On October 17, Sri Lanka’s national power grid collapsed, plunging the country’s 23 million people into darkness for more than five hours. As authorities scrambled to restore electricity, the National System Operator (NSO), which manages the country’s power grid, issued an unusual directive: rooftop solar operators were asked to switch off their systems until further notice.The directive highlighted the challenges of integrating large volumes of rooftop solar into the power grid. This is not the first time Sri Lankan authorities have raised concerns about rooftop solar. During a similar blackout last year, rooftop solar was identified as a contributing factor. The latest incident has renewed questions about how growing volumes of decentralised solar generation can be managed without compromising grid stability. The issue is relevant for India, too, as it rapidly expands rooftop solar capacity, which currently stands at 33.41 gigawatts (GW).So, how can rooftop solar affect grid stability? What role did it play in Sri Lanka’s latest blackout? And what lessons does the incident hold for India? We explain.What exactly happened in Sri Lanka’s blackout?Sri Lanka’s installed power generation capacity comprises 51% renewable energy, with hydropower accounting for 21% of the total, followed by oil-based power plants at 17% and coal at 11%. Rooftop solar accounts for 48% of the country’s total installed renewable energy capacity, amounting to 2,499 MW.While the extent to which distributed rooftop solar contributed to the grid failure remains unclear, the preliminary report on the October 17 power system failure, prepared by the NSO, has indicated the possible role of “non-dispatchable generation sources” in triggering the disturbance. Non-dispatchable generation sources include renewable energy sources such as rooftop solar and wind power, whose generation depends on weather conditions and cannot be ramped up or down by grid operators as easily as conventional power plants.The report suggests that a sudden loss of generation from such sources may have triggered the sequence of events that eventually led to the nationwide blackout.Story continues below this adAlso in Explained | Power subsidies are slowing household solar adoption. A solution: More subsidiesAccording to the report, the disturbance began at 11:43:29 am. Data recorded at the National System Control Centre showed that LVPP Unit 1 (Lakvijaya Power Plant), the country’s largest coal-based power plant, rapidly reduced its net power generation from 187 MW to 5 MW within 450 milliseconds. This was followed by a sharp decline in the grid’s frequency — a measure of the balance between electricity supply and demand. The frequency fell from 49.906 Hz to 48.75 Hz within just two seconds, the report noted.The report noted that no dispatchable power generation units — conventional plants whose output can be controlled by grid operators — had tripped at the initial stage of the disturbance. It said the sudden drop in frequency might instead have been caused by the rejection of non-dispatchable generation sources such as rooftop solar connected to the grid at the time.In a power system, frequency falls when electricity supply suddenly drops below demand. The preliminary report suggests that the initial disturbance may have triggered the disconnection of non-dispatchable generation sources, causing a further loss of supply and accelerating the decline in frequency.Rooftop solar systems are connected to the grid through inverters, which convert the direct current generated by solar panels into the alternating current used by the power system. These inverters have built-in protection mechanisms that can disconnect the systems if grid voltage or frequency moves outside specified limits. If a large number of rooftop solar systems disconnect simultaneously, the resulting loss of generation can further destabilise the grid.Story continues below this ad A 500 kW rooftop solar power plant at the Delhi Assembly. Managing distributed solar would be the next challenge as India rapidly expands rooftop installations. Photo: Amit MehraAs the frequency continued to fall, all five stages of the Under Frequency Load Shedding (UFLS) scheme — which automatically disconnects selected loads to reduce demand and stabilise the grid — were activated. However, the safeguards failed to prevent a total system collapse. At 11:43:59 am, all dispatchable generating units tripped due to under-frequency or under-voltage conditions, except two units that remained connected but operated at no load. Grid restoration began immediately, and the entire national transmission system was restored by 4:58 pm, five hours and 15 minutes after the blackout, the report said.Why rooftop solar poses challenges to grid stabilityRooftop solar systems, particularly their inverters, had also come under scrutiny when Sri Lanka’s power grid suffered a similar failure in February 2025.While rooftop solar was not identified as the initiating cause of the 2025 blackout, its high penetration and the response of its inverters were found to have contributed to the rapid escalation of the disturbance into a total system failure.Story continues below this adA report investigating the February 2025 blackout, prepared by the country’s Ceylon Electricity Board (CEB), found that a fault in a busbar — a rigid metallic strip or bar made of copper or aluminum that collects, carries, and distributes high-current electrical power — at the Panadura Grid Substation triggered localised voltage drops. This prompted a large number of rooftop solar inverters to automatically disconnect from the grid.The report noted that some inverters had non-standard or overly sensitive protection settings, with disconnection thresholds set at frequencies as high as 49.5 Hz or voltage levels as low as 94% of the nominal value.More in Explained | Kerala is seeing record rooftop solar adoption. So why is it still facing power supply issues?The nominal value here refers to the target or designed value of a characteristic (like voltage or frequency) that a system is built to operate at under normal conditions. It is the standard baseline from which tolerances and allowed variations are measured: so, a mere 6% drop in voltage levels means the inverter immediately disconnects rather than riding through the minor dip.The sudden loss of rooftop solar generation increased the burden on the remaining conventional power plants, which had to compensate for the shortfall. This further strained the system and accelerated the decline in grid frequency, contributing to the cascading failure.Story continues below this adFollowing the February 2025 blackout, utilities required rooftop solar inverters to adopt low-voltage and frequency ride-through settings, said Kumar M, founder and CEO of Smart Grid Analytics, which provides grid management solutions to renewable energy operators. These settings allow inverters to remain connected during certain voltage and frequency disturbances instead of disconnecting immediately. However, Kumar estimated that less than 50% of the installed rooftop solar capacity had adopted these settings. He attributed the gap to older inverters, firmware that could not be updated and weak enforcement.“If a large share of non-compliant inverters tripped together, they would have deepened the generation deficit. The inquiry can settle this with disturbance recorder data, inverter OEM cloud logs and smart meter data,” he said.India, according to him, has already faced similar grid integration challenges at the utility scale, with the Central Electricity Regulatory Commission (CERC) recording 68 disturbances in renewable complexes between January 2022 and June 2025, involving generation losses of 1 GW to 7 GW.Story continues below this adHe said stricter enforcement had helped reduce such incidents, but managing distributed solar would be the next challenge as India rapidly expands rooftop installations.Kumar said every state should assess how its electricity grid would perform on days when power demand is at its lowest and solar generation is at its highest. This would involve studying how much rooftop solar each local distribution feeder can safely accommodate, simulating how the grid responds to sudden disturbances using combined models of rooftop solar and other distributed resources, and setting limits on system inertia and on how fast grid frequency can change without destabilising the system. States should also determine the impact of the sudden loss of the largest source of electricity supply that the grid may reasonably be expected to withstand. He said under-frequency load-shedding mechanisms need to be redesigned for feeders that export electricity during solar hours, as disconnecting these feeders could remove generation rather than reduce demand.Black-start (the process of bringing an electrical grid back to life after a major blackout) and grid-restoration plans should also account for how rooftop solar systems reconnect after a blackout, he added.