]
I dug into the source for that claim. Basically, the article forgets to mention that it is comparing an open field in warmer climates to a greenhouse in colder climates.
The source article[0] cites a report by the The Food and Agriculture Organization of the United Nations[1] for that claim. Going into the source:
> Energy inputs can be around 40MJ/kg for fresh produce, such as tomatoes or peppers (FAO, 2011a). In countries such as China and South Korea, the area used for simple shade houses is increasing, but their energy inputs are low compared with energy-intensive heated greenhouses used in other parts of the world. In general, crops grown in greenhouse can have energy intensity demands around 10 to 20 times that of the same crops grown in open fields (Saunders and Hayes, 2009)
[0] http://www.lowtechmagazine.com/2015/12/fruit-walls-urban-far...
http://www.lowtechmagazine.com/2015/12/fruit-walls-urban-far...
At least in this case the original article is properly referenced, but the Low Tech Magazine version is much more elaborate and also has many links to sources.
https://www.commitchange.com/ma/cambridge/prx-inc/campaigns/...
