When the power goes out or your tools grind to a halt mid-job, having the right generator can be a lifesaver. But here’s the thing: choosing the wrong size generator can end up costing you more than the outage itself.
If you’re preparing your home for unexpected power cuts or ensuring a busy construction site stays on track, getting the generator size right is key. Too small, and you’ll overload the system. Too big, and you’ll waste fuel and money.
That’s why choosing the right generator isn’t just a technical decision, it’s a practical one. A generator that’s too small won’t handle your load and could cause breakers to trip or equipment to fail. A unit that’s too large might run inefficiently, guzzling more fuel than needed and driving up your running costs.
In this guide, we’ll walk you through exactly how to choose the right generator size for home and site use, so you’re always powered up, safely and efficiently.
This guide will help you:
- Understand what “generator size” really means
- Learn the difference between running watts and starting watts
- Identify common mistakes in generator sizing
- Choose the ideal generator for your home or site
What Does “Generator Size” Actually Mean?
When we talk about generator size, we’re referring to its electrical output capacity, measured in watts (W), kilowatts (kW), or kilovolt-amperes (kVA). Think of it like a water tap; it’s the maximum flow rate the generator can deliver to your appliances and tools.
But here’s where it gets interesting: generators have two key power ratings. Running watts (or continuous power) is what the generator can produce steadily over long periods. Starting watts (or peak power) is the short burst of extra power needed when motors kick in, like when your fridge compressor starts up or you fire up a circular saw.
This distinction is crucial because many people only consider running watts and end up scratching their heads when their generator trips out every time they start a power tool.
Key Risks of Under or Over-Sizing
Get your generator sizing wrong, and you’re asking for trouble. Undersizing leads to overloaded circuits, which means constant tripping and potential damage to your expensive tools or appliances. Imagine your generator cutting out just as you’re making a critical cut with an angle grinder, not ideal for safety or productivity.
Oversizing might seem like the safe option, but it’s wasteful and expensive. A 10kW generator sipping fuel to power a single floodlight is like using a Ferrari for the school run; it works, but it’s hardly efficient.
You’ll burn through diesel unnecessarily and pay higher hire rates for capacity you’ll never use.
Step-by-Step Generator Sizing Process
Step 1: Assess Your Total Power Requirements
Start by making a comprehensive list of everything you need to power. For homes, this includes essential appliances like fridges, boilers, lights, and maybe a kettle for that all-important brew. On construction sites, list every tool, light, and piece of equipment you’ll use.
Next, determine both running watts and starting watts for each item. This information is usually found on the appliance nameplate or in the manual. If you can’t find it, a quick online search for your specific model will usually do the trick.
Sample Power Usage Tables
Home Use Appliances
| Appliance | Running Watts | Starting Watts |
| Fridge/Freezer | 700W | 2200W |
| Boiler Pump | 250W | 500W |
| Kettle | 3000W | 3000W |
| WiFi Router + TV | 150W | 200W |
Site Tools
| Tool | Running Watts | Starting Watts |
| Circular Saw | 1400W | 2300W |
| Cement Mixer | 700W | 1500W |
| Grinder | 2000W | 3000W |
| Floodlight | 500W | 500W |
Step 2: Include Starting Loads & Peak Surge Wattage
Here’s where many people trip up. When a motor starts, it draws significantly more power for a few seconds than it does while running. A cement mixer might only need 700W to keep turning, but it needs 1500W to get going in the first place.
The key is to add the highest single starting load to your total running watts. So if you’re running a 700W cement mixer, 500W floodlight, and 150W radio simultaneously, that’s 1350W running load. But when the mixer starts up, you need 1350W (other running loads) plus 1500W (mixer starting load) = 2850W total.
Don’t add up all the starting watts; that would assume everything starts simultaneously, which rarely happens in practice.
Step 3: Add a 20–25% Safety Margin
Never run a generator at 100% capacity continuously. It’s like redlining your car engine, technically possible, but you’ll wear it out quickly and risk breakdowns at the worst possible moment.
Adding a 20-25% safety buffer serves multiple purposes: it allows for unexpected additions (like plugging in a phone charger or adding an extra light), provides surge protection, and ensures your generator runs efficiently within its optimal range.
The calculation is simple: multiply your total calculated load by 1.25. So our 2850W example becomes 2850W × 1.25 = 3563W, meaning you’d want at least a 4kW generator.
Step 4: Match Generator Type to Your Use Case
Generator for Home Use
For domestic backup power, you’ve got two main options: portable generators and standby generators. Portable units are versatile and cost-effective for occasional use, whilst standby generators automatically kick in during outages but require professional installation.
Consider whether you need emergency essentials only (lights, fridge, heating) or whole-home coverage. Most UK homes can maintain essentials with a 3-5kW generator, whilst whole-home backup might require 7-10kW or more.
Petrol generators are lighter and cheaper to buy, making them ideal for occasional use. LPG models offer cleaner burning and longer storage life, perfect if you’re keeping the generator for emergency use only.
Generator for Site Projects
Construction sites have different demands entirely. Diesel generators are the workhorses here, offering superior fuel economy and durability for long shifts. They’re built to handle the dust, weather, and constant use that site work demands.
Look for features like weatherproof enclosures, towable chassis for easy repositioning, and robust construction that can handle the inevitable knocks and scrapes.
Many UK sites also require specific safety certifications, so check compliance with current regulations.
Step 5: Single-Phase vs Three-Phase Power
Most UK homes use single-phase 230V power, which is fine for domestic appliances and small tools. However, larger machinery, industrial compressors, and heavy-duty site equipment often require three-phase 400V power.
If you’re powering anything with a three-phase motor, you’ll need a three-phase generator; there’s no way around it. Single-phase generators simply can’t provide the power configuration these machines need.
Construction sites typically use three-phase power, whilst homes almost always use single-phase. When in doubt, check your main electrical panel or ask an electrician.
Step 6: Fuel Type, Runtime, and Efficiency
Petrol generators start easily in cold weather and are readily available, but they’re less fuel-efficient and require more frequent refuelling. Diesel generators offer better fuel economy and longer runtime, making them ideal for extended use despite being harder to start in winter.
LPG generators provide the cleanest burning option with minimal maintenance, plus LPG doesn’t deteriorate in storage like petrol does.
Modern generators often include eco-modes that adjust engine speed to match demand, significantly improving fuel efficiency during light loads. Low-oil shutdown protection prevents expensive engine damage if oil levels drop.
Common Generator Sizing Mistakes to Avoid
1. Undersizing for Startup Loads
The biggest mistake? Forgetting about starting watts entirely. You size for running loads, then wonder why your generator trips every time you start the compressor. Always account for motor starting requirements.
2. Ignoring Simultaneous Use
Another common error is calculating each tool separately, but forgetting that you might run several simultaneously. That angle grinder, site radio, and LED floodlight might seem modest individually, but together they can push your generator to its limits.
3. Forgetting to Plan for Growth or Expansion
Projects evolve, and so do power needs. That small extension might grow into a major renovation. Site work often expands beyond the initial scope. Build in capacity for reasonable growth rather than sizing to the absolute minimum.
4. Misjudging Fuel Consumption
Running an oversized generator for light loads is expensive and inefficient. A 10kW generator powering a single 500W floodlight will guzzle fuel unnecessarily. Match your generator size to typical loads, not worst-case scenarios.
Power Up with the Right Generator with Hire It Direct
Picking the wrong generator can do more harm than good, think damaged equipment, wasted fuel, and costly project delays.
So why take that risk?
At Hire It Direct, we take the guesswork out of generator sizing. Whether you need a reliable generator for home use, power for a busy construction site, or a short-term hire for an event, we’ve got the right solution, backed by expert advice and outstanding customer service.
We offer a wide range of generator options, including portable, diesel, petrol, and standby models, with flexible rental terms to suit daily, weekly, or long-term needs. Plus, we provide full tool and equipment hire to support every aspect of your project.
Our team is always on hand to help you choose the perfect fit. Call us today on 020 8427 6604 to speak directly with one of our friendly experts, or fill out our quick enquiry form, and we’ll be in touch to help you get powered up, safely and efficiently.
FAQs
1. What size generator do I need for my house UK?
Most UK homes need a 3-5kW generator for essential backup power (lights, fridge, heating, WiFi) or 7-10kW for whole-home coverage. Calculate your specific needs using running watts plus the highest starting load, then add 25% safety margin.
2. How do I calculate my generator power needs?
List all appliances/tools, find their running and starting watts, add up running watts for simultaneous use, add the single highest starting load, then multiply by 1.25 for a safety margin.
3. Can I use a portable generator to power a house?
Yes, but with limitations. Portable generators work well for essential circuits but require proper transfer switches for safety. Never connect directly to your mains supply without appropriate switching equipment.
4. Do I need a three-phase generator for site equipment?
Large site machinery, industrial compressors, and heavy-duty tools often require three-phase power. Check your equipment specifications; single-phase generators cannot power three-phase equipment.
5. Is a 3kW generator enough for home backup?
A 3kW generator covers essential items like a fridge, some lights, a boiler pump, and electronics. However, high-power appliances like kettles, electric heaters, or washing machines will require a larger capacity.