What Size Diesel Generator Would You Need to Run a House?

If you’re thinking about getting a diesel generator for your home, the first question is probably “how big does it need to be?” The answer depends on what you want to power and how much electricity your household actually uses. Getting this right is crucial, too small and you’ll overload the generator, too big and you’ll waste money on fuel and upfront costs.

Let’s break down how to work out what size generator you actually need for your home, from covering just the essentials to running everything as normal.

Understanding Your Home’s Power Needs

The average UK household uses around 2,700 kWh of electricity per year, which works out to about 7.5 kWh per day. But that’s just an average; your actual needs could be quite different depending on your house size, how many people live there, and what appliances you use regularly.

Here’s how power consumption typically breaks down by house size:

• Small homes/flats: Around 1,800 kWh per year (5 kWh daily)
• Medium homes (3-4 bedrooms): Around 2,900 kWh per year (8 kWh daily)
• Large homes (5+ bedrooms): Around 4,300 kWh per year (12 kWh daily)

But daily averages don’t tell the whole story. Your actual power demand at any given moment could be much higher, especially when multiple appliances are running at once. That’s what determines the size generator you need.

Essential vs Full House Power

Before sizing a generator, you need to decide what you actually want to power during an outage.

Essential Power Only

Most people don’t need to run everything during a power cut. Essential items might include:

• Refrigerator and freezer (to keep food safe)
• Basic lighting in key rooms
• Heating system (boiler and pumps)
• Phone chargers and an internet router
• Maybe one TV

For essentials only, you’re looking at roughly 3-7 kW of power demand, so a generator in the 5-10 kVA range would typically cover this.

Partial House Power

This adds some comfort items like:

• Washing machine
• Microwave
• More lighting throughout the house
• Desktop computers
• Maybe a small electric heater

You’d be looking at 8-15 kW of demand, requiring a generator in the 10-20 kVA range.

Full House Power

Running everything as normal, including:

• Electric shower
• Tumble dryer
• Air conditioning (if you have it)
• Electric cooker/hob
• All lights and sockets
• Hot water immersion heater

This could easily require 20-30 kW or more, meaning you’d need a generator of 25-40 kVA capacity.

Working Out Your Specific Needs

Rather than guessing, it’s worth calculating your actual requirements. Here’s how to do it properly:

Step 1: List Your Essential Appliances Write down everything you absolutely need to keep running, then add anything you’d really prefer to have working.

Step 2: Find the Power Ratings. Check the power consumption of each appliance. This is usually on a label on the device or in the manual, listed in watts (W) or kilowatts (kW).

Step 3: Account for Starting Power. Many appliances need extra power to start up, especially anything with a motor, like fridges, washing machines, or power tools. This “surge” power can be 2-3 times the running power, but only lasts for a few seconds.

Step 4: Add It All Up. Add the running power of everything you want to operate simultaneously. Then check that your generator can also handle the highest starting power requirement.

Here’s a table of typical power requirements for common household items:

Converting Between kW and kVA

Here’s where it gets slightly technical. Household appliances are rated in kilowatts (kW), which is the actual power they use. But generators are usually rated in kilo-volt-amperes (kVA), which accounts for electrical inefficiencies.

The relationship between them depends on something called power factor, but for most household loads, you can use this rough conversion:

• kW × 1.25 = kVA (or divide kVA by 0.8 to get kW)

So if your appliances need 10 kW of power, you’d want a generator rated at about 12-13 kVA to handle it comfortably.

Practical Generator Sizes for Different Needs

Based on typical UK homes, here’s what size generator you’d need:

5-7 kVA Generator (4-6 kW usable power)

• Small flat or house
• Essential items only: fridge, freezer, lighting, heating controls
• Perfect for keeping food safe and basic comfort during outages

10-15 kVA Generator (8-12 kW usable power)

• Medium-sized house
• Most appliances except high-power items like electric showers
• Good compromise between capability and running costs
• Generators like the Perkins PTG-T550S fall into the higher end of this range and can handle substantial household loads

20-30 kVA Generator (16-24 kW usable power)

• Larger house or full power requirements
• Can handle most household demands including some high-power appliances
• Suitable for running a business from home or a workshop equipment

30+ kVA Generator (24+ kW usable power)

• Large house with high electrical demands
• Multiple high-power appliances running simultaneously
• Commercial or industrial applications

Why Diesel Makes Sense for House Generators

Diesel generators have several clear advantages over petrol alternatives when it comes to home backup power. They’re typically 20-40% more fuel-efficient, which really matters when you’re running the generator for days during extended outages. Diesel engines are also built for continuous operation and tend to be more reliable than petrol engines, especially when you need them to run for long periods.

From a practical standpoint, diesel fuel stores much better than petrol and is less volatile, making it safer to keep larger quantities on your property. You’ll also get more power per unit size with diesel generators compared to petrol equivalents, which is useful when space is limited.

The main downsides are higher upfront costs and slightly more noise, but for serious backup power where reliability and efficiency matter, diesel is usually the better choice.

Installation and Safety Considerations

Remember that generators need proper installation to be safe and legal:

• Location: Must be outside, at least 1 metre from any opening (doors, windows, vents) and on a level, well-ventilated area.
• Electrical Connection: Needs a proper changeover switch installed by a qualified electrician to prevent back-feeding into the grid.
• Fuel Storage: Diesel storage has regulations—typically up to 2,500 litres for domestic use, but check local authority requirements.
• Noise: Consider neighbours—most residential areas have noise limits, especially at night.
• Maintenance: Regular servicing is essential for reliability when you need it most.

Don’t Oversize Your Generator

It’s tempting to buy a bigger generator “just in case,” but oversizing actually creates more problems than it solves. Large generators running light loads are incredibly inefficient with fuel; you’ll burn much more diesel than necessary just to power a few appliances. They also cost significantly more to buy, install, and maintain, so you’re paying extra money upfront and ongoing for capacity you don’t use.

Perhaps most importantly, diesel engines need proper loading to run cleanly and reliably. If you’re only using 20-30% of a generator’s capacity, it can actually cause mechanical problems over time. A properly sized generator running at 70-80% capacity will be more efficient, quieter, and more reliable than an oversized one struggling with light loads.

Read: Common Generator Failures and How to Prevent Them

Find Your Perfect Generator Solution

Ready to secure reliable backup power for your home? Powertech Generators specialises in matching diesel generators to your exact power requirements. We stock a full range of new and used generators to suit every budget, from compact units for essential loads to larger systems for whole-house power.

Our team helps you get the sizing right, so you’re not paying for capacity you don’t need or ending up with a generator that can’t handle your requirements.

Contact Powertech Generators today for expert advice on choosing the perfect diesel generator for your home. With our comprehensive range and professional guidance, you’ll have reliable power when you need it most.