Why Generator Sizing Matters More Than You Think

Getting your generator size wrong is expensive. Too small and you’ll trip it every time the kettle boils. Too large and you’ve wasted thousands on capacity you’ll never use—plus you’ll burn more fuel for the privilege.

I’ve seen businesses buy 50kW units when 20kW would’ve done the job. That’s around £15,000 down the drain before they’ve even paid for installation.

Here’s the good news: sizing a generator isn’t rocket science. You just need to understand your actual load, not what you think you need.

Understanding Your Power Requirements

The Three Load Types You Need to Know

Your generator doesn’t just power things—it starts them. And that’s where most people get caught out.

Running Load (Continuous Power) This is what your equipment draws during normal operation. Your fridge humming away, lights on, computer running. Simple enough.

Starting Load (Surge Power) This is the beast. Motors need 2-3 times their running power just to get going. Your 1kW fridge might pull 3kW for two seconds at startup. That two seconds will kill an undersized generator.

Peak Load When everything that could possibly start does start—at the same time. It won’t happen often, but your generator needs to handle it without crying.

The Simple Home Load Calculation Method

Let’s work through this step by step. Grab a pen.

Step 1: List Your Essential Appliances

Walk around your property and write down everything you need to run during a power cut. Not everything you own—what you actually need.

Typical UK home essentials:

• Boiler (150-250W)
• Fridge (150-300W running, 450-900W starting)
• Freezer (100-200W running, 300-600W starting)
• Lighting (200-400W for LED, more for halogens)
• WiFi router (20-50W)
• TV and media (200-400W)
• Kettle (2-3kW if you absolutely must)

Step 2: Apply the Starting Load Multiplier

For anything with a motor or compressor, multiply the running wattage by 3. That’s your starting load. You don’t add this to the total—it’s the peak your generator must handle when that specific item starts.

Step 3: Calculate Your Total Load

Add up all the running loads. Then identify your highest single starting load. Your generator needs to handle the total running load PLUS that highest starting surge.

Here’s a worked example for a typical 3-bed UK semi:

Required capacity: 1,080W + 600W = 1,680W = 2kW minimum

I’d recommend a 3-4kW unit for this setup. Why? Because generators run most efficiently at 60-80% load, and you want headroom for the kettle.

SME Load Calculation: It Gets More Complex

Business power needs vary wildly. A corner shop and a joiners’ workshop aren’t even playing the same game.

Office Environment (Typical Small Office)

You’re looking at:

• Computers and monitors (200-400W each)
• Server/NAS (300-500W)
• Lighting (500-1000W)
• Air conditioning (1-3kW per unit)
• Printers (300-500W)
• Kettle and fridge (add 3kW if people are involved)

A 10-person office typically needs 5-8kW continuous power with 10-12kW surge capacity.

Light Industrial/Workshop

Now we’re talking serious kit:

• Compressors (1.5-7.5kW)
• Welders (2-5kW)
• Machine tools (2-10kW each)
• Extraction systems (1-3kW)
• Lighting and office (2-3kW)

You’ll want a three-phase supply for anything over 15kW. More on that in our single-phase vs three-phase guide.

The 80% Rule for Commercial Installations

Never run a commercial generator above 80% of its rated capacity continuously. A 20kW generator should only supply 16kW max. This gives you:

1. Longer engine life
2. Cooler running temperatures
3. Surge capacity when you need it
4. Compliance with most manufacturer warranties

Common Sizing Mistakes (And How to Avoid Them)

Mistake 1: Forgetting About Power Factor

Motors don’t just draw watts—they need reactive power too. That’s power factor. Most appliances are 0.8-0.9 PF. Industrial motors can be worse.

If you’re running lots of motors, add 20% to your calculated load. Just trust me on this one.

Mistake 2: The “Start Everything at Once” Panic

You don’t need to start your fridge, freezer, and boiler simultaneously. Stagger them. Wait 30 seconds between starts. Your generator will thank you.

That said, your generator MUST handle the largest single motor starting whilst everything else runs. That’s non-negotiable.

Mistake 3: Ignoring Future Growth

Planning to add a hot tub? Workshop equipment? Another fridge in the garage? Buy the bigger generator now. Upgrading later costs double.

Fuel Type Affects Your Decision

Different fuels deliver different performance characteristics.

Petrol Generators Perfect for smaller loads (up to 7kW). Our Honda and Yamaha ranges excel here. They’re portable, start easily, and don’t mind sitting idle.

Diesel Generators The commercial workhorse. More efficient for continuous running, better for larger loads (10kW+). Check our diesel generator range for heavy-duty options from Cummins and FG Wilson.

Quick Reference Sizing Chart

Getting Professional Advice

Still unsure? That’s fine. Undersizing costs you in downtime. Oversizing costs you in capital and running costs.

For standby installations, check our standby generator range with models from CAT, SDMO, and Pramac.

Leisure users heading to campsites or onto boats should explore our leisure generator options—they’re sized specifically for caravan and marine applications.

The right generator isn’t the biggest one. It’s the one that matches your actual load with enough headroom for real-world operation. Calculate honestly, add 20-30% for safety, and you’ll be sorted.