Using a home battery in a power cut

One of the most common questions we are asked is whether you can run your home off a home battery in a power cut. The comments section on our Fully Charged episode on the Tesla Powerwall2 was absolutely packed with questions on this issue. Not all battery systems automatically offer this facility, although there have been options available for decades.

It sounds easy – there's a power cut and so you just run your home off the battery instead. Sadly, it's a little trickier than that, so here are the key things you'll need to consider.

How does a home backup battery system work?

The main complexity with using batteries for backup power is that they have to comply with strict safety requirements. If there's a power cut, engineers will need to work on the electricity lines on the grid, and there cannot be the risk of any form of generation or batteries making this live while they carry out their works. It is for this reason that solar inverters turn off in the event of a power cut.

It is possible to run a home off a battery during the power cut, but the controls for a conventional home battery are not currently good enough – if they don't operate 100% perfectly, there would still be the risk of electrocution, and if the battery does not operate properly it does not 'fail safe'.

Instead, backup battery systems have a relay to physically disconnect the electricity supply in a building from the grid (called islanding). It's essentially a big switch, which detects that the drop in voltage on the grid in the event of a power cut, and disconnects the home from the grid. It needs to be located between your incoming supply and your main fuseboard, so you will need space for this.

There are only a few suppliers currently offering this system in the UK, which we look at in more detail below.

How much storage capacity do I need for a back-up battery in my home?

You'll need to decide what percentage of your storage capacity you want to reserve for backup. This means you keep your battery partially full with your emergency electricity. Most people are using home batteries for storing solar electricity, but battery levels can be low in the mornings. The worst thing would be to invest in a back-up battery system and have no stored electricity in the event of a morning power cut!

Getting the balance right will be crucial, and the reserve will be a portion of the battery capacity you do not use every day. If you set the reserve capacity too low, you won't have much energy in the event of a power cut. If you set the reserve capacity too high, you won't have the room to fully utilise the battery for its main job of storing excess solar electricity, thereby damaging your return on investment. In practice, getting a back-up option is likely to require a bigger battery than just using it for load shifting.

What appliances can I run off the battery during a power cut?

Home batteries have an integrated inverter that produces AC power for use in the home. The higher the rated power output of the battery inverter, the higher instantaneous power can be delivered to appliances at any one time. If you want to run lots of power hungry appliances from the battery during the power cut you'll need a high-powered battery inverter.

When running a conventional on-grid battery system, peaks in power are often met by the grid. If your battery has a 2kw inverter, and you turn on a 3kW kettle, then it will source 2kW of power from the battery and 1 kW from the grid. During a power cut this wouldn't be possible.

So, if you want the battery to run during a power cut, it will need a large inverter – for example the 5kW of power available from the Tesla Powerwall2 would be able to run the kettle.

What if there's not enough power from the battery?

If you do try to use more power than the battery inverter can provide, you might trip the battery inverter, and still end up with no power during the power cut! And of course, as the transition from grid to battery is smooth, you may not realise there is a power cut actually occurring. So how do you stop yourself turning on high power loads?

The only way to make this fool-proof is to separate critical and non-critical loads within the house, and run them off different circuits. All your LED lighting, efficient fridges, gas boilers, laptops, phones and even modern LED TVs take very little electrical energy to run and can be powered by most battery systems for extended periods of time. High powered appliances such as power showers, kettles, microwaves and electric cookers would be on a non-critical circuit that the battery cannot power.

However, this could mean a major re-wire for your building. Separating critical from non-critical loads in your house could be expensive and time consuming. In a recent situation, we found a tumble dryer was on the same circuit as a fridge freezer, so to separate them we'd need to run two separate ring mains to the kitchen. It's not always the case, but separating appliances to different circuits could involve long cable runs back to the fuse board, re-chasing cables into walls and even re-decoration.

Changeover switches

Instead of separating critical loads, in some situations it may be possible to fit a physical changeover switch. In the event of a power cut you would turn off the non-essential loads and physically flick a switch to get back-up power.

Will solar panels charge the battery when the grid goes down?

You'd assume it would, right? Unfortunately, it's not always the case. More 'cost efficient' backup systems will just discharge their stored energy when the grid goes down and the solar will turn off. This is partly due to the fact that if the battery inverter is smaller than the solar inverter then the solar inverter could overload the battery system. More expensive battery systems can be charged by the solar when the grid goes down. To do this, the battery inverter will have a higher capacity than the solar. Once the battery is full, the solar will either turn off or, if the systems can talk to each other, the solar will ramp down to match the building's load exactly.

Are there any other changes needed?

The UK and Europe have different earthing regulations to the US and Australia. Back-up systems in the UK will need to be able to swap to a local earth if the house normally uses the grid as its earth.

So is it worth it?

In 2015, there were 640 power outages affecting over 2.5m people for an average of 50 minutes.

So although power cuts are rare, they are disruptive when they do occur. Furthermore, some areas, especially rural locations, are more prone to power cuts than others.

You'll need to balance the extra cost of a back-up battery system against the benefits of riding through the power cut.

Current back-up battery systems available in the UK

Victron and SMA

Victron are old hands in the inverter back-up and off grid market and offer a huge range of back-up capable battery inverters, starting at 500W of power. They can be used in conjunction with their own Lithium Ion batteries or third-party batteries. If you are looking for a small, relatively inexpensive backup system then it would be good to consider Victron.

Victron inverters are UPS standard. This means that the changeover to backup is fast enough to avoid data loss on servers.

SMA Sunny Island systems can be used for back-up and start around 3 kW of power. SMA do not produce their own batteries but can be used with a huge range of third party battery suppliers.

Forthcoming back-up battery systems available in the UK

Tesla (unofficially due summer 2019)

Ok, this is what everyone is really waiting for. Tesla don't yet supply a back-up system in the UK but it is available in the USA and Australia. This is due to European wide earthing regulations which means that the Tesla Powerwall 2 back-up interface (called the gateway by Tesla) does not yet comply in the UK.

Roll out of a new European compliant back-up gateway is unofficially due to be available in Q2 2018. It will come at a higher price than the standard battery gateway and there will be higher installation costs associated with it. It is important to note that the battery itself will remain the same and only the gateway changes.

The back-up gateway contains a substantial relay which completely islands the house in the event of a power cut. It will need to be positioned on the grid side of the main consumer unit (or fuse board).

As installers, we'll determine what type of earthing you have and, if necessary, install a local earth for when the grid goes down.

The integrated 5 kW battery inverter is large enough to cover most electrical loads in a modest sized house. For some properties, it would therefore be possible to wire the Powerwall to cover the whole house in a power cut.

Larger houses will have the option of separating critical loads for back-up or physically turning off the higher loads and waiting a few minutes for the battery to kick back in.

Further Reading

Find out more about our battery offers

Here’s our guide to how a home battery works

How we work with you on your residential energy project


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