Tag: rooftop solar troubleshooting

Your Rooftop Solar Is Generating Less Than Promised: 8 Reasons Why

Short answer: under-performing rooftop solar is almost always a stack of specific, findable losses — soiling, unmodelled shading, a dead or mismatched string, inverter clipping, temperature derating, undersized cabling, grid outages during peak sun, and genuine module degradation. Each one is measurable, and most are correctable. The difficulty is that none of them announces itself. The plant runs, the inverter shows green, and the monthly number is simply lower than the projection in the proposal. Here is how to work through it in order of likelihood. First, Compare the Right Numbers Monthly kWh is not a diagnosis — irradiation varies. Compare specific yield (units generated per kW installed) against the same month last year, and against the plant’s performance ratio, which accounts for available sunlight. A sudden drop points at a fault; a gradual decline points at soiling, degradation or a developing problem. What You Observe Likely Cause How to Confirm Gradual decline over weeks, recovers after rain Soiling — dust, pollution, bird droppings Compare output before and after a cleaning cycle Output dips at the same time each day Shading from a structure, pole, parapet or new construction Site walk at that hour; string-level data One inverter or MPPT consistently lower Failed string, blown fuse, connector fault, module failure String current measurement and IR testing Output flat-tops at midday Inverter clipping — DC capacity exceeds inverter limit Check DC-to-AC ratio against the inverter rating Lower generation in peak summer Temperature derating — module output falls as cell temperature rises Compare winter and summer specific yield Loss worse at high output Undersized DC or AC cable — losses rise with current Voltage drop measurement at full output Zero generation in blocks during the day Grid outage, inverter trip on grid parameters, or anti-islanding Inverter event log Small, steady annual reduction Normal module degradation Year-on-year specific yield trend against warranty curve Soiling Is the Biggest Avoidable Loss in Indian Conditions Dust, construction activity, industrial emissions and bird droppings accumulate quickly, and the loss is continuous rather than dramatic. Cleaning frequency should be set by site conditions, not by a generic schedule — a plant next to a cement unit or an unpaved road needs far more attention than one in a clean coastal area. Bird droppings matter more than their area suggests, because a fully blocked cell can force a bypass diode to conduct and knock out a whole sub-string. Shading That Was Not in the Original Study Shadow analysis is done at design stage, against the site as it was. Then a water tank arrives, a neighbour builds a floor, a chimney is extended, or a row of panels is added too close to the one in front. Because strings are series circuits, a small shadow on one module can affect output far beyond its own area — which is why shading complaints often sound exaggerated until the data is examined. The Faults Only String-Level Monitoring Will Find A plant monitored only at the inverter output can lose a string for months without anyone noticing — the number simply looks a little low. String-level or module-level monitoring converts that invisible loss into an alarm. If your plant was built without it, periodic string current measurement with a clamp meter under stable sun is the manual equivalent and should be part of the O&M routine. Design Decisions You Are Now Living With DC-to-AC ratio: some clipping is deliberate and economically sound; excessive clipping means the array was oversized for the inverter Cable sizing: DC and AC cables sized only for current, not for loss, cost output every sunny hour Tilt and row spacing: tight spacing increases inter-row shading in winter months Ventilation under modules: flush-mounted arrays run hotter and derate more Earthing and surge protection: weak protection means inverter trips and failures during the monsoon What a Proper Performance Audit Produces A performance assessment should end with measured string currents and voltages, IR test results, thermal imaging of modules and junction boxes to find hotspots, inverter event log analysis, a soiling loss estimate from a cleaning trial, and a loss breakdown that assigns the shortfall to specific causes with the cost of correcting each. That turns a vague complaint into a work order. VTPL executes solar EPC projects and plant rectification works as part of its renewable energy portfolio, alongside the electrical distribution and protection works these plants connect into. With 33+ years of electrical EPC experience across 19+ states and clients including PSUs and industrial groups, our teams approach solar as a power system, not a panel installation. Frequently Asked Questions How often should solar panels be cleaned in India? Frequency depends on local dust, pollution and bird activity rather than a fixed rule. The practical method is to measure output before and after a cleaning cycle and set the interval where the recovered generation justifies the cleaning cost. What is a performance ratio in solar? The ratio of actual generation to what the plant should have produced given the available irradiation and installed capacity. It allows fair comparison between months and between plants, unlike raw kWh. Why does solar generation drop in summer? Module output falls as cell temperature rises, so peak-heat months can yield less per kW than clear, cool months despite longer sunlight hours. Poor ventilation under the array worsens the effect. Can one faulty string affect the whole plant? A faulty string reduces that MPPT’s contribution, and mismatched modules within a string drag the whole string down. Without string-level monitoring, the loss can continue unnoticed for months. Is it worth upgrading an older solar plant? Often yes — particularly where the loss is from undersized cabling, failing inverters, weak surge protection or a lack of monitoring. An audit that quantifies each loss tells you which corrections pay for themselves. VTPL delivers solar power plant EPC, rectification and grid interconnection works across India. Plant not meeting projections? Request a project consultation →