Shade reduces solar output in steps rather than gradually, because a single shaded cell on a standard panel can switch off a third of it. On most UK homes the cheapest fixes are design decisions that cost nothing: mounting panels to suit the direction the shadow runs, and putting shaded panels on their own inverter input. During testing, micro inverters recovered 40 to 50% of the output lost to shade, so they are worth fitting only when that extra generation pays back comfortably inside the warranty period.
A solar panel's power output is its voltage multiplied by its current. A typical 500 W panel runs at around 40 V and 12.5 A.
The cells inside a panel are wired in series. Their voltages add up, but the current is the same all the way along, and the weakest cell sets it. When one cell is shaded, it can no longer produce the current the others are generating, so it limits the output of every cell in the chain.
Panels are wired together in a chain called a string, so that restriction carries through to the panels on either side as well.
The unshaded cells still push current through the shaded one, but instead of turning it into electricity, the shaded cell turns it into heat. This is called a hot spot, and if it gets hot enough it can crack the cell or damage the panel.
Bypass diodes give the current a route around shaded cells before a hot spot forms. Almost every panel sold today has three, dividing it into three sections side by side. Each section is then split into a top half and a bottom half.
Shade enough of one section and its diode switches on, costing roughly a third of the panel. Shade a band along the bottom of all three sections and the top halves keep working, so you keep about half. It takes shade over nearly the whole panel to drop it to zero.
Spirit Energy's surveyors are asked this on almost every shaded roof, and nine times out of ten the answer is to fit it.
Panels are the cheap part of a solar installation. Scaffolding, labour, electrical work and design time cost the same whether the array has 10 panels or 20. Once the scaffold is up, one more panel is a marginal cost.
A well-oriented roof in central and southern England produces around 950 kWh a year for every kWp installed. One 500 W panel with a clear view of the sky gives roughly 475 kWh. Take 30% off for shade and you still have about 330 kWh.
The average price cap unit rate for October to December 2026 is 26.32p/kWh for households on a default tariff. At that rate, 330 kWh is worth around £88 a year if you use it at home, or closer to £40 if it is all exported. Against a panel costing a couple of hundred pounds fully installed, it pays for itself. A shaded panel is a slightly less productive panel, not a wasted one.
Mounting panels in portrait or landscape changes how many of the three sections a shadow falls across. Getting it right costs nothing.
Take a chimney partway up a south-facing roof. As the sun moves from east to west, its shadow sweeps across the panels below as a roughly vertical band. In portrait, the three sections stand side by side like fence panels, so the stripe covers one section at a time and you lose about a third.
Mount the same panels in landscape and the sections are stacked on top of each other. The vertical stripe now crosses all three, and you keep only about half.
Flip the shadow and the answer flips too. Shade from something low and wide, such as a parapet wall on a flat roof, a neighbour's extension or snow along the bottom edge, lands as a horizontal band. Landscape then holds the loss to a third, while portrait drops you to half.
The rule is to arrange the panels so the shadow lands on as few sections as possible. Where landscape fits more panels but the shade favours portrait, only proper shade modelling will show which layout produces more across the year.
Most inverters take more than one string through separate maximum power point trackers (MPPTs). Each tracker constantly adjusts its string's voltage and current to find the combination that produces the most power.
Every panel in a string shares the same current, so the tracker has one dial to turn. Put 12 clear panels and two chimney-shaded panels on one string and it has to compromise. Push the current up for the clear panels and the shaded ones lose more. Pull it down and the clear panels never reach full output.
Give the shaded panels their own tracker and both groups run at their best. If the inverter has a spare input, this costs nothing beyond design time.
Modern inverters make this far easier. The UK Powerwall 3 has three MPPTs, with a tracking range starting at 60 V DC, which is as few as two panels in reasonable light. Older inverters typically had two inputs and a higher start-up voltage, which is why installers once reached for optimisers on complicated roofs.
Back contact panels, such as AIKO's all back contact (ABC) range and LONGi's back contact modules, move every electrical contact to the rear of the cell. That gives much finer control over individual cells.
Rather than tripping a bypass diode as soon as one cell is shaded, the shaded cell enters a non-destructive breakdown state that lets current pass around it. AIKO says shading one cell costs only a single-digit percentage, with the bypass diode activating only once four cells are shaded.
A leaf, a bird dropping or a small shadow therefore costs a few per cent rather than a third of the panel. The gain is clearest in light or localised shade. Heavier shade from trees or roof obstructions still brings the diodes into play.
An optimiser is a small converter fitted behind each panel. It finds that panel's best operating point on its own, then passes the power to a standard string inverter. A weak or shaded panel no longer drags down the strong ones in the same string.
An optimiser can't generate power from a panel in hard shade. If the bypass diodes have already switched a section off, there is no current left for it to work with. Its job is to stop one struggling panel holding back the rest.
A micro inverter goes a step further than an optimiser. It converts the panel's DC electricity to AC on the roof, so there is no string inverter and no high-voltage DC running across the roof. Each panel works completely independently.
A study with two identical 8 kW arrays found that micro inverters added around 4% a year under light shading, 8% under moderate and 12% under heavy, recovering 40 to 50% of what shade had taken. The median home in the underlying survey fell into the light shading category.
At around £120 per micro inverter, plus the scaffold and labour to replace one if it fails, that gain is worth modelling rather than assuming. If an installer recommends optimisers or micro inverters, ask for two performance models, one with and one without. Divide the extra cost by the annual value of the extra kWh. If the payback doesn't sit comfortably inside the warranty period, think carefully before fitting.
The cheapest answer is to match panel technology and layout to the shadow on your roof, and to keep shaded panels on their own strings. Panel-level electronics are proven, but they need a cost-benefit check first. Where the source of shade can be removed, that beats any mitigation.