No. A standard grid-tied solar system stops when the grid fails, even on a bright day. Your panels may be generating, but the inverter shuts down because it is built to. To keep any power flowing in an outage, a site needs a battery, an inverter that can run without the grid, and a separate circuit for the loads you want to keep alive.
This post explains why the shutdown happens, what backup actually takes, and how to decide whether your business needs it.
Why does solar switch off when the grid goes down?
It is a safety rule. When a network fails, engineers work on the lines as if they were dead. If your inverter kept feeding power into that line, it would be live without anyone knowing. That could kill someone.
So every grid-tied inverter must spot the loss of the grid and disconnect. This is called loss-of-mains or anti-islanding protection. The Energy Networks Association sets the rules for connecting generation to the network. Engineering Recommendation G98 covers small installations, up to 16 amps per phase in total, which is 3.68kW on a single phase. G99 covers anything bigger, which includes almost every commercial roof. Our guide to G99 and grid connection covers the wider process.
The inverter reconnects only after the grid has been back and stable for a set time. You cannot switch this protection off, and a good installer will never offer to.
What keeps the lights on instead?
Three things have to work together.
| Part | Job |
|---|---|
| Battery | Holds the energy. Once the grid is gone, it is the only source of power. |
| Hybrid or battery inverter with backup output | Detects the cut, splits your site from the grid, and runs the backup circuit from the battery |
| Separate backup circuit | A small distribution board for the loads you want kept alive |
The split from the grid is the key. Because the site is cut off from the network, the backup system can run safely without sending power down the dead line. The panels can then recharge the battery during the day, which stretches the backup time.
Not every battery system can do this, because some batteries are wired only to save money on bills and will shut down with the grid. If backup matters to you, put it in writing in the specification and ask what the changeover time is. Some systems switch in a fraction of a second. Others take longer, and sensitive equipment may restart when they do.
Will backup run the whole building?
Almost never, since sizing a battery to carry a full commercial site is expensive. A common approach is to move a short list of essential circuits onto the backup board and leave the rest on the main supply.
Typical essential loads include:
- server and network equipment
- alarms, door access and emergency systems
- refrigeration compressors
- a few lights and sockets so staff can work safely
- on farms, milking, ventilation or water supply
How long it lasts is simple sums. A battery holding 100kWh will run a 10kW essential load for about ten hours, less allowance for losses. Add a 40kW machine to the backup board and that falls to about two. This is why the choice of circuits matters more than the size of the solar array.
Who actually needs this?
Ask what an outage costs you. If the answer is a few lost minutes, backup is rarely worth the extra capital. If it is spoiled stock, lost data, harmed animals or a stopped production line, the sum changes.
The strongest cases are cold stores and food sites, IT-dependent businesses, farms with livestock, and sites with safety systems that must keep running. Our posts on cold storage and refrigeration and poultry farms show how round-the-clock loads change the picture.
Many sites already have a diesel generator for this job. A battery does not usually replace a generator for long outages. It can cover the first hours, cover the gap while a generator starts, or run small loads without fuel. Whether it beats a generator on cost depends on your outage history and how long you need cover.
How often do cuts happen, and how long do they last?
Most customers see few, and most are short. Network operators report their performance to Ofgem each year as the number of interruptions and the minutes lost per customer. Your own network operator publishes its figures, and your own records of past outages are more useful than any national average. Check your network operator’s published results and your site log before you size anything.
Long cuts can also trigger payments. Ofgem sets guaranteed standards for power cut payments, but check them against what a lost day actually costs you.
Is it cheaper to design backup in from the start?
Usually, yes. Because a string inverter cannot be changed into a backup-capable one, it has to be replaced. The backup board is also much easier to wire before a site’s electrics are finished than afterwards. If resilience is on your list, say so in the first brief.
Backup also adds cost, so it should be priced as its own line. That lets you see what resilience costs and what it saves. Our battery storage guide looks at the bill-savings side, and the two cases can add up, though resilience should not be hidden inside the payback figure.
Questions to put to a supplier
- Does the quoted system keep running with the grid down, and for which circuits?
- How long does it take to switch over?
- How many hours of backup do the chosen loads get, with the numbers shown?
- What happens to the panels’ output during the outage?
- What does the backup add to the price, line by line?
Vague answers are a warning sign. A supplier that has done this before will give you figures.
Where we can help
We are an independent adviser. We do not sell panels or batteries, so the resilience case is built from your loads and your outage costs rather than a supplier’s default package. Our energy and ROI modelling service puts backup next to the normal payback numbers so you can see both.
Sources: Energy Networks Association, connecting generation to the electricity networks; Ofgem, power cut payments under guaranteed standards.
Because backup changes the inverter, the battery and the wiring, plan it before you buy. Our energy and ROI modelling sets the cost of backup against what an outage costs you. A feasibility study checks space and the grid, while procurement and tender support puts the backup design into the specification. If a funder owns the system, PPA and financing advice checks what happens in a power cut. Manufacturers should also read our guide to solar for factories.