Understanding · Energy
Why doesn't my solar power the house in a blackout?
Almost every new solar owner finds this out the hard way. The panels are producing full power, the sun is on the roof, the grid goes down and the house goes dark with it. There is a good reason and it is worth understanding before you buy, not after.
Tuesday, 1 p.m. · sun on the roof · grid up
A normal Tuesday afternoon. Panels generating, house running, surplus going to the grid at a modest export price.
What actually happens in the first second of a cut.
When the grid goes down, your inverter detects that the network voltage has collapsed and shuts itself off inside a few tenths of a second. This is not a fault. It is designed behaviour, mandated by the standard every grid-tied inverter in Britain is certified against; Engineering Recommendation G99, issued by the Energy Networks Association. The technical name is anti-islanding, and every domestic solar inverter connected to a UK Distribution Network Operator's grid must do it.
The reason is safety. During a fault or planned outage, engineers work on the assumption that the network is dead. A house pushing power back onto a "dead" line could injure or kill them. So the moment the grid drops, every grid-tied inverter in the country simultaneously stops exporting, and stops feeding the house it sits on.
Tuesday, 1 p.m. · sun on the roof · grid down
Sunlight is still hitting the roof; the inverter has isolated within about 200 milliseconds. The house is dark until the grid returns.
The panels aren't broken. The inverter is doing exactly what its certification requires.
Three things installers usually don't spell out.
The first is that this is a hardware limit, not a firmware setting. A standard grid-tied string inverter has no mechanism to run the house in island mode; the circuitry to do it safely simply isn't in the box. Adding backup capability means specifying different hardware at install time, not enabling a feature later.
The second is that a "hybrid" inverter is not automatically a backup inverter. Many hybrid inverters manage a battery beautifully in normal operation and still shut down in a cut, unless they include an EPS (Emergency Power Supply) output. EPS is the specific feature that keeps a dedicated essentials circuit alive during a grid outage; it needs to be spelled out in the quote by name.
The third is that a whole-home switchover is a different, larger project again. An ATS (Automatic Transfer Switch) plus a battery sized for the whole load lets the entire house ride through a cut, including heavy loads like an electric oven or a heat pump. It is possible; it is not what a standard installation delivers.
Same afternoon · hybrid inverter with EPS
Same house, hybrid inverter with EPS. The essentials circuit; fridge, boiler pump, a few lights, the router; runs from battery and live solar. The rest of the house stays dark.
None of this is a design flaw. It is what happens when a network built for one-way flow starts having small generators plugged into it.
Verdict
Where the decision actually breaks.
If backup matters, it's a hardware choice at install time. Retrofitting it later is a full inverter swap.
The honest question isn't whether solar can power a blackout; it's whether backup is the primary job you're asking the system to do. If it is, spec a hybrid inverter with EPS from day one, and let the installer size a small essentials circuit around it. If a whole-home ride-through matters, that's an ATS plus a much larger battery, and a different conversation about cost and scope. If backup is nice-to-have but not essential, a standard grid-tied install remains the cheapest and highest-return option; and you accept that a cut is a cut.
Same evening · sun gone · battery carrying the essentials
Evening of the same cut. Sun gone, essentials still running from battery. This is what EPS is designed to do.
Why we think thisOpenClose
Reasoning
G99 is the current Engineering Recommendation governing the connection of distributed generation to UK distribution networks, published by the Energy Networks Association on behalf of the DNOs. Every domestic solar inverter installed in Britain since 2019 is certified to it. Its predecessor, G83, imposed the same anti-islanding requirement on earlier systems.
The EPS and ATS distinctions are hardware-level, not sales copy. The MCS installer standard covers the connection of solar and storage but does not require backup capability; that is an installer-designed feature you have to specify and pay for. The Solar Energy UK guidance for households summarises the same trade-off in consumer language.
Assumptions
- Grid-tied domestic solar installed to MCS standards since 2019, certified under G99.
- Typical 4 to 8 kWp domestic array with a string inverter or hybrid inverter with a battery.
- Householder on a standard distribution network connection; islanded or off-grid systems are a different design brief entirely.
Sources
- Engineering Recommendation G99: Requirements for the connection of generation equipment — Energy Networks Association (current issue)The standard every grid-tied inverter in the UK is certified against; anti-islanding is mandated in section 10.
- Solar and battery storage; guidance for households — Solar Energy UKPlain-English summary of what a standard install does and doesn't cover in a power cut.
- MCS 020: Standard for solar photovoltaic systems — MCSThe installer standard that governs the design and commissioning of domestic solar; does not require backup capability.
If this were our house
If this were our house, we'd decide the backup question before the quotes come in, not after.
- 1Ask the installer to price both. A quote with a standard string inverter and a quote with a hybrid inverter with EPS on the same essentials circuit. The delta is typically £1,500 to £3,000 and is the honest cost of the feature; not something to negotiate away later.
- 2Design the essentials circuit deliberately. Fridge, freezer, router, boiler pump, one lighting circuit, one socket ring. Not the oven, not the shower, not the heat pump. A small circuit runs for days on a modest battery; a whole-house circuit runs for hours.
- 3If backup genuinely matters, consider a whole-home ATS. Different budget, different install, worth pricing separately. Only worth it when medical, work continuity or a rural distribution network makes a "few hours" cut a real problem.
We would still recommend a standard grid-tied install for most homes; cuts are rare and short in most of Britain. The point is to make it a decision, not a discovery.
This is the surprise every new solar owner discovers on the wrong afternoon. Now you know before the quote arrives.
Related questions this page hasn't answered.
Can I fit a portable generator instead?
What about vehicle-to-load from an EV?
How often do UK homes actually lose power?
- Last reviewed
- 4 July 2026
- Evidence quality
- High· Sits on top of the ENA Engineering Recommendation G99 and the MCS installer standard.
- Engineering Recommendation G99: connection of generation equipment · Energy Networks AssociationThe standard every grid-tied inverter in the UK is certified against; anti-islanding is mandated in section 10.
- MCS 020: solar photovoltaic systems · Microgeneration Certification SchemeThe installer standard governing domestic solar design and commissioning; does not require backup capability.
- Solar and battery storage: guidance for households · Solar Energy UKPlain-English summary of what a standard install does and doesn't cover in a power cut.
- Electricity distribution network performance reports · Ofgem (annual)Published customer-interruption statistics by DNO region.
- Grid-tied domestic solar installed to MCS standards since 2019, certified under G99.
- Typical 4 to 8 kWp array with a string inverter, or hybrid inverter with battery.
- Standard distribution network connection; islanded and off-grid systems are a different design brief.
- 4 July 2026First publication with full evidence apparatus.