What Size Generator Do I Need?

At a glance: choosing the right generator size

• Start with what you need to power. Your generator size should be based on the appliances, tools or equipment you'll actually use.
• Add up the running watts. This tells you how much power your generator needs to provide during normal use.
• Check the starting watts. Motors, pumps, compressors and some power tools need a short burst of extra power when they start.
• Leave some headroom. Choose a generator that can comfortably handle your expected load rather than running at its limit.
• Think about what you'll run at the same time. This gives you a more realistic idea of the output you need.
• Petrol and dual-fuel generators suit a wide range of jobs. They're practical choices for backup power, outdoor use and powering tools.
• Consider diesel for more demanding use. A diesel generator may be worth looking at for heavier-duty or prolonged power requirements.

If you’re asking “what size generator do I need?”, start with what you actually want to power.

Add up the wattage of the appliances, tools or equipment you expect to use at the same time, then check whether any of them need extra power when they start.

From there, you can choose a generator with enough output to handle the job comfortably, plus some sensible headroom.

You don’t need to get bogged down in technical details. Use our simple generator wattage calculator to give you a good idea of the output you need and help you avoid choosing something that’s either underpowered or unnecessarily large.

How to calculate what size generator you need

Generator output is normally measured in watts (W) or kilowatts (kW). One kilowatt equals 1,000 watts.

Make a list of everything you want to power and check the wattage of each item. You'll usually find this on its rating plate, in the manual or in the manufacturer's specifications.

Then:

1. Add up the running wattage of everything you'll use at the same time.
2. Check whether any equipment has a higher starting wattage or surge requirement.
3. Look for a generator that can comfortably cover that demand, with some spare capacity.

For example, if the equipment you want to run needs 2,500W in total, don't automatically choose a generator because its headline output is 2,500W. Check the rated and peak output to make sure it can cope with your normal load as well as any extra demand when equipment starts.

This gives you a much better idea of how many watts you need from a generator than choosing based on size or price alone.

What's the difference between running and starting watts?

Running watts are the amount of power an appliance or tool uses during normal operation.

Starting watts, sometimes called surge watts, are the extra power some equipment needs briefly when it's switched on. You'll often need to account for this with equipment that uses an electric motor, pump or compressor.

A power tool, for example, may run comfortably once its motor is up to speed but need a higher burst of power to get started.

Check the manufacturer's figures for the equipment you want to use rather than relying on estimates. This will give you a more accurate idea of the generator output you need.

What size generator do you need for different jobs?

There's no single generator size that's right for everyone. A generator for occasional outdoor use will have very different demands from one powering tools on a job site or providing backup power at home.

Generator for light and occasional use

If you only want to run a few lower-powered devices, you may not need a particularly large generator.

A smaller portable petrol generator can be a practical choice when you want easy access to power away from the mains. Check the total wattage rather than simply counting how many devices you're plugging in, as several smaller items can still add up to a significant load.

Generator for home backup and power cuts

If you're choosing a generator for home use or backup during a power cut, decide which appliances and devices you actually need to keep running.

You may only need to cover a few essentials rather than everything in your home. Check their combined wattage and any starting requirements to get a clearer idea of the generator output you'll need.

If a generator is being connected to your home's electrical installation, use a suitably qualified electrician. Never connect a portable generator directly to household wiring unless the correct professionally installed equipment is in place.

Generator for power tools

If you need a generator for power tools, check both the running and starting requirements of the tools you'll be using.

This matters particularly for equipment with electric motors. A generator may have enough rated output to keep a tool running but still need additional peak capacity to handle the initial startup demand.

Think about how you work, too. If several tools or pieces of equipment will be running together, you'll need to account for their combined load.

Generator for outdoor and event use

A portable generator can be useful when mains electricity isn't available, whether you're setting up outdoors or need temporary power in a remote location.

Look at the whole setup when calculating your requirements. Lighting, equipment and other electrical devices all contribute to the total load. Portability, fuel choice and noise may also matter depending on where you'll be using the generator.

Petrol or dual-fuel: which generator is right for you?

For many home, outdoor and general-purpose jobs, a portable petrol generator is a good place to start. With different power outputs available, you can choose a model that suits the equipment you need to run without paying for far more capacity than you'll use.

A dual-fuel generator gives you the option to use petrol or LPG. That extra flexibility can be useful if you want a choice of fuel sources rather than relying on petrol alone. LPG can also offer several practical advantages, including cleaner handling, easier long-term storage and reduced fuel-related maintenance.

Unlike petrol, LPG doesn’t degrade in the same way during storage, making it a convenient option when a generator may sit unused for extended periods. LPG can also help reduce deposits within the engine, potentially keeping the fuel system cleaner and reducing maintenance requirements. Depending on the generator and load, LPG may also provide longer or more predictable running times, giving you another useful option for extended backup power or regular use.

Keep an eye on the specifications when comparing dual-fuel models. Generator output can differ depending on whether you're running on petrol or LPG, so check the figures for the fuel you're most likely to use.

Whatever fuel you choose, the same rule applies: make sure the generator has enough rated and starting capacity for your equipment.

When should you consider a diesel generator?

A diesel generator may be worth considering if you need power for a more demanding or prolonged application.

Sizing works in the same way: check the load you need to power and make sure the generator can handle any startup demand. For many domestic, portable and occasional jobs, petrol or dual-fuel will be the more natural starting point. For heavier-duty requirements, it's worth comparing diesel options as well.

Do you need a bigger generator than your calculated wattage?

A little spare capacity is sensible. A lot of unused capacity usually isn't.

Choose too small and the generator may struggle with your equipment, especially when something needs extra power to start. Go much larger than you need and you could end up paying for output you aren't going to use.

The aim is to choose a generator that can comfortably handle your expected load and startup requirements, with enough headroom to do the job properly.

Generator sizing checklist

Before choosing your generator, ask yourself:

• What appliances, tools or equipment will I power?
• What is the running wattage of each item?
• Which items will I use at the same time?
• Does anything have a higher starting or surge wattage?
• Does the generator's rated output cover my normal load?
• Can its peak output handle the startup demand?
• Have I allowed some spare capacity?
• Would petrol or dual-fuel suit how I'll use it?
• Do I need to consider diesel for heavier-duty use?
• How important are portability and noise?

With those answers, it becomes much easier to narrow down the right generator for your needs.

Find the right size generator for the job

The right generator isn't necessarily the one with the biggest kW figure. It's the one that gives you enough power for the equipment you actually need to run.

Check your running wattage, account for any extra starting power and leave yourself some sensible headroom. You can then compare petrol and dual-fuel generators, or diesel for more demanding applications, knowing what level of output you're looking for.

Still not sure what size generator you need? Check out our Generator Selector Guide which will help you choose the generator most suited for you.