What Size Generator Do I Need? kW, kVA and Surge Watts Explained for Farm and Workshop Use
Buying the wrong size generator is one of the most expensive mistakes in farm electrical planning. Buy too small and it won't start your compressor. Buy too large and you've wasted money. This guide covers exactly how to size a generator correctly — what kW vs kVA means, why surge watts are what really matters, and what size you need for common farm and workshop loads.
Free Tool
Generator Size Calculator
Add your equipment, account for surge loads, and get a kVA recommendation instantly — with diesel vs petrol fuel estimate and PDF download.
The Single Most Important Concept: Surge Watts
The number that determines your generator size is not the running wattage — it is the startup surge wattage.
Every piece of equipment with an electric motor — compressors, water pumps, angle grinders, feed augers, milk vacuum pumps — draws two to three times its normal running current for a fraction of a second when it starts. This startup surge is what most undersized generators fail to handle.
| Equipment | Running Watts | Surge Factor | Peak Surge |
|---|---|---|---|
| Air compressor 2.2 kW | 2,200 W | ×3.0 | 6,600 W |
| Water pump 1.5 kW | 1,500 W | ×3.0 | 4,500 W |
| Milk vacuum pump 2.2 kW | 2,200 W | ×2.5 | 5,500 W |
| Angle grinder 2.3 kW | 2,300 W | ×2.0 | 4,600 W |
| MIG welder 140A | 4,000 W | ×1.2 | 4,800 W |
A 2.2 kW compressor needs a generator capable of delivering nearly 7 kW for the instant it starts. If your generator cannot supply that surge, the motor will hum, stall and potentially burn out — or the generator will trip its overload protection and shut down.
Motor loads always surge
Compressors, pumps, grinders, augers, milk plant — any equipment with an electric motor has a startup surge. The heavier the motor and the longer it has been standing, the higher the surge tends to be.
Resistive loads have no surge
Electric heaters, incandescent lights and kettles draw the same wattage the moment they switch on. They do not surge and can be sized on running watts alone.
What Is the Difference Between kW and kVA?
kW — Real Power
What actually runs your equipment
Kilowatts are the actual useful work the generator does. This is the number that matters for running your loads.
kVA — Apparent Power
What the generator sees as total load
Kilovolt-amperes include reactive power from motors and transformers. Most generators are rated in kVA.
The relationship
kW = kVA × Power Factor (0.8)
For most farm and workshop generators, the power factor is 0.8. This means a 10 kVA generator produces only 8 kW of usable power. A 5 kVA generator does not produce 5 kW — it produces 4 kW. If you size on kVA alone you will systematically undersize every generator you buy.
| Generator Rating | Actual kW Output (PF 0.8) |
|---|---|
| 5 kVA | 4 kW |
| 6.5 kVA | 5.2 kW |
| 8 kVA | 6.4 kW |
| 10 kVA | 8 kW |
| 13 kVA | 10.4 kW |
| 15 kVA | 12 kW |
| 20 kVA | 16 kW |
When a supplier tells you a machine is "5 kVA," always ask for the kW output at 0.8 power factor. Reputable suppliers will state this clearly in the specification sheet.
What Is Power Factor in Plain English?
Power factor is a measure of how efficiently the electrical load uses the power supplied to it.
Resistive loads (PF ≈ 1.0)
Electric heaters, incandescent bulbs and kettles use power very efficiently. All the power supplied does useful work. Power factor close to 1.0.
Inductive loads (PF 0.6–0.9)
Motors, transformers and fluorescent lighting create a phase difference between voltage and current. Some power goes into maintaining the magnetic field rather than doing useful work.
The practical implication is simple: always size your generator on kW output, not kVA rating, and use 0.8 as your assumed power factor unless the manufacturer specifies otherwise. Inverter generators typically have a better power factor (some approach 0.95–1.0), which is one reason they can be more compact for a given kW output.
Running Watts vs Starting Watts: How to Calculate Generator Size
List all equipment you might run simultaneously
Be realistic. On a farm this might be the milk vacuum pump, bulk tank cooler and workshop lighting. In a workshop, a compressor, welder and grinder.
Find the running wattage of each item
Check the equipment nameplate — on the motor housing or back panel. It will state input power in watts (W) or kilowatts (kW). If it states amps and volts, multiply together and multiply by 0.8 for single-phase motors.
Apply surge factors to motor loads
| Load Type | Surge Factor |
|---|---|
| Pumps and compressors | × 3.0 |
| Large motors (augers, conveyors) | × 2.5 |
| Small motors (grinders, drills) | × 2.0 |
| Welders | × 1.2 |
| Resistive loads (heaters, lights) | × 1.0 |
Calculate peak surge load
Add up all running loads, then subtract the running watts of your largest motor and replace it with the surge watts of that same motor. This gives the worst-case scenario — when everything is already running and the largest motor kicks in.
Add 20% headroom
Multiply your peak surge figure by 1.2. Running a generator at or near maximum rating continuously accelerates wear, reduces voltage stability and shortens service life.
Convert to kVA
Divide by 1,000 to get kW, then divide by 0.8 to get kVA. Round up to the next standard generator size.
Worked Example — Farm Workshop
Air compressor 2.2 kW + angle grinder 750W + workshop lighting 300W
| Item | Running W | Surge W |
|---|---|---|
| Compressor 2.2 kW | 2,200 W | 6,600 W |
| Angle grinder 750W | 750 W | 1,500 W |
| Lighting 300W | 300 W | 300 W |
| Total running | 3,250 W | — |
Worst-case surge: 3,250 W − 2,200 W + 6,600 W = 7,650 W peak surge
With 20% headroom: 7,650 × 1.2 = 9,180 W = 9.18 kW
Divide by 0.8 power factor: 9.18 ÷ 0.8 = 11.5 kVA minimum
Recommendation: 13 kVA generator
The Generator Size Calculator does all of this automatically — enter your equipment and it works out the recommendation.
Open →What Size Generator Do I Need For...
A Farm Milking System
A typical 100-cow dairy milking system includes a 2.2 kW vacuum pump, a 3 kW bulk tank compressor and milking parlour lighting. The bulk tank compressor is the critical load — it surges at 2.5× its running load.
For larger herds with multiple vacuum pumps and automatic cluster removers, allow 15–20 kVA. Always check with your milking equipment supplier.
A Farm Workshop With a Welder and Compressor
A MIG welder at 140A (4 kW input) plus a 2.2 kW compressor plus lighting is the most common workshop combination. The compressor surge (6,600 W) dominates. If only welding without the compressor, a 6.5 kVA machine handles a 140A MIG comfortably.
A Single Water Pump for Farm Emergency Use
For a 1.5 kW pump with a surge of 4,500 W, plus a safety margin. A petrol generator is adequate for occasional emergency water pump use.
For regular use, step up to a 6.5 kVA diesel.
A Pressure Washer
A 2.5 kW pressure washer has a surge factor of approximately 2.0, giving a peak of 5,000 W. With 20% headroom that is 6 kW, or 7.5 kVA. A smaller 5 kVA machine may start it but will run close to its limit.
A Grain Store or Drying Fan
Grain drying fans are among the highest-load items on a farm. A 5.5 kW fan motor surges at 2.5×, giving a peak of 13,750 W. Multiple fans, conveyors and augers multiply this further.
Grain drying operations often require 25–30 kVA or larger three-phase machines. Consult your grain equipment supplier.
Diesel vs Petrol Generator — Which for Farm Use?
Choose Diesel If:
- ✓Regular use (more than 100 hours/year)
- ✓You need 6 kVA or larger
- ✓You have diesel on the farm already
- ✓Maximum reliability and service life
- ✓Extended continuous running periods
Diesel is approximately 30% more fuel-efficient than equivalent petrol. A good diesel generator on a working farm will run 15,000–20,000 hours with proper servicing.
Choose Petrol If:
- ✓Occasional emergency backup only (< 50 hrs/year)
- ✓You need something lightweight and portable
- ✓Your loads are 5 kVA or under
- ✓Initial purchase cost is the primary concern
Petrol generators are cheaper to buy and easier to move. Fine for infrequent backup but become expensive and maintenance-intensive if used regularly.
Inverter Generators
Inverter generators produce cleaner, more stable power through electronic frequency regulation. They are quieter, more fuel-efficient at partial load and better suited to sensitive electronics — laptops, modern control systems, some welders. For general farm use with motors and compressors a conventional generator is fine, but for modern variable-speed drives or electronic controls, an inverter generator is worth the extra cost.
Generator Safety on Farm — Critical Points
Never run a generator indoors or in an enclosed space
Carbon monoxide poisoning from generator exhaust is a genuine and regular cause of death. Always site generators outdoors with exhaust pointing away from doorways and windows.
Never connect to fixed farm wiring without an approved transfer switch
Backfeeding live power into the mains can kill electricity board engineers working on the distribution network. This is a legal requirement. An automatic transfer switch (ATS) or manual changeover switch must be installed by a qualified electrician.
Earth your generator properly
Portable generators must be earthed to the manufacturer's specification. Failure to earth correctly creates electrocution risk, particularly in wet farm environments.
Store fuel safely
Store generator fuel in approved containers away from heat sources and equipment with open flames. Keep a fire extinguisher accessible when refuelling.
Service and run regularly
Run your generator under load for at least 30 minutes every three months to keep fuel fresh, seals lubricated and batteries charged. A generator that won't start in an emergency is no generator at all.
Quick Reference — Generator Size by Application
| Application | Typical Load | Recommended Generator |
|---|---|---|
| Emergency lighting only | 500 W | 1–2 kVA petrol |
| Single water pump (1.5 kW) | 4,500 W surge | 5–6.5 kVA |
| Workshop: welder only (140A MIG) | 4,800 W surge | 6.5–8 kVA |
| Workshop: compressor + tools | 8,000–12,000 W surge | 13–15 kVA diesel |
| Milking system (100-cow) | 10,000–15,000 W surge | 15–20 kVA diesel |
| Grain handling + drying fan | 15,000–25,000 W surge | 20–30 kVA diesel |
| Whole farm backup | Variable | 25–50 kVA diesel |
Diesel Generator 6–10 kVA
Reliable diesel generator for regular farm and workshop use.
Inverter Generator 2–3 kVA
Quiet inverter generator — cleaner power for welders and sensitive electronics.
Automatic Transfer Switch
Approved changeover switch for connecting a generator to fixed farm wiring.
Affiliate disclosure: AgriOps participates in the Amazon EU Associates Programme. Links are affiliate links — we may earn a small commission at no extra cost to you.
Frequently Asked Questions
Related Tools
All wattage figures are typical values for guidance. Always check your specific equipment nameplate for actual input power. Generator installation must be carried out by a qualified electrician. Never connect a generator to fixed wiring without an approved isolation transfer switch. This guide does not constitute electrical engineering advice.