What Size Generator Do I Need? Complete UK Guide
Choosing the wrong size generator is one of the most expensive mistakes you can make. Too small and it overloads, trips out, or damages your equipment. Too big and you waste thousands on a unit that burns fuel inefficiently and never runs at its proper load.
This guide cuts through the confusion. Whether you need a generator for your house, your farm, a building site, or a commercial building, we'll walk you through exactly how to work out the right size in kVA.
Quick Answer: What Size Generator Do I Need?
If you just want a fast answer, here's what most UK customers end up buying based on their application:
| Application | Typical Size | Phase | What It Powers |
|---|---|---|---|
| Small house / flat | 5–8 kVA | Single | Lights, fridge, boiler, TV, charging |
| Medium house | 10–15 kVA | Single | All above plus oven, washing machine, power tools |
| Large house / multiple circuits | 15–25 kVA | Single | Whole house backup including electric heating |
| Small farm / rural property | 15–30 kVA | Single / 3 | Livestock housing, lighting, water pumps, fridges |
| Large farm / dairy / poultry | 30–100 kVA | 3 Phase | Milking parlour, grain dryers, feed systems, workshops |
| Small building site | 20–40 kVA | 3 Phase | Power tools, site cabins, lighting, small plant |
| Large construction site | 50–150 kVA | 3 Phase | Cranes, hoists, concrete pumps, welfare units |
| Small business / shop / office | 15–30 kVA | Single / 3 | Computers, tills, lighting, HVAC, fridges |
| Commercial / industrial | 50–500 kVA | 3 Phase | Production lines, large HVAC, heavy machinery |
Not sure where you fit? Keep reading and we'll help you calculate your exact requirements.
How to Calculate What Size Generator You Need
Working out the right generator size comes down to three things: what equipment you need to run, how much power each item uses, and whether anything has a high starting current that temporarily spikes the load.
Step 1: List Everything You Need to Power
Write down every piece of equipment or appliance you want the generator to run at the same time. Be realistic here — you probably don't need to run every single thing simultaneously. Focus on the essentials that must stay on during a power outage or that you need on site at any given moment.
Step 2: Add Up the Running Watts (or kW)
Every electrical device has a wattage rating, usually on a label or in the manual. Add up the running wattage of everything on your list. This gives you your continuous load in watts or kilowatts.
Common UK Appliance and Equipment Wattages
| Item | Running Watts | Starting Watts |
|---|---|---|
| LED Lighting (10 bulbs) | 100W | 100W |
| Fridge / Freezer | 150W | 600W |
| Gas Boiler (pump + controls) | 200W | 400W |
| Television | 100W | 100W |
| Microwave | 1,000W | 1,500W |
| Electric Oven | 2,500W | 2,500W |
| Washing Machine | 500W | 2,000W |
| Kettle | 3,000W | 3,000W |
| Immersion Heater | 3,000W | 3,000W |
| Circular Saw (1.4kW) | 1,400W | 2,800W |
| Cement Mixer | 800W | 2,400W |
| Welder (MIG 180A) | 4,000W | 6,000W |
| Air Compressor (3HP) | 2,200W | 6,600W |
| Submersible Pump (1HP) | 750W | 2,250W |
| Portable Heater (Fan) | 3,000W | 3,000W |
| Site Cabin (lights + sockets) | 3,000W | 3,000W |
Note: Starting watts matter. Motors in fridges, pumps, compressors and power tools draw 2–3 times their running wattage for the first few seconds when they kick in. Your generator needs to handle this spike without tripping.
Step 3: Convert Watts to kVA
Generators are rated in kVA (kilovolt-amperes), not kW (kilowatts). The difference is power factor. For most diesel generators the power factor is 0.8, which means:
kVA = kW ÷ 0.8
So if your total running load is 8 kW, you need a minimum of 10 kVA. Simple as that.
Step 4: Add a Safety Margin
Never run a generator at 100% of its rated output continuously. It reduces lifespan, increases fuel consumption, and leaves no headroom for starting surges. The rule of thumb is to run at 70–80% of rated capacity. So take your kVA figure and multiply by 1.25 to give yourself a proper margin.
Required kVA = Total Load (kW) ÷ 0.8 × 1.25
Example: Your equipment totals 16 kW running load. 16 ÷ 0.8 = 20 kVA. 20 × 1.25 = 25 kVA. You need a 25 kVA generator.
Single Phase or Three Phase: Which Do You Need?
This is one of the most important decisions and getting it wrong means the generator simply won't work for your application.
Single phase (230V) is what most UK homes and small businesses run on. If you plug things into standard 3-pin sockets, you're on single phase. Single phase generators typically range from 5 kVA up to about 25 kVA.
Three phase (400V) is used in commercial and industrial settings where heavy machinery, large motors, or high-power equipment is involved. If your premises has a red 3-phase socket or a 3-phase supply from the grid, you need a 3-phase generator. Three phase units start from around 10 kVA and go up to 500 kVA and beyond.
Important: You cannot run single phase equipment directly from a 3-phase generator without a transformer or a generator that has a dedicated single-phase output. Always check before you buy.
Generator Sizing by Application
Generators for Houses and Homes
Most UK homes have a single phase supply. For basic backup power covering lighting, the fridge, your gas boiler, charging devices and a television, a 5–8 kVA generator will do the job. If you want to run heavier loads like an electric oven, washing machine, or electric shower, you're looking at 10–20 kVA. For full whole-house backup including electric heating, go 20–25 kVA.
The key with home generators is that you rarely run everything at once. Work out your essential loads — the things you absolutely cannot be without during a power cut — and size based on those.
Generators for Farms and Agricultural Use
Farms are one of the most common applications we supply generators for, and the sizing varies massively depending on your operation. A small livestock farm with basic lighting, water pumps and electric fencing might get by on 15–20 kVA single phase. A dairy farm with a milking parlour, bulk tank cooling, and feed systems will typically need 30–60 kVA three phase. Large arable operations with grain dryers and heavy workshop equipment can need 80–150 kVA.
For farms, always factor in starting currents from motors. Milking vacuum pumps, compressors, and grain augers all have high inrush currents that can trip an undersized generator.
Generators for Construction Sites
Construction sites almost always need three phase power. A small site with hand tools, a site cabin and basic lighting can work with 20–30 kVA. Medium sites running concrete mixers, small cranes, hoists and multiple welfare cabins will need 50–80 kVA. Large sites with tower cranes, concrete pumps and multiple active work zones typically need 100–200 kVA or more.
On construction sites the biggest mistake is undersizing. Equipment gets added as the project progresses. Always buy bigger than you think you need today because the load almost always grows.
Generators for Businesses and Commercial Premises
Business backup generators range hugely. A small office or shop with computers, lighting and a till system can get by on 10–20 kVA. A restaurant or pub needing to keep fridges, freezers, kitchen equipment and lighting running during a power cut will need 20–40 kVA. Larger commercial operations like warehouses, factories and data centres can need anything from 50 kVA right up to 500 kVA.
For businesses, the calculation is simple: what does a power outage cost you per hour? If the answer is hundreds or thousands of pounds, the generator pays for itself the first time you need it.
Understanding kVA Ratings
Generator sizes are expressed in kVA, which stands for kilovolt-amperes. This is the apparent power output. The usable power in kW is always lower than the kVA rating because of power factor. For diesel generators, the standard power factor is 0.8.
| Generator Size | Usable Power (kW) | Max Amps (230V SP) | Max Amps (400V 3P) |
|---|---|---|---|
| 5 kVA | 4 kW | 22A | – |
| 10 kVA | 8 kW | 43A | 14A |
| 15 kVA | 12 kW | 65A | 22A |
| 20 kVA | 16 kW | 87A | 29A |
| 30 kVA | 24 kW | 130A | 43A |
| 50 kVA | 40 kW | – | 72A |
| 100 kVA | 80 kW | – | 144A |
| 150 kVA | 120 kW | – | 217A |
| 200 kVA | 160 kW | – | 289A |
5 Common Generator Sizing Mistakes
- Ignoring starting currents. Motors in compressors, pumps, and power tools draw 2–3x their running wattage on startup. If you only size for running watts, your generator will trip every time a motor kicks in.
- Buying too small to save money. An overloaded generator burns out faster, uses more fuel under strain, and can damage your equipment. The few hundred pounds you save buying smaller costs you thousands in the long run.
- Confusing kW and kVA. A 10 kVA generator delivers 8 kW, not 10 kW. If you need 10 kW of usable power, you need a 12.5 kVA generator. Always check the kW rating, not just the kVA.
- Getting the phase wrong. Buying a single phase generator when you need three phase, or vice versa, means it simply won't power your equipment. Check your supply before you buy.
- Not planning for future growth. Especially on sites and farms, the load almost always grows over time. Buy one size up from what you think you need today.
Still Not Sure? We'll Help You Size It
If you're still unsure what size generator you need, just get in touch. Tell us what you want to power and we'll recommend the right unit for your application. No hard sell, just honest advice from people who supply generators every day.
Call us: 01254 476 679
WhatsApp: 07484 413 610
Email: info@affordablegenerators.co.uk
We stock diesel generators from 6 kVA to 275 kVA, single phase and three phase, all silent with electric start. UK stock, fast delivery, and the best prices you'll find anywhere.
Related Guides
- Diesel Generator Fuel Consumption Guide by kVA
- Single Phase vs 3 Phase Generator: Which Do You Need?
- Best Generator for Home Backup UK
- Best Generator for a Construction Site UK
- Best Generator for Farm Use UK