Solar, batteries and power cuts

This is the thing people are most surprised by, and they are usually surprised at the worst possible moment. You have panels on the roof and a battery in the garage, the street loses power, and your house goes dark along with everyone else's.

The system has not failed. Disconnecting itself is exactly what it is required to do when the grid goes down.

Why it shuts down

When the grid fails, engineers go out to work on the lines. If houses along that street kept pushing electricity back into the network, those lines would still be live, and the person working on them would have no way of knowing.

So every grid-connected solar system is required to detect that the grid has gone and disconnect itself within a fraction of a second. It is called anti-islanding, and it is not optional.

The unfortunate side effect is that your inverter switches off too, taking your own supply with it — even though the sun is shining and the battery is full.

Why the battery does not help either

A battery seems like it ought to save the day, and by default it does not.

An ordinary home battery is designed to work alongside the grid. It pushes power into your house's wiring assuming there is a grid there setting the rhythm — the voltage and frequency everything else follows. Take the grid away and it has nothing to follow, so it stops.

To run without the grid, a battery needs an inverter that can create that rhythm itself, and a way of physically disconnecting your house from the street so nothing leaks back out. That is extra equipment, and it costs more.

How to get backup

Ask for it explicitly, before anything is installed. It is much cheaper designed in than retrofitted.

What you are asking for is usually described as backup, islanding, or emergency power supply. Two things have to be true: an inverter capable of running on its own, and a changeover arrangement that isolates your house from the network when the grid drops.

A few things to establish while you are asking.

What does it actually power? Most backup arrangements cover a subset of circuits — lights, sockets, the fridge, the router — and not the whole house. Electric showers and cookers are usually left out, because the battery cannot supply that much at once.

How long for? A battery holding ten kilowatt-hours will run a fridge, some lights and a router for a day or two. It will run an electric shower for a few minutes. Backup is about keeping essentials going, not carrying on as normal.

Does it switch over automatically? Some systems change over in a fraction of a second so you barely notice. Others need somebody to flip a switch. Both work; they are different products at different prices.

Will the panels recharge it? During an outage lasting more than a night, this matters a great deal. Not every backup arrangement keeps the solar running while islanded, and one that does turns a day of backup into an indefinite one on a sunny week.

Is it worth paying for?

Depends entirely on where you live and what depends on the electricity.

In most British towns the power goes off rarely and briefly. Backup adds cost and saves nothing on a bill, so the question is what avoiding a few dark evenings is worth to your household.

It becomes a much easier decision if you are at the end of a rural line where winter storms take the supply out for hours, if somebody in the house depends on powered medical equipment, if you work from home and cannot afford to be offline, or if you keep anything in a freezer that would be expensive to lose.

If any of those apply, say so at the quotation stage. If none do, it is a reasonable thing to skip — as long as you skip it knowingly and do not discover it during the first storm.