After Spain's blackout, British sites should buy inverters that can hold voltage

What happened in Iberia was a voltage failure

At 12:33 CEST on 28 April 2025, continental Spain and Portugal lost power. The final report presentation published by ENTSO-E traces the final sequence through three major generation trips in south-west Spain within seconds, losses of 355 MW, 727 MW and 928 MW, each pushing voltage higher and triggering more disconnections. The 49-member Expert Panel, drawn from transmission operators, regional coordination centres, ACER and national regulators, found no single cause. It points to oscillations, gaps in voltage and reactive power control, differing voltage regulation practices, rapid output reductions and uneven stabilisation capability.

Britain's system operator summarised the stakes plainly. In its reflections on the event, NESO noted that parts of the peninsula were without power for up to 19 hours. Its technical assessment records that the generation mix at the time was 82% renewable and 18% non-renewable, and that in Spain thermal plant had an obligation to regulate voltage dynamically, while renewable generation did not yet have the same obligation.

Fixed settings were part of the problem

The detail that matters for anyone buying an inverter is in the ENTSO-E root cause tree. The presentation explains that renewables operating at a fixed power factor provided no dynamic voltage control: their reactive power moved in step with active power, not with voltage. When output changed quickly, voltage changed quickly too. It also found that many protection settings diverged from requirements or were not aligned with system needs, and that the design of voltage control in some local generation networks behind the connection point was not aligned with what the system needed.

The presentation also notes different voltage limits either side of the border, with an upper limit on the 400 kV network of 435 kV in Spain against 420 kV in Portugal. None of this was exotic hardware failure. It was a matter of settings, obligations and coordination, most of which were decided long before the day.

Britain already requires the capability

NESO's assessment is reassuring on the headline point. It says the lack of dynamic voltage regulation from renewables is not the case in Great Britain, because the Grid Code requires reactive capability and voltage control from all generation, including non-synchronous renewables, with compliance testing before connection and periodically afterwards. NESO can limit the operation of non-compliant generators.

For most business installations the relevant rulebook is the Energy Networks Association's Engineering Recommendation G99, whose Issue 2 was published on 10 March 2025. It defines a Type A power generating module as one connected below 110 kV with a registered capacity from 0.8 kW up to 1 MW, which covers most commercial rooftop and car park solar. A Type A module must be capable of operating at a power factor between 0.95 lagging and 0.95 leading at full output, and the generator must agree with the distribution network operator the set points for voltage control, power factor control or reactive power control, recorded in the connection agreement.

Capability is not the same as behaviour

That last clause is the gap businesses should notice. G99 requires an inverter to be able to supply or absorb reactive power. It leaves the control mode, voltage control, power factor control or reactive power control, as a setting agreed with the network operator. An inverter that can only run at a fixed power factor may meet the letter of a connection while behaving, in a fast voltage event, like the plants that tripped in Spain.

NESO itself flags two emerging issues close to business sites. Its assessment says fault ride through compliance is an emerging issue with distributed generators, and that all asset owners, control points and investors must be clear about their Grid Code obligations. It also reports regional instances of reactive power injection from distribution networks at grid supply points, which it is working with distribution network operators to address before it becomes a problem, and foresees a trend towards increasing voltage fluctuations and high voltages. Rules for distributed kit are therefore likely to tighten, not relax.

The strongest objection is that one rooftop cannot destabilise a grid

The reasonable objection is that a 200 kW warehouse roof is irrelevant to national grid security. Britain's transmission system is planned and operated by NESO with dedicated reactive equipment and new reactive power markets, and NESO says GB is well protected against the sequence of events seen in Iberia. Paying more for advanced inverter functions, and for an engineer to configure them, looks like gold plating for a business whose main concern is its electricity bill.

The first half of that is true. No single business site will cause a blackout, and NESO's report should give confidence. But the Iberian findings are about aggregation. Thousands of small sites behaving the same way at the same moment add up to the behaviour of a power station, which is why ENTSO-E singled out local generation networks and protection settings. Inverters are only one part of a project that also pays for panels, batteries, mounting and installation, so choosing a model that supports voltage control modes is a narrow decision with long consequences. Buying the capable model is insurance against a later requirement to retrofit or derate, and the network operator decides the mode, so the business is not committing to anything it cannot change.

Settings drift is a business risk too

Protection settings are part of the same story. ENTSO-E found many settings diverged from requirements. On a business site, that typically happens when an installer replaces an inverter, an engineer changes a parameter to clear a nuisance trip, or a battery is added years after the solar. Each change should be agreed with the network operator under G99, which says the control scheme, its settings and any changes must be agreed and recorded. A business that keeps that record avoids being caught out by a compliance check, and avoids its own system tripping off in a voltage disturbance just when on-site power is most valuable.

What a UK business should do before its next installation

For any solar or battery project going out to tender in 2027, the facilities or energy manager should require inverters that support voltage control, reactive power control and power factor control modes, with the ability to change mode remotely, and should ask suppliers to confirm compliance with G99 Issue 2 in writing.

Before connection, make sure the agreed control mode and protection settings are written into the connection agreement and handed over in the operation and maintenance file. Ask the installer to explain which mode the inverters will run in and why.

For existing sites, ask the installer or maintenance contractor by the end of the first quarter of 2027 to check that current inverter and protection settings match those recorded with the network operator, and to correct and re-record any that have drifted. Treat future inverter replacements as changes that need the same agreement. The Iberian blackout was the first of its kind. Businesses that buy kit able to hold voltage, and keep its settings honest, help make sure Britain does not have the second.