Most solar systems go dead in a power cut, and that is deliberate

The inverter is required to shut down

It surprises almost every new owner. The sun is shining, the panels are making power, the lights are out, and the system has switched itself off. This isn’t a fault and it is not a design oversight. A grid-connected inverter is required by the connection rules of essentially every network in the world to detect a loss of supply and disconnect within a fraction of a second.

The reason is safety. If a network section is de-energised for work or after a fault, anyone touching those conductors expects them to be dead. A house pushing power back into that section would make it live, without warning, at a point where the utility believes it has isolated everything. The consequences are obvious and the rule is not negotiable.

There is a second, more technical reason. A conventional inverter is grid-following: it takes its voltage and frequency reference from the network and synchronises to it. With the network absent there is nothing to follow, and the inverter cannot simply invent a reference without being built to do so.

What backup actually requires

Turning a grid-tied system into one that keeps working needs three things, and a battery alone is only one of them. It needs an inverter capable of grid-forming operation, which establishes its own voltage and frequency rather than tracking someone else’s. It needs a means of storing energy, because the sun is not obliging enough to shine when the network fails.

And it needs a changeover arrangement — a switch, contactor or gateway that physically separates the house from the network before the inverter energises anything. That device is what makes the whole thing legal, because it guarantees that the island created inside the property cannot extend past the meter.

Retrofitting all of this to an existing installation is possible and it is rarely trivial. The changeover device sits between the incoming supply and the distribution board, so the work is at the heart of the electrical installation rather than out at the edge of it, and it usually requires notifying the network operator.

Whole house or essential circuits

Backing up an entire house means the inverter must be able to supply whatever the house asks for at any instant. That is a demanding specification, because the peak matters more than the average: an electric shower, an oven, an immersion heater or an induction hob can each ask for several kilowatts on their own, and motors draw a large surge on starting.

The usual compromise is a separate backup distribution board carrying selected circuits. Lighting, sockets in a couple of rooms, the router, the heating controls and the refrigeration. Everything heavy stays on the unbacked side. That approach fits a modest inverter and it makes the runtime arithmetic far more comfortable.

The rewiring is not free. Circuits have to be identified and moved, which means work at the consumer unit and sometimes new cable runs, and it is much cheaper to plan for at installation than to add later. Anyone who thinks backup might matter one day should say so before the electrician arrives, not afterwards.

What backup will and will not run

The illustrative arithmetic is easy and clarifying. A refrigerator, a router, a few lamps and a heating pump might together average a couple of hundred watts, so a domestic battery of ordinary size will carry them through a night comfortably and probably rather longer. That is a genuinely useful outcome.

Now add a single electric shower drawing several kilowatts. It empties the same battery in well under an hour, and it may exceed the inverter’s output limit before it does. Electric heating, cooking and vehicle charging all belong to that category, and expecting a house battery to cover them during an outage is expecting the wrong thing entirely.

Whether the solar array can recharge the battery during an outage depends on the system architecture, and it is worth asking directly. Some backup arrangements allow the panels to continue feeding the island, which transforms a multi-day outage. Others keep the array offline and give you the battery and nothing more.

Whether it is worth buying at all

This is a resilience purchase, not an economic one. Backup capability generally does not lower a bill by a single unit, and a battery bought for backup will do the same energy arbitrage that a battery bought without it would do. The added hardware buys continuity of supply and nothing else.

So the question is what an outage costs you, and that is entirely local. Where the network is reliable and cuts last minutes a few times a decade, the honest answer is usually that backup is not worth the extra hardware and complexity. Where storms take the supply down for days each winter, or where the grid is unreliable as a matter of course, the calculation is completely different.

Medical equipment changes the picture immediately and should never be balanced against convenience. Anyone dependent on powered equipment needs a considered plan, made with the supplier of that equipment and with the network operator, who in many countries maintain a register of vulnerable customers and prioritise accordingly.