A balcony in high summer is a harder environment than the same air temperature in a garden, and the reasons are structural rather than meteorological.
I am not going to give you a temperature. I could not verify one today, and an invented threshold on this subject would be exactly the sort of number that gets repeated. What I can give you is why a balcony is hotter than it looks and which positions on it are not.
What makes a balcony hot beyond the weather
Four things, all of them about the construction rather than the climate.
Hard surfaces on every side. Concrete, tile, brick, glass, metal railings. There is no soil, no lawn and no planting to moderate anything, and hard surfaces absorb and re-radiate.
A wall behind. The building is on one side, and on a sunny aspect that wall takes sun all day and gives it back.
Reflection. Light bounces from the floor, the wall and any glass, so a plant on a balcony receives light from directions it never would in a garden, including from below.
A pot instead of the ground. A container is not insulated the way ground is. This is the largest single difference between a plant on a balcony and the same plant in a garden bed, and it is why balcony conditions are more extreme in both directions.
None of that needs a figure to be actionable, and all of it points the same way.
The convenient part: shade solves both problems at once
The source's recommendation for light is already shade. Iowa State University Extension: houseplants "will do best if they 'vacation' in a full-shade or part-shade location outdoors", because "nearly all houseplants are tropical understory plants" used to "low light and warm temperatures."
Note that phrase: low light and warm temperatures. Warmth is not the enemy for these plants. Direct sun is, and direct sun is also what turns a balcony's hard surfaces into radiators.
So the position that answers the light question, back from the edge, under cover, against a shaded wall, is the same position that answers the heat question. You do not have to trade one against the other, which is unusual and worth knowing.
The positions to prefer when it is hot
Under cover, at the back. Shaded all day, and out of the reflected light from the open edge.
On the shaded side. If part of the balcony is shaded by the building or a neighboring structure, that part is a different environment from the sunny part.
Off the floor, if the floor is dark and in sun. A hard surface in full sun is not somewhere a container wants to sit. Raising a plant onto something is a placement change, and the structure that does the raising is another site's subject.
Not against a sunlit wall. The building side is the sheltered side for wind and the worst side for radiated heat if that wall is in sun. Which wall is which changes with aspect, so check yours rather than assuming the back is always best.
If your balcony faces south you are working with the most intensity available; what a south aspect delivers covers that.
When there is no shade at all
The source has a direct answer and it is worth using rather than improvising past:
"Protect plants with some cloth or shade material if a full-shade location cannot be found."
That is the recommendation. How you rig it is a structural question on a balcony and belongs to people who cover that, but the principle is stated by the source and it is the right response to an exposed balcony in summer.
The alternative response, which people take instead, is to put the plant out anyway and hope. The source's warning covers what that risks: "Never move houseplants from indoors into full sun. Leaf burn, discoloration, and drop can occur. On some species, especially succulents, the damage to the fleshy stems and leaves is permanent."
The signal to watch
One, and it is the same one as everywhere else in this cluster: "burning or discoloration of the foliage – this could mean the light is too intense."
On a hot exposed balcony that signal can appear quickly. Check daily during the first hot spell after a plant goes out, and move it back toward shade at the first sign rather than waiting to be sure.
What heat changes that is not mine to tell you
A pot in a hot position dries differently, and what a plant needs to drink changes with it. That is real, it matters, and it is another site's subject. It is worth reading about separately rather than taking a reassuring sentence from a placement page.
What is mine: do not treat a hot balcony as a lower-maintenance version of indoors. People move plants out partly to reduce fuss, and the hot exposed position is the one where that assumption fails hardest.
The seasonal reality on a balcony
A balcony's conditions swing more than any room's, and heat is only one end of that. The same space is the coldest part of your home at night and can be the hottest in the afternoon.
Choose plants for the variability rather than for the peak, and prefer the position that is least extreme in both directions, which on almost every balcony is the shaded back rather than the open edge.
If you have pets
Animals seek out balconies in warm weather and will lie where the shade is, which is exactly where you are putting the plants. Check anything you put out, by botanical name, against the ASPCA's toxic and non-toxic plants database.
Frequently Asked Questions
Why is a balcony harder on plants than a garden at the same air temperature?
Because of the construction rather than the weather. Hard surfaces on every side, concrete, tile, brick, glass and metal, absorb and re-radiate, with no soil or planting to moderate anything. The building wall behind takes sun all day on a sunny aspect and gives it back. Light reflects off the floor, the wall and any glass, so the plant receives light from directions it never would in a garden. And a container is not insulated the way ground is, which is the single largest difference and why balcony conditions are more extreme in both directions.
Is heat itself the problem for houseplants?
Not by itself. Iowa State University Extension describes houseplants as tropical understory plants used to low light and warm temperatures, so warmth is not the enemy. Direct sun is, and direct sun is also what turns a balcony's hard surfaces into radiators, which is why the two problems have the same solution.
Which spots on a hot balcony are the best?
Under cover at the back, shaded all day and out of the reflected light from the open edge. The shaded side, if part of the balcony is shaded by the building or a neighboring structure. Off the floor, if the floor is dark and in sun, since a hard surface in full sun is not somewhere a container wants to sit. And not against a sunlit wall, because which wall is sheltered and which is radiating changes with aspect, so check yours rather than assuming the back is always best.
What if my balcony has no shade at all?
The source's answer is to protect plants with some cloth or shade material if a full-shade location cannot be found. That is the recommendation rather than an improvisation, and the alternative people take instead, putting the plant out anyway and hoping, runs into the warning that moving houseplants from indoors into full sun can cause leaf burn, discoloration and drop, with permanent damage on some species and especially succulents.
What should I watch for during a hot spell?
Burning or discoloration of the foliage, which the source says may mean the light is too intense. On a hot exposed balcony it can appear quickly, so check daily during the first hot spell after a plant goes out and move it back toward shade at the first sign rather than waiting to be sure.
The part worth remembering
A balcony is hot for structural reasons: hard surfaces, a wall behind, reflection, and a container instead of ground.
The good news is that the answer is the one you already had. Shade solves the light problem and the heat problem with the same square foot, and if your balcony has none, the source's own answer is to make some rather than to proceed without it. The indoor half of the same habit is the room-by-room survey.