How to Choose the Right Generator Size for Your Needs: A Complete Guide

By Finning Power Rental

At Finning Power Rental, we understand that choosing the right generator size is critical - whether you're powering a construction site, hosting an event, or preparing for an emergency. As part of Finning, the world’s largest Caterpillar dealer, we’re here to help you make the right choice with confidence.

Why Generator Sizing Matters

Selecting the correct generator size ensures:

Reliable power without overloads or failures
Fuel efficiency and reduced running costs
Longer equipment lifespan
Compliance with UK and EU safety, emissions, and operational standards

Whether you need diesel generator hire, event power hire, or emergency generator hire, getting the size right is the first step to a successful project.

Step 1: Calculate Your Power Requirements

Before arranging your generator hire, list all the equipment or appliances you plan to power. For each item, identify: 

Starting (surge) wattage – the power needed to start the equipment
Running (continuous) wattage – the power needed to keep it running

This is especially important for construction site generator hire, where heavy-duty tools often require high starting loads. If you’re not sure, give your local Finning Power Rental branch a call and we’ll be happy to assist.

Step 2: Use Caterpillar’s Generator Sizing Tools

We recommend using Caterpillar’s trusted tools to size your generator accurately: 

Three Step Sizer – Ideal for smaller or straightforward projects: https://power.cat.com/simple-sizer?language=en_GB
SpecSizer – Designed for larger or more complex applications: https://specsizer.cat.com/

These tools are perfect whether you're looking for standby generator rental, mobile generator hire, or temporary power solutions.

Step 3: Add a Safety Margin

We recommend including a 10–20% buffer above your total load to accommodate future expansion and load fluctuations, so if you’re doing your own calculations, be sure to add this on. This ensures your generator hire remains reliable even under variable conditions.

Step 4: Understand Your Equipment and Site Needs

Beyond the generator, you’ll need the right setup to ensure safe and efficient operation. At Finning Power Rental, we supply everything required, including: 

Cables and distribution boards for safe power delivery
Bunded fuel tanks and fuel management solutions
Delivery, installation, and commissioning 
We’ll help you assess your site and provide a complete power package—so you’re fully covered from start to finish.

If you're searching for generator hire near me, Finning Power Rental offers nationwide coverage, with branches in Cannock, Aberdeen, and Dublin.

Step 5: Speak to a Power Expert

Even with the best tools, expert advice is invaluable. Our team at Finning Power Rental can:

Assess your site or event
Recommend the most suitable generator
Ensure compliance with UK and ROI regulations and safety standards

We specialise in temporary power solutions for every industry, from construction and utilities to healthcare and events.

Final Tips

Avoid running a generator at full capacity for extended periods
Use Cat’s tools to simulate real-world load conditions
Always follow safety and maintenance guidelines

Need Help Sizing Your Generator? 

At Finning Power Rental, we offer expert advice and a wide range of Cat generators for hire across the UK and Ireland. Whether you need event power hire, construction site generator hire, or emergency backup, we’ve got you covered. 

Need help choosing the right generator?

Get in touch with your local branch today.