why does my solar system (inc battery) go off when we have a power cut?

[Charles Brown]

It’s a requirement that your system as a whole isolates itself from the grid in the event of a grid failure. Most PV + battery installations are done simply as an adjunct to the household wiring – off to one side. A combined PV+battery controller could in theory cut itself off from your house and continue storing PV production in the battery until the battery was full but I’m not sure that any of them actually does that.

What is needed to allow things to continue working usefully is some means of disconnecting your entire house – PV, battery and everything else – from the grid. Some battery systems can do that, the obvious one being the Tesla Powerwall. With a Tesla Powerwall you have a Powerwall Gateway which actually sits between your house and the grid. In the event of a grid outage it disconnects your house from the grid entirely and will keep you going for as long as the battery and PV hold out. This however can only be done if you have something which provides that same functionality – disconnecting you from the grid.

[Charles Bell]

It’s primarily because in the UK, we get the earth connection from the grid. If the cable is cut you have no earth for safety. Off grid backup systems have a ‘switch’ that cuts you off from the grid, and you also need a local earth ( typically a long metal road driven down into the ground)

[Charlie Jones]

It seems most systems sold in the UK are sold to price rather than requirements. Most systems can operate, and supply power, whilst the grid is down but this is an additional cost and is rarely recommended or sold by suppliers competing for a sale.

The kit supplied obviously is relevant and some have built-in functionality for this whereas others require extra switching and protection devices.

[Dave Roberts]

You require a separate earth rod to be installed on the property grounds, you also require a changeover panel to switch earths if grid detection is lost.

[Edward Johnson]

There are two answers to this.

The first is what everyone says is that it is to protect grid workers from being zapped by power backfed from solar to the grid which is true and a good enough reason for shutdown but in practice I think it is mostly a marketing excuse. A grid tied inverter is literally connected to the grid. I needs to sync to grid frequency and output a voltage just a tiny bit above grid voltage to be able to push power out. If the grid is down then there is nothing to phase lock to so the inverter just cannot export anything as it has nothing to work with.

A grid tied inverter cannot run your house during a power cut as it directly connects to the mains so either it shuts down or tries to power the entire local grid which is clearly impossible unless you have a solar farm and even then the frequency locking would make this very problematic.

Summary – a grid tied inverter really has no option apart from shutting down in a power cut but even if it had some fancy electronics to allow it to create a local grid phase it would be trying to power the entire area or possibly the entire country so it is pointless to try.

[Ed Nelson]

Because you can export power, you would fry any one fixing the power lines! I chose a Tesla Powerwall specifically because it comes with a gateway that isolates the house from the grid in a power cut, all our solar and battery continues to work normally, you barely see a flicker from the lights, it’s all automatic, it just works.

[Eddie Cooper]

G98 / G99 regulations. Amazes me some other installers never explain this fact on a site survey to upsell EPS changeover switches. We do, and sell a lot of EPS upgrades.