Does Commercial Solar Keep Working in a Power Cut?
Updated 23 October 2026 · SEO Dons Editorial
Why does grid-tied solar stop in a power cut?
Because the rules say it must. The Electricity Safety, Quality and Continuity Regulations 2002 allow a source of energy to run in parallel with a distributor's network only if the owner has "the necessary and appropriate equipment to prevent danger or interference with that network". For the small-scale route (no more than 16 amperes per phase at low voltage), regulation 22 adds that the source must be "configured to disconnect itself electrically from the parallel connection when the distributor's equipment disconnects the supply of electricity to the person's installation".
The network operators' conditions say the same thing in plain terms. National Grid Electricity Distribution lists "The system will shut down during a power outage" among the criteria for a G98 connection, and the Energy Networks Association (ENA) includes "The generating units will not operate when there is a loss of mains situation" in its fast-track conditions for small-scale generation and storage. The technical name is loss-of-mains protection, or anti-islanding. An island, in the IET's definition, is an operating state in which part of an installation "continues to operate in the absence of the grid supply".
The Energy Saving Trust gives the household version: "Typically solar panels can't power your home during a power cut." It adds that a battery "also does not normally provide power in a power cut unless additional equipment and configuration is included in the installation". A business array follows the same principle, so panels alone are not backup. The battery storage page covers when storage itself is worth modelling.
What does backup power actually need?
A system designed to island, and the equipment that lets it do so safely. Regulation 21 of the same Regulations treats a source operated "as a switched alternative to a distributor's network" separately: the owner must "ensure that that source of energy cannot operate in parallel with that network". The IET's answers to a battery storage webinar held on 2 November 2021 put it this way: "A switching arrangement is usually required to form island mode, so that 'maintained loads' are powered from the grid-connected system in normal operation and local generation (including batteries) in island mode." Maintained loads are the circuits chosen to stay live. In the IET's published guidance, the parts of such a design include:
- A switching device for islanding. IET Wiring Matters (July 2026) says one is required to connect and disconnect the installation from the grid, and that it "shall be suitable for isolation (Regulation 826.1.1.5 of BS 7671)".
- An earthing arrangement for island mode. The same article says "A common earthing arrangement for an islanded PEI is TN-S" and that this requires "a local earth electrode to provide a reference to the general mass of Earth". PEI means prosumer electrical installation, one that both uses and generates power.
- Fault protection that works on limited current. "One of the technical issues associated with islanded PCE, is the limited fault current available", which is why the article turns to residual current devices for disconnection times.
- A grid notification that covers island operation. "If the system switches between connected and island mode, G98 (or more likely G99) notification is usually necessary."
- Safe working arrangements. "To work safely on a point in an installation with island-mode capability is likely to require multiple points of isolation", with labelling and information to match.
Two cautions. The July 2026 article states that it "is dedicated to single residential dwellings with one local PCE", so a commercial site with several inverters needs its own design under BS 7671 and the IET Code of Practice for Electrical Energy Storage Systems, now in its 3rd edition. An IET preview of that edition listed "An update to the requirements for island mode isolators". And an installation with backup is a different design from the grid-tied array on the hybrid inverter and AC and DC-coupled battery pages, which explain the hardware routes to it.
What do manufacturers publish about backup ratings?
Backup figures are listed separately from grid-tied figures. The table shows what three makers' own product pages stated on 22 October 2026. These are datasheet statements, not a statement of what any site will do in an outage.
| Maker and range | Backup-related figures stated on the page |
|---|---|
| SolaX X3-ULTRA (15 to 30 kW) | Peak EPS output power of 2 times rated power for 10 s; switchover time under 10 ms |
| GoodWe ET 15-30 kW | For the GW25K-ET: 27,500 VA maximum apparent power to the grid, 25,000 VA back-up nominal apparent power |
| Solis S6-EH3P(30-50)K-H-ND | "160% overload" backup capability; UPS-level switching listed among load options |
Three points follow. First, the backup rating can sit below the grid-tied rating on the same datasheet, as in the GoodWe row, so the circuits to be backed up have to fit inside the lower number. Second, short-term overload ratings exist because some loads draw more when they start; check the figure against the largest motor or compressor on the backed-up circuits. Third, "UPS-level" is a maker's phrase. The IET's answer on whether a battery system works like an uninterruptible supply is: "Not with most simple electrical energy storage systems", because the parts concerned "would also have to comply with BS EN 62040 series for UPS".
Which loads are worth backing up?
Only those where an outage costs more than the extra equipment, and the list should be short. Backing up a whole site is a larger design than backing up a panel of chosen circuits. Questions that narrow the list:
- What must keep running: refrigeration, server racks, alarm and access systems, tills, control gear for a process?
- For how long? Duration drives battery energy, and it is a risk decision for the business, not a figure the datasheet supplies.
- Which phases do those loads sit on? Some makers state how their backup output handles uneven loads; the Solis page says its units "readily adapt to three-phase unbalanced loads and half-wave loads".
- Do any need no break at all? A changeover of milliseconds is still a changeover.
Is backup sizing the same as peak-shaving sizing?
No, because the two answer different questions. Peak shaving asks how much power a battery must deliver at the moments of highest demand to lower the peaks bought from the grid; GoodWe describes it as peak shaving that "balances power demand and grid power imported". Backup asks which circuits must stay live, at what power and for how long, while the network is down. The GoodWe datasheet rows show the split: grid output and back-up nominal apparent power are listed as separate figures.
A battery sized for shaving may be too small in energy, or too small in output, to carry a chosen set of loads through an outage, and a battery sized for backup may sit idle for most of the year if shaving was the aim. State which job comes first before asking for a quote through the site's enquiry form.
What should a quote say about backup?
- The circuits backed up, by name, and the circuits that are not.
- The backup output rating in kVA and the short-term overload rating, taken from the datasheet.
- The switching device for islanding and how it is interlocked with the island-mode earthing arrangement.
- How the grid notification covers island operation.
- How the changeover is tested at commissioning.
- Any statement about what the system will do in an outage, with the conditions attached, such as battery charge at the time. A statement with no conditions deserves a question.
Sources
- The Electricity Safety, Quality and Continuity Regulations 2002, regulation 21 and regulation 22, legislation.gov.uk, checked 22 October 2026.
- Micro generation (G98), National Grid Electricity Distribution, checked 22 October 2026.
- Connecting generation to the electricity networks, Energy Networks Association, checked 22 October 2026.
- Solar power fact check, Energy Saving Trust, checked 22 October 2026.
- Battery storage webinar questions and answers (webinar of 2 November 2021), IET, checked 22 October 2026.
- Automatic disconnection of supply for island mode prosumer electrical installations, IET Wiring Matters, July 2026, checked 22 October 2026.
- Have your say! Drafts for public comment coming soon, IET Wiring Matters, July 2023, checked 22 October 2026.
- X3-ULTRA product page, SolaX; ET 15-30kW product page, GoodWe; S6-EH3P(30-50)K-H-ND product page, Solis (each used only for facts about that maker's own product), checked 22 October 2026.
Common questions
Do the panels still produce electricity when the supply fails?
The disconnection happens at the inverter's connection to the site circuits, not at the panels. The small-scale route in regulation 22 requires the source to be configured to disconnect itself when the distributor's supply is lost, and the ENA's fast-track conditions say the units will not operate in a loss of mains situation, so the site's circuits receive nothing from a grid-tied array.
Does a battery on its own keep a site running?
Not normally. The Energy Saving Trust says a battery system does not normally provide power in a power cut unless additional equipment and configuration is included in the installation. The IET describes the switching arrangement that forms island mode.
Does a system with backup need a different grid notification?
The IET says that if a system switches between connected and island mode, G98 or, more likely, G99 notification is usually necessary. A system that is totally grid-independent is a different case, for which the IET says no notification is required.
Can any three-phase hybrid inverter carry a whole commercial site?
Check the backup rating, which is listed apart from the grid-tied rating. On the GoodWe ET page, for example, the GW25K-ET lists 25,000 VA of back-up nominal apparent power against 27,500 VA to the grid.