Backup Power with Home Batteries in Scotland: Simple, Safe, and Actually Useful When the Lights Go Out

Power cuts are annoying at best and genuinely stressful at worst. If you live somewhere windy or rural in Scotland, you've probably had that moment where everything clicks off and you're standing in the dark thinking& right, what now. A home battery can make that moment a non-event. Lights stay on. WiFi keeps humming. Boiler controls don't throw a wobbly. Let's talk about how to set that up properly, without the tech headache.

First, the Two Big Choices: Critical Circuits or WholeHome

Here's the honest version I share at kitchen tables.

Critical circuits: we create a small "essentials" fuseboard for the stuff you really care about in a cut. Fridge and freezer. Lighting. Boiler or heat pump controls. Router and a couple of sockets so the laptop and phone don't die. The battery's backup output feeds just that board. It switches over fast& usually in about 10 to 30 milliseconds, which is quick enough that most things don't even blink. It's simple, safe, and costeffective.

Wholehome: everything in the house flips onto the battery using a changeover switch. Sounds great. But it needs careful design: switching the neutral as well as the live, making sure the earthing is right when you're running "islanded" off grid, and checking the battery inverter can handle big startup surges. Think heat pumps, well pumps, big ovens. They can gulp power when they kick in. Many hybrid inverters also have limited shortcircuit current, so we check fault levels and motor starting before promising wholehome UPSlike performance.

Most homes I see are happiest with the essentials board. You get resilience where it matters. You don't spend money oversizing a system to run an electric shower during a storm.

Quick Glossary You Can Skim and Move On

EPS means Emergency Power Supply. It's the feature that runs your essentials when the grid fails, but it must be safely isolated from the street network.

Island mode is just your home running by itself during a cut. No connection to the grid. On purpose.

Transfer time is how fast the switch happens. 10 to 30 milliseconds is common for that dedicated backup output, but always check the specific inverter.

DNO is your local network company. SSEN up north and the islands. SP Energy Networks in the Central Belt and south.

Open PEN fault is a nasty one. On a common Scottish supply type called PME, if the combined neutral and earth conductor breaks, metalwork can become live. That's why earthing in backup needs care.

Safety Rules That Actually Matter in Scotland

You don't need to love standards. I don't either. But they're the guardrails that keep us out of trouble.

BS 7671 is the UK wiring rulebook. The current version we work to is the 18th Edition with Amendment 2:2022 and a bolton update called Amendment 3:2024. It applies in Scotland. It tells us how to wire, protect and label backup systems so they're safe. A3 also clarifies that protective devices must suit bidirectional power flow, which is relevant for PV and batteries.

G98 and G99 are the grid connection rules. They say your kit must disconnect from the public grid when there's a power cut. No backfeeding the street. Full stop. G99 Issue 2 was published on 10 March 2025 and includes clear guidance on running a "customer's installation island" as long as it's fully isolated from the public network.

Earthing during island mode on Scottish homes

• Many homes are on PME. During a cut, you can't rely on the network's neutral and earth. Your backedup section needs its own safe reference.
• Two common ways: the inverter creates a neutral to earth link internally when it's islanded and removes it when power returns, with a switched neutral so you're not tied to the network. Or the essentials board acts like a small standalone system with its own earth electrode during a cut. Both are valid when designed properly.

EV chargers and backup

• Tempting to include the charger on backup. I get it. But EVs have extra rules. If you do back one up, you need either a separate earth electrode for that circuit or a compliant open PEN protection device that's tested for PME conditions. Most families just leave the charger off the backup board. Keeps things simple and safe.

Design Choices That Make or Break the Experience

Changeover done right: for wholehome, use a properly interlocked switch that changes all conductors including the neutral, so there's never any parallel with the grid. For essentials only, the hybrid inverter's backup output feeds the small board and the main house stays on the normal supply.

The right safety switches: inverters can leak a little direct current that confuses oldschool RCDs. Use the right type for the job, usually Type A, sometimes Type B if the manufacturer requires it. And set them up so a fault on the essentials board trips that board first, not the whole house.

Transfer you can live with: if you've got a work PC or a security system you can't have dropping, pick an inverter with that fast EPS output. If you go wholehome with a manual switch, accept it'll take a moment. That's normal.

Labels and paperwork: boring but important. We label alternative supplies and keep a oneline diagram with the consumer unit. If you ever sell the house or call out another electrician, they'll thank you.

What Actually Goes on the Essentials Board

Here's my usual shortlist. Tweak to taste.

Must haves: lighting on each floor, fridge and freezer, boiler and heating controls or heat pump controls, broadband and a couple of sockets for the router and IT, maybe a utility circuit if you've got a sump pump.

Usually skip: electric showers, ovens and hobs, immersion heaters, the EV charger, large heat pumps or electric boilers unless you've sized the inverter and battery with a big safety margin.

Rule of thumb: plan your typical backup load to sit well under the inverter's continuous rating. I like 30 to 60 percent so you've got headroom for things starting up.

Common Mistakes I See and How to Dodge Them

Not switching the neutral during wholehome changeover. That can leave you tied to the network when you're meant to be islanded. Fix is simple. Use a switch that changes the neutral as well.

No neutral to earth link in island mode, or leaving one in permanently on a PME supply. Both are wrong in different ways. The fix is either the inverter handles the link only while islanded, or we create a small standalone earth for the essentials board during a cut.

Putting the EV charger on backup without the extra protection. Honestly, just leave it off backup unless you're doing a proper TT or a tested open PEN device& and you're happy with the cost.

Using the wrong RCD type. If your installer writes "Type AC everywhere," that's a red flag on modern systems. We check manufacturer guidance and coordinate the devices so the right one trips.

Assuming any battery can run the whole house like a datacentre UPS. Some can handle a lot. Many can't. We check transfer time, shortcircuit current and motor starting on the specific appliances you care about. Then we test it. With you watching.

Paperwork, Approvals, and the NotSoFun Admin

If your system connects to solar or can export to the grid in normal use, we still follow the G98 or G99 process with your DNO. That's separate from the backup feature. If there's an export limit set to keep the local network happy, that's about daytoday running, not outages. During a cut, the system must be fully isolated from the grid anyway.

A Simple Plan You Can Follow

1. Decide: essentials only or wholehome. If you're not sure, start with essentials. You can always rethink later.

2. Check your supply type with your installer. PME or TNS or TT. Then pick the right earthing strategy for island mode. On PME, treat open PEN risks with respect.

3. Choose the inverter with a fast backup output if you care about seamlessness. Confirm the RCD type it needs. Write the transfer time into the spec so everyone's on the same page.

4. For wholehome, specify a neutral switching changeover and clear labels. For essentials only, plan the circuits you'll move across and keep it tidy.

5. Leave the EV charger off backup unless you're ready to do the extra safety work. Your future self will be less stressed.

6. Do the G98 or G99 paperwork for the gridparallel side. If an export limit is needed for normal operation, agree it early with SSEN or SP Energy Networks.

7. Test day. Simulate a power cut, watch the switchover, check the earthing behaves, press the RCD test buttons, and make sure the things you care about actually stay on.

A Quick Word on Sources, Because Facts Matter

BS 7671 is the rulebook we follow in Scotland. The current edition includes Amendment 2:2022 and Amendment 3:2024, which adds requirements relevant to bidirectional systems like PV and batteries.

The grid connection rules are ENA G98 and G99. Issue 2 of G99 was published on 10 March 2025 and it directly covers storage and how to run a customer installation as an island internally, provided you're properly isolated from the public network.

Export limiting is covered by G100. Different topic to backup, but often present alongside solar and batteries in daytoday operation.

EV open PEN protection has fresh guidance from the IET. If you're mixing EVs and backup, it's worth doing properly.

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Mackie Electrical - NICEIC-approved electrical contractors serving Central Scotland. Specialists in home battery backup systems, EPS installation, PME earthing compliance, and BS 7671-compliant backup power for homes in Stirling, Perth, Falkirk, and throughout Scotland.