Shade costs a balcony solar panel more than the shaded area suggests, because a photovoltaic module is a series circuit and the least-lit cell sets the current for every cell wired behind it. Bypass diodes contain the damage inside a panel, and per-panel MPPT stops it spreading between panels. Neither recovers the light you lost.
Almost all advice about the effects of shading on solar panels is written for roofs, where the offenders are a chimney, a vent stack or a neighbour’s tree, and the fix is to move the solar array. A balcony gives you no such option. What can cause shading there is structural and usually permanent.
That doesn’t make a plug-in solar panel system a bad idea, but it does mean the shading question deserves a proper answer before you spend anything, rather than the marketing claim that microinverters have solved it.
The short version
Inside a panel, shade is handled by bypass diodes, and the granularity is coarse: a shadow over one solar cell can cost a third of that panel. Between panels, it’s handled by per-panel MPPT, which stops one panel in shade dragging down its neighbour. Neither creates energy generation the light didn’t deliver.
What Partial Shade Does Inside a Solar Panel
A solar panel is several dozen solar cells wired in series. Each cell passes a current roughly proportional to the light falling on it, and the same current has to pass through all of them, so the least-lit cell throttles the string to whatever it can carry. That’s why the impact of shading is so disproportionate: cover 5% of a panel with something opaque and you can lose far more than 5% of its output.
It gets worse before the mitigation kicks in. The lit cells behind the shaded cell keep pushing current, so that cell ends up reverse-biased and dissipates the power as heat. That’s a hot spot, and hot-spot endurance is a design-qualification test in IEC 61215 because the failure mode is real.
Bypass diodes exist to stop both problems. A crystalline module is split into sub-strings with a bypass diode wired across each. When a sub-string’s output collapses, its diode conducts, current routes around it, and the rest of the panel carries on. Conventional solar panels usually use three sub-strings, though the count is a datasheet detail worth checking rather than assuming.
So partial shading losses come in steps rather than in proportion. Shade one cell hard enough and you lose its whole sub-string, roughly a third of the panel. Shade a cell in each of three sub-strings and you can lose the entire panel. Two shadows of identical area can cost wildly different amounts.
How Solar Panel Shading Depends on Shadow Shape
Sub-strings run as bands across the PV module. A shadow lying along one band costs one diode’s worth. A shadow crossing all of them costs everything. The shading effect depends on which way a shadow runs relative to the cell layout, not on how large it is.
Balconies differ from roofs here. The characteristic balcony shadow is horizontal shading: a band rising from the bottom edge of the panel, thrown by a solid balustrade infill, a parapet, the lip of the floor slab or the mounting rail. Whether that band takes one sub-string or three depends on how the panel is hung.
Half-cut cells are common on modern solar panels, and they change the arithmetic, but in one orientation only. PVsyst’s modelling documentation describes a twin half-cut module as "2 sets of 3 strings of half-cells connected in parallel", where "each pair of strings shares the same bypass diode", and says that "when the bottom half-module is shaded, the upper sub-module continues operating at half the total module current". The catch is the next sentence: "This is valid when modules are positioned in portrait. For landscape orientation, they will behave like standard modules."
Balcony railings are wide and short, so panels usually go up landscape, and mounted that way a half-cut module gives no shade-tolerance advantage against a band rising from the bottom edge. That is exactly the shadow a balcony throws, so if you can mount portrait, portrait earns its awkwardness. Our guide to mounting balcony solar panels covers what railing hardware allows.
One more shape to know. A thin, light stripe from a bar railing, a washing line or a badly routed DC cable costs more than its area suggests, because it derates every cell it crosses without pushing any sub-string far enough for its diode to conduct. Diodes only help once the mismatch is severe, so a gentle, wide shadow sits exactly where nothing protects you.
Four Types of Shading a Balcony Faces
The shading to consider when installing on a balcony falls into four groups, sorted by where the obstruction sits rather than by season, because each behaves differently through the day and the year.
1. The railing and the balustrade
Hang the panel on the outside face of the railing and the railing is behind it, shading nothing. Move it inboard, onto the inside face or a floor stand set back from the edge, and a solid or frosted balustrade clips the bottom of the panel for much of the day. Perforated and bar balustrades produce the striped, diode-defeating shadow described above.
The parts people forget are small: a top handrail sitting proud of the panel, clamp brackets, a tilt frame’s upright, and the DC cable if it’s looped across the face rather than dressed behind. All are cheap to fix at install time and expensive to ignore.
2. The balcony above
An overhanging slab removes the upper part of the sky, and what it costs depends almost entirely on how far the panel sits from the slab edge above. A panel on the outer face of the railing sits roughly under that edge and loses little. A floor stand set a metre back loses much more, because the same overhang covers a far larger slice of sky.
Since the sun is high in summer and low in winter, an overhang takes its biggest bite around midday in summer. At London’s latitude the noon sun sits at roughly 62 degrees above the horizon at midsummer and roughly 15 at midwinter, which is 90 degrees minus the latitude, plus or minus the Earth’s 23.4-degree tilt. Run that for your own latitude before assuming an overhang is harmless.
3. Neighbouring blocks and the street
This decides whether a balcony is viable at all, and it’s the hardest thing to change. A building opposite blocks the sun whenever the sun’s altitude in that direction is below the angle to its roofline. Measure that angle with a phone clinometer from where the panel will sit: if the roofline to the south is 30 degrees above your horizontal, nothing below 30 degrees of altitude reaches you from there.
Then apply the season. Shadow length is the obstruction’s height divided by the tangent of the sun’s altitude, so an object casts a shadow about half its height at midsummer noon in London and close to four times its height at midwinter noon. Winter shading isn’t a slightly worse version of summer shading, and it lands in the months when a balcony solar system generates least anyway. Lower floors take the worst of it.
4. Glass and glazed balconies
Glass shows up twice, and both are easy to miss. The first is an obscured or tinted balustrade panel in front of the array. The second, common in newer flats, is an enclosed balcony where the panel ends up behind a window.
Modern glazing in flats is often solar-control or low-emissivity, and the point of those coatings is to reject solar heat gain, which is the same energy your PV panels want. The figure to look for is the unit’s g-value, the fraction of solar energy it passes: a low number is good for overheating and bad for panel performance. An enclosed balcony also runs hot, and solar efficiency falls as cell temperature rises. Juliet balconies are the special case, since the glazing and panel position are fixed by the building.
Microinverters, String Inverters and What MPPT Really Does
Plug-in kits on the UK route are built around a microinverter, since the device has to be one unit with a factory-fitted plug rated at 800 VA. The string comparison still matters as the mental model for what happens between panels.
With a string inverter, several panels are wired in series into one input, and the array is then current-limited by its worst panel in exactly the way a panel is current-limited by its worst cell. Traditional solar panels live with this, because a rooftop solar installation usually has room to dodge the shadow. A balcony doesn’t. Our breakdown of balcony solar inverter types sets out the full picture.
Maximum power point tracking is the part that gets oversold. An MPPT circuit continuously adjusts the voltage it presents to its input, hunting for the point where volts multiplied by amps is highest. A two-input microinverter with two independent trackers allows each panel to operate independently, so a shaded panel and one in full sun are optimised separately.
Three honest qualifications, none of which tend to appear in the marketing.
MPPT works per input, not per cell. A tracker sees a panel’s terminals, with no visibility of what’s happening inside the laminate and no way to address cell-level mismatch. Inside a module, bypass diodes are the entire mitigation, at sub-string granularity.
Under partial shade the tracker can settle on the wrong peak. Once diodes start conducting, a module’s power-voltage curve stops having one hump and grows several, and a hill-climbing algorithm that nudges the voltage and keeps whichever direction improved things can lock onto a local maximum and stay there. This isn’t a fringe worry: SMA built a function called OptiTrac Global Peak to deal with it, describing it on its own site as recognising "the global maximum performance of the affected module" and optimised for system shading of 15 to 20 percent. Whether a balcony microinverter runs that kind of periodic full-curve sweep is a question its datasheet usually doesn’t answer.
A published MPPT efficiency figure tells you nothing about this. Hoymiles’ datasheet for the HMS-800W-2T gives a nominal MPPT efficiency of 99.80% alongside two MPPTs with one input each. That describes how tightly the tracker holds the maximum it found, not whether it found the right one across a partly shaded solar PV array.
The specification that tells you something about performance under shading is the "number of MPPTs" row, which is separate from the number of DC inputs. Our microinverter comparison sets out how the main brands publish theirs.
What per-panel MPPT does not fix
Independent tracking stops one panel in shade dragging an unshaded one down. It doesn’t recover that panel’s own output, it does nothing about mismatch between cells inside a module, and it can’t be worked around by adding a second device. From 27 August 2026 the network rules permit one plug-in device per household, not one per shaded corner.
Surveying Your Balcony for Solar Shading
Do this before choosing a kit. It costs an afternoon.
Stand where the panel will sit and photograph the view at mid-morning, midday and mid-afternoon on a clear day. You’re looking for what covers the sky, not where the sun happens to be. Note the angle to the top of anything solid: the block opposite, the slab above, a stair tower, a lift overrun.
Then correct for the season you’re not in. Survey in July and December shadows will be roughly seven times longer at noon. Survey in December and the summer sun will clear obstructions you currently see no way past, though at an angle a near-vertical panel handles less well. How tilt and orientation interact is covered in our guide to the best direction for balcony solar.
Two potential shade issues get missed when people plan a solar PV system. First, a panel is affected by shade on overcast days too: an obstruction removes part of the sky dome, and on a dull day most of the light reaching the panel is diffuse light from that dome. A heavy overhang still costs you in November, when nothing is casting a visible shadow at all.
Second, watch for dynamic shading: washing on a line, a parasol, planters that grow through the summer, and bird mess, a small, hard shadow of exactly the kind bypass diodes handle badly.
When Shade Rules It Out
Our position on this is unglamorous. If shading issues are structural rather than seasonal, and the panel sits in shadow through the middle of the day for most of the year, a plug-in kit won’t pay for itself. The government’s own estimate of annual bill savings is £70 to £110, and a balcony already sits at the low end, because the upper figure models a 30-degree south-facing installation rather than a vertical railing. Take a large bite out of that in lost solar generation due to shade and the payback maths stops working. Our savings guide sets out the assumptions.
Work through the cheaper fixes first. Move the panel outboard of the railing rather than inboard. Raise it, if railing height and your freeholder allow. Split the panels across two faces so one produces while the other is shaded, remembering the route caps you at four modules with no more than two in series and 120 V DC open circuit at the inverter inputs. And weigh shade against direction honestly: an unshaded west-facing panel will often out-produce a heavily shaded south-facing one.
If the numbers still don’t work, that’s a real answer rather than a failure. Like any renewable energy purchase, site conditions decide it. Whether a kit is lawful yet is a separate question worth settling first: see is balcony solar legal in the UK.
Frequently Asked Questions
Do solar panels work if partially shaded? Yes, but not in proportion to the shaded area. Because the cells are wired in series, the least-lit one limits the current, and bypass diodes then isolate whole sub-strings at a time. Expect a step-change in output rather than a gentle taper.
How much does shading affect solar panel output? It depends on which sub-strings the shadow crosses, not how much area it covers. A hard shadow on a single cell can remove roughly a third of a conventional three-diode panel. Shadows in all three sub-strings can take nearly all of it. No single percentage holds across balconies.
How do bypass diodes work in a solar panel? Each diode is wired across a sub-string of cells and does nothing under normal light. When shading occurs and that sub-string’s output collapses, the diode conducts, current routes around the shaded cells, and the rest of the module keeps generating. It also stops that cell overheating as a hot spot.
Do microinverters solve shading on a balcony? They solve one part of it. Independent per-panel tracking means a panel in shade doesn’t drag down its neighbour, which a string setup would allow. It doesn’t recover that panel’s own generation, and it does nothing about mismatch between cells inside a single panel. Treat it as damage limitation.
What is the 33% rule for solar panels? There isn’t one 33% rule. The phrase circulates in at least three unrelated senses: a US fire-code limit on roof coverage, a rule of thumb that an east or west array gives up about a third against south, and a habit of oversizing panel capacity against AC output. None is a shading test, and none is a UK requirement we can point you to.
Can you put a solar panel on a balcony in the UK? From 27 August 2026 a compliant plug-in solar device can lawfully be sold and used, subject to the product meeting the interim specification and to your building, lease or tenancy allowing it. Shading is a separate question from legality, and it’s the one that decides whether the kit is worth buying.
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