In photovoltaics, shading costs far more than the area actually masked, because the cells in a module are wired in series: a shadow over 10% of the surface costs up to 25% of output with a string inverter. Moving to micro-inverters or optimisers brings that loss back to around 10%, without ever cancelling it. Shading analysis is done before installation, allowing for the low winter sun that lengthens the shadow cast by a tree or a chimney, and for the tree's growth over the life of the installation. An output quoted without a shading survey commits the firm from the first winter, and is easily challenged with meter data.
Understanding shading in photovoltaics: what it really changes
Partial shading vs total shading: the effects on an array of modules
In photovoltaics, total shading on a module can cut production for its entire string, depending on the wiring and inverter. Partial shading acts more insidiously. It reduces power without necessarily dropping it to zero, and it can create imbalances between modules.
Why a small shaded area can cause a large production loss
The cells of a module are connected in series. A shaded area on a few cells limits the current for the whole module, a bit like a bottleneck in a pipe. The result: a small leaf, an aerial stub, or the corner of a dormer window can trigger losses far greater than the surface actually shaded.
Bypass diodes, strings, MPPT: the concepts to know for an accurate diagnosis
Bypass diodes protect the module and bypass some of the shaded cells, but at the cost of a voltage drop. The string groups several modules together. The inverter's MPPT tracker(s) look for the best operating point. To diagnose, check orientation, string configuration, and if shading is recurring, consider optimisers or micro-inverters. To complete the analysis, you can also estimate solar yield by climate zone and exposure.
Nearby tree: season, growth and yield losses
Shade depending on time of day and season: the typical winter/summer case
In winter, the low sun stretches shadows further. A branch that is actually far away can cut production early in the morning or late in the afternoon. In summer, the sun climbs higher. The same tree canopy causes less obstruction between late morning and mid-afternoon. On a photovoltaic installation, these differences show up most on cold, clear days, precisely when good output is expected.
Foliage, branches, maintenance: when the tree becomes a real weak point
Over the years, the tree grows and the foliage thickens the shade. Even minor shading can disrupt a string of modules and cause production to drop. Add leaves, pollen and droppings that dirty the glass. Regular maintenance, with sensible pruning and removal of dead branches, limits the loss without turning your roof into a construction site.
Measuring and proving the loss: records, photos, and production data
To make the loss objective, combine dated photos at several times of day, production monitoring records (power curve, kWh per day) and, if possible, a shading diagnosis by a professional. Compare similar weather days before and after pruning. You will have clear evidence to decide on pruning, repositioning, or optimisers. To go further on measurement methods, also see consumption monitoring after the works.
Chimney, parapet, aerial: "hard" shadows and their impact
Sharp, recurring shadows: why they penalise more than you might think
On a photovoltaic installation, a "hard" shadow created by a chimney, a parapet or an aerial cuts light on a few cells. The result is immediate losses and, above all, losses that recur every day at the same time. The bypass diodes kick in, the string ends up throttled, and production can drop far beyond the surface actually shaded.
Identifying risk areas on the roof: ridge, gable ends, technical obstacles
Sensitive areas are often near the ridge and gable ends, where shadows lengthen in winter. Also spot mechanical ventilation outlets, guardrails, roof lights, and flat-roof parapets. A layout that keeps sufficient distance from obstacles, and optimisers or micro-inverters where needed, limits the domino effect on the array.
Special cases: smoke, soot and module fouling
A chimney can add a second problem. Smoke and soot foul the modules, reduce light transmission and promote localised overheating. Plan for cleaning access and an appropriate frequency, especially during the heating season.
Site solutions to limit shading and loss in 2026
Optimising layout: orientation, spacing, and choice of strings
On a photovoltaic job site, the first savings come from layout. Orient the modules according to the roof, then adjust spacing to avoid self-shading and keep maintenance access simple. On the wiring side, group panels that see the same shade on the same string, otherwise the most penalised module drags the whole string down.
Micro-inverters, optimisers, multi-MPPT inverter: how to choose based on shading
If shading is localised and variable (chimney, tree), micro-inverters or optimisers limit losses by working module by module. With two distinct sunlight zones, an inverter with several MPPT trackers is often enough. Aim for consistent sizing rather than "the most expensive".
Pruning, moving an obstacle, raising the mounting: weighing cost, feasibility, effectiveness
Before changing the electronics, price out three options. Pruning or moving a small obstacle can bring a lasting gain. Raising the mounting sometimes helps, but watch out for wind, loads and waterproofing. Look for the best cost-benefit ratio, not perfection.
Diagnostic method: from on-site survey to customer report
Site surveys: solar mask, photo, drone, and checking cast shadows
On a photovoltaic roof, everything starts with reliable surveys. Record orientation, pitch, parapet heights and nearby obstacles. Produce a solar mask at several points, supplement it with geolocated photos, then use a drone if access is risky or to check hidden areas. Check cast shadows at different times of day, especially in winter, when the sun is low.
Simulation and production estimate: announcing the loss without getting it wrong
The simulation must reflect the site. Enter the site data into a recognised tool, then apply realistic losses. Account for shading, temperature, soiling, and inverter efficiency. Present an annual range and a shading-related loss with clear assumptions, to avoid promises that overheat too quickly. Panel layout roof plane by roof plane, expected output by orientation and tilt, self-consumption and resale are all simulated in the same photovoltaic workflow, with the real roof obstacles and the sun path.
Writing a clear report: assumptions, limits, and recommendations
The report fits in a few pages. Note the method, the date of the surveys, the calculation assumptions, and the limits. Add a roof plan, shadow views, and concrete actions. For example, moving a row of modules, planning for optimisers, or dealing with a pruning zone. The customer leaves with useful recommendations and an informed decision.
