At a glance: generator transfer switches
- A generator transfer switch controls the change between your mains electricity supply and backup generator power.
- Transfer switches can be manual or automatic, depending on whether you want to manage the changeover yourself or have a compatible system do it for you.
- An automatic transfer switch (ATS) can detect a mains failure and trigger a compatible generator to start, before transferring the electrical load.
- Your generator still needs to be correctly sized. A transfer switch manages the power source; it doesn't increase generator output.
- Compatibility matters with automatic systems. Electric or remote start doesn't necessarily mean a generator supports ATS.
- Petrol generators can be used for many backup applications, including automatic backup where the generator has the required ATS compatibility.
- Installation should be handled by a suitably qualified electrician where a transfer switch connects a generator to your mains electrical system.
A generator transfer switch is an important part of many backup power setups. It provides a controlled way to change the electrical supply from the mains to a generator during a power cut, then back again when mains electricity returns.
In the UK, you'll come across two approaches: manual changeover and automatic transfer. A manual system puts you in control of the switch, while an automatic transfer switch can manage the process with a compatible generator.
The setup that’s best for you depends on how you use your backup power, what needs to keep running during an outage and whether you need the generator to respond when nobody is there.
What does a generator transfer switch do?
A transfer switch allows an electrical installation or selected circuits to receive power from either the mains supply or a backup generator through an appropriate changeover arrangement.
It doesn't produce electricity itself. Instead, it controls which power source is supplying the connected electrical load.
When planning backup power, you'll need to think about the generator and transfer switch as separate parts of the same setup:
- The generator provides the backup power.
- The transfer switch manages the change between power sources.
The generator therefore needs enough rated output for everything you expect it to support during an outage.
What types of generator transfer switches are there?
For most backup-power setups, the main distinction is between manual and automatic transfer.
Manual transfer switch
With a manual transfer switch, you manage the changeover yourself.
When mains power fails, someone needs to start the generator and operate the changeover correctly. When mains electricity returns, the supply is switched back and the generator can be shut down.
This can work well for occasional backup where someone is available to manage the process.
Automatic transfer switch
An automatic transfer switch (ATS) monitors the mains electricity supply. With a compatible setup, it can detect a mains failure, signal the generator to start and transfer the electrical load to generator power.
When mains electricity returns, the system can transfer the load back before the generator shuts down.
Automatic transfer is especially handy when you don't want backup power to depend on someone being there to start the generator.
Manual or automatic: which transfer switch do you need?
Neither option is automatically better. It depends on what you expect from your backup setup.
A manual transfer switch may be enough if power cuts are occasional, you're likely to be there when one happens and you're happy to start and switch over the generator yourself.
An automatic transfer switch makes more sense when you want backup power to respond without manual intervention. That could be useful if essential equipment needs to remain available while you're away or you simply want a more automated backup setup.
If you're weighing up the two options, we've compared the practical differences separately.
Does every generator work with a transfer switch?
No. The type of transfer switch makes a difference here.
With an automatic system, the generator needs to support ATS operation. Look for a dedicated ATS connection and confirmation that the generator is compatible with the transfer equipment you're considering.
Don't rely on the starting method alone. An electric-start or remote-start generator isn't necessarily capable of responding automatically to an ATS.
Manual changeover works differently because the generator doesn't need to receive an automatic start signal from the switch. Even then, the generator, transfer equipment and wider electrical installation still need to be suitable for one another.
If you're shopping specifically for an automatic setup, compatibility should be one of the first things you check.
Can petrol generators work with a transfer switch?
Yes. A petrol generator can form part of a backup-power setup, provided the generator and changeover arrangement are suitable for one another.
Petrol is a practical choice for home backup and other occasional power requirements. If you want automatic transfer, look specifically for a petrol generator with ATS compatibility rather than assuming you'll need to move to diesel.
For a manual setup, you'll still need to make sure the generator has enough output for the equipment you want to support and that the electrical installation has been properly designed.
What about dual-fuel generators?
A dual-fuel generator gives you the choice of petrol or LPG.
That can give you more flexibility over fuel supply, but fuel type and transfer-switch compatibility are separate issues.
Being dual-fuel doesn’t automatically mean a generator can work with an ATS.
If automatic changeover matters, check the specific model’s ATS compatibility. Also check its rated output on the fuel you intend to use because output can differ between petrol and LPG.
When might you use a diesel generator?
A diesel generator is the more practical choice where backup requirements involve larger electrical loads, sustained running or heavier-duty applications.
Some diesel generators are designed specifically with standby applications and ATS connectivity in mind. But choosing automatic transfer doesn't mean you automatically need diesel.
For occasional or domestic backup, a petrol generator may be perfectly suitable if it provides enough output and supports the changeover arrangement you need.
Choose the generator around the load and intended use first. Fuel type comes after that.
What size generator do you need with a transfer switch?
A transfer switch doesn't change the amount of power available from your generator.
Before choosing the generator, decide what you actually want to keep running during a power cut. That could include essential heating equipment, refrigeration, lighting or other important appliances rather than everything in the property.
Then work out:
- The combined running wattage
- Any additional starting wattage
- Which equipment may run at the same time
- The generator’s rated output
This gives you a much better idea of the generator capacity you need.
There’s no need to oversize simply because you’re installing a transfer switch. The generator still needs to match the actual backup load.
What about single-phase and three-phase power?
Your transfer equipment needs to suit the electrical supply and generator it's being connected to.
Single-phase power is common in UK homes, while three-phase supplies are more often found in commercial, industrial and other higher-demand installations.
If you’re dealing with three-phase equipment, generator output and load distribution need closer attention. Don’t assume the generator’s full rated output can be drawn from any individual phase or connection.
The transfer switch also needs to be rated correctly for the voltage, current and phase configuration of the installation.
If you’re unsure which supply you have, establish that before choosing the generator or transfer equipment.
Where should a generator transfer switch be installed?
The exact arrangement depends on the property and what you want the generator to power.
Some backup systems are designed around selected essential circuits, while others may be specified differently depending on the generator capacity and electrical installation.
Either way, the transfer arrangement needs to prevent the generator supply and mains supply from being connected incorrectly.
This isn't something to work out just from the transfer switch. The generator output, circuits being backed up and electrical installation all need to be considered together.
Who should install a generator transfer switch?
Where a transfer switch forms part of your mains electrical installation, use a suitably qualified electrician.
They can assess:
- Your existing electrical supply
- The circuits or equipment requiring backup
- Generator output
- Transfer-switch rating and compatibility
- Single-phase or three-phase requirements
- The appropriate changeover arrangement
- Electrical protection and installation requirements
If you’re planning a backup system before buying the generator, involving an electrician early can prevent you choosing equipment that doesn’t suit the installation.
Generator transfer switch checklist
Before choosing a transfer switch, check:
- Do I want manual or automatic changeover?
- What equipment needs backup power?
- How much generator output do I need?
- Is the generator compatible with the transfer system?
- If I want ATS, does the generator have the required ATS connection?
- Is my electrical supply single-phase or three-phase?
- Does the transfer equipment have the appropriate electrical rating?
- Will I be there to manage a power cut manually?
- Who will install the system?
- Have the generator and transfer switch been considered as one backup setup?
Getting those answers first makes it much easier to avoid compatibility problems later.
Need help planning your backup power setup?
A transfer switch is only one part of a backup-power system.
The generator needs enough output for the equipment you want to run. The changeover equipment needs to match the generator and electrical supply. The installation then needs to bring everything together correctly.
If you’re working through a generator and transfer-switch setup, use our chat system to tell us what you need to keep powered and whether you’re considering manual or automatic changeover and we can narrow down the options with you.
