23 Jul 2026
WHAT SIZE GENERATOR FOR I NEED? | A COMPLETE UK GUIDE (2026)
Choosing the right generator doesn't have to be complicated. This guide explains how generator sizing works, how to calculate your power requirements, and how to choose a generator that delivers reliable performance without paying for more capacity than you actually need.
Quick answer
The right generator size depends on the total wattage of the equipment you need to power, including both the running watts and the starting (surge) watts of any equipment with electric motors.
As a general guide:
These figures are general guidance only. Always check the running and starting wattage of the equipment you intend to power.
The safest approach to calculate your total electrical load and then choose a generator with approximately 20-25% spare capacity. This helps prevent overloading and allows for future equipment requirements.
Browse our full range of portable generators and inverter generators to compare outputs, fuel types and features before calculating the right size for your needs. Shop Portable Generators & Inverter Generators →
WHY GENERATOR SIZE MATTERS
Selecting a generator that is too small can lead to overloaded circuits, nuisance shutdowns, and equipment that fails to start correctly.
Choosing an excessively large generator isn't always the answer either. Oversized generators can be less fuel efficient, more expensive to purchase, and unnecessarily heavy to transport.
A correctly sized generator should:
- Start connected equipment reliably
- Deliver stable electrical output
- Operate within its designed capacity
- Provide good fuel efficiency
- Allow for occasional increases in demand
Whether you're powering a construction site, workshop, outdoor event, or emergency backup system, correct sizing is one of the most important decisions you'll make.
RUNNING WATTS VS STARTING WATTS
One of the most common mistakes is calculating only the running power.
Many electrical tools and appliances require additional power for a few seconds while starting.
This is known as:
- Starting watts
- Surge watts
- Peak load
For example:
Note: Starting (surge) watts are only required for a few seconds while equipment starts up. Always size your generator to accommodate the highest starting load.
If your generator cannot supply this temporary surge, the equipment may fail to start or the generator's overload protection may activate.
HOW TO CALCULATE THE GENERATOR SIZE YOU NEED
Follow these four simple steps.
STEP 1: LIST EVERYTHING YOU NEED TO POWER
Write down every appliance, machine or tool you expect to operate at the same time.
For example:
- Site lighting
- Battery chargers
- Circular saw
- SDS drill
- Mobile phone chargers
- Compressor
STEP 2: FIND THE RUNNING WATTAGE
Most equipment displays its power rating on the manufacturer's label.
If only amperage is shown, you can estimate wattage using:
Watts = Volts × Amps
For example:
230V × 5A = 1,150W
STEP 3: ALLOW FOR STARTING LOADS
Identify any equipment containing electric motors.
These typically require additional power during startup.
Examples include:
- Compressors
- Pumps
- Refrigeration equipment
- Circular saws
- Mitre saws
- Cement mixers
Only the largest starting load normally needs to be added, as most equipment is not started simultaneously.
STEP 4: ADD A SAFETY MARGIN
Professional users rarely size a generator to operate continuously at 100% capacity.
Leaving around 20–25% spare capacity helps improve reliability and allows flexibility if additional equipment is needed.
GENERATOR WATTAGE GUIDE FOR COMMON EQUIPMENT
The figures below are approximate. Always check the manufacturer's specifications for your equipment.
Important: These figures are approximate. Check the equipment rating plate or manufacturer’s specification before choosing a generator, and allow for any additional starting or surge wattage.
Looking for a generator that can comfortably power your equipment? Our collection includes compact inverter generators through to heavy-duty site generators. Explore Our Generator Range →
EXAMPLE CALCULATIONS
Example 1
DIY Project
Equipment
- Circular saw – 1,500W
- Work light – 100W
- Battery charger – 250W
Total running load
1,850W
Allowing for startup demand and spare capacity.
Example 2
Construction Site
Equipment
- SDS drill – 1,200W
- Angle grinder – 1,400W
- Lighting – 300W
- Battery chargers – 400W
Total running load
3,300W
Example 3
Emergency Home Backup
Essential equipment
- Fridge freezer
- Internet router
- Lighting
- Boiler controls
- Mobile phone charging
A quality 3–5kW generator is often sufficient for essential household appliances during an outage. High-demand appliances such as electric ovens, immersion heaters and electric showers can significantly increase the capacity required.
If your calculations are similar to the examples above, you can browse generators in the recommended output range to find the right model for your application. Find the Right Generator →
SHOULD I BUY AN INVERTER GENERATOR?
An inverter generator is ideal when powering sensitive electronic equipment.
Unlike conventional generators, inverter models electronically regulate their electrical output to produce a stable voltage and frequency.
This makes them suitable for:
- Computers
- Laptops
- TVs
- Medical devices
- Testing equipment
- Battery chargers
- Audio equipment
Many inverter generators are also quieter, lighter and more fuel efficient because they automatically adjust engine speed to match the electrical demand.
If your primary requirement is powering modern electronics, an inverter generator is often the preferred option.
COMMON GENERATOR SIZING MISTAKES
Avoid these common errors when choosing a generator.
BUYING BASED ON PRICE ONLY
The cheapest generator isn't always the most suitable. Capacity, reliability and intended use should always come first.
FORGETTING START UP LOADS
Motor-driven equipment often needs considerably more power when starting than when running.
RUNNING CONTINUOUSLY AT MAXIMUM OUTPUT
Operating close to full capacity for extended periods can reduce efficiency and increase wear.
NOT PLANNING FOR FUTURE REQUIREMENTS
Many users add extra tools or equipment over time. Choosing a generator with reasonable spare capacity provides useful flexibility.
Still unsure which generator is best for your equipment? Browse our complete collection to compare specifications, outputs and applications. View All Generators & Inverter Generators →
FREQUENTLY ASKED QUESTIONS
WAHT SIZE GENERATOR DO I NEED FOR POWER TOOLS?
For one or two common DIY power tools, a generator between 3kW and 5kW is often suitable. Larger construction equipment or multiple tools operating together may require significantly more output.
WHAT SIZE GENERATOR DO I NEED FOR HOME BACKUP?
This depends on the appliances you intend to power. Essential lighting, refrigeration, broadband equipment and heating controls generally require less capacity than electric cooking or whole-house backup systems.
CAN A GENERATOR BE TOO LARGE?
Yes. Oversized generators cost more to buy, consume more fuel under some operating conditions and may be less practical to transport. Selecting a generator matched to your expected load usually provides the best balance between performance and efficiency.
IS A LARGER GENERATOR SAFER?
Not necessarily. Safety depends on correct installation, proper ventilation, appropriate extension leads, overload protection and following the manufacturer's operating instructions. A correctly sized generator used according to its instructions is generally the best approach.
CAN I RUN MULTIPLE TOOLS AT ONCE?
Yes, provided the combined running and starting loads remain within the generator's rated capacity.
NEED HELP CHOOSING A GENERATOR?
At LES - Lifting Equipment Store, we supply a carefully selected range of portable generators and inverter generators suitable for construction, industrial, commercial and backup power applications.
Whether you're powering professional tools, supporting remote work or preparing for emergency situations, selecting the correct generator starts with understanding your power requirements.
If you're uncertain which model is right for your application, our team can help you compare generator capacities, understand electrical loads and choose a solution that matches your operational needs.
FIND THE RIGHT GENERATOR FOR YOUR NEEDS
Now that you know how to calculate your running and starting wattage, you're ready to choose a generator with confidence.
At LES - Lifting Equipment Store, we stock a wide range of portable generators, inverter generators and site-ready power solutions suitable for DIY projects, construction sites, emergency backup and commercial use.
