Which Diesel Welder or Generator? A Buying Guide

Not sure yet whether you need welding capability or just site power? Start with our Diesel Welder Generators & Site Power Guide.

Before You Buy: Four Questions to Answer

What Size Generator Do I Need?

Sizing is where most buying mistakes happen. An undersized generator trips out or struggles the moment a motor starts. An oversized diesel generator that spends its life on light loads can develop engine problems of its own (see the used buying section below). Aim for a machine that runs comfortably at around half to three-quarters of its rated output for most of the working day.

How to Work Out Your Load

1. List everything that will run at the same time. Take the power rating (in watts or kW) from each tool’s rating plate.
2. Allow for starting surge. Anything with an electric motor (grinders, compressors, pumps, saws) draws two to three times its running power for a moment as it starts. Size for the largest motor starting while everything else is already running.
3. Convert to kVA. Generators are usually rated in kVA, while tools are rated in kW. For a typical site load, kW is roughly 0.8 × kVA, so a 10kVA generator delivers around 8kW of usable power.
4. Add 20–25% headroom. This covers extra tools appearing on site and keeps the engine away from its limit.

What Will Each Size Run?

What Size Generator Do I Need to Run a Welder?

Running an inverter welder from a standalone generator is common on site, but it's the case where getting the generator wrong costs the most. Size and power quality both matter.

Size from the welder's input rating, not its welding amps. The rating plate shows the maximum input current (I1max). Work it out in three steps:

1. Multiply the supply voltage by I1max to get kVA. For example, 230V × 25A ≈ 5.8kVA.
2. Add around 30% for surge.
3. Round up to the next generator size. The example above comes out at about 7.5kVA, so choose an 8kVA generator.

In practice, a 160A inverter welder usually needs a generator of around 6–8kVA, and a 200A welder around 8–10kVA. Input current varies between models, and welders with power factor correction (PFC) need less, so the rating plate calculation always beats a general guide.

Use a generator with AVR, or an inverter generator. Inverter welders are controlled by IGBTs; highly sensitive electronics. A cheap, unregulated generator might produce "dirty" power (voltage swings and high harmonic distortion, for example), and that can destroy an inverter welder's circuit boards. Look for a generator with AVR or an inverter generator, and check the welder manufacturer's guidance. Many state "generator compatible" and a maximum total harmonic distortion (THD), commonly around 6% or less. Older transformer welders are far more tolerant of generator power.

Start the generator before switching on the welder. Then ensure you switch the welder off before stopping the engine. Start-up and shutdown fluctuations are one of the most common causes of dirty power.

If you need to weld daily, a dedicated welder-generator is usually a better buy than a generator plus a separate welder. It's designed to handle welding loads continuously, while still providing auxiliary power for tools.

Choosing a Diesel Welder: Output, Duty Cycle and CC/CV

When choosing a diesel welder, the welding specifications matter as much as the engine. These are the specs to compare.

Welding Output

Duty Cycle

Duty cycle is the percentage of a 10-minute period a machine can weld at a given output without overheating. A machine rated at 100% at 300A can weld continuously at 300A, while one rated 60% at its maximum needs regular breaks at that level. For all-day site welding, compare duty cycle at the amperage you’ll actually use, not just the headline maximum.

CC or CV Output?

This is the most common specification mismatch on diesel welders.

• Constant current (CC) output suits stick/MMA and TIG welding.
• Constant voltage (CV) output is needed to run a MIG or flux-cored wire feeder.

Many larger machines offer both (multi-process), but some smaller or older units are CC only. If you plan to run a wire feeder, confirm CV output before buying. For more on the processes themselves, see Arc Welding Processes Explained, or our General MIG Welder Buying Guide if MIG is your main process.

Auxiliary Output

Size the auxiliary output to the tools you’ll run alongside welding, using the same load calculation as above. Most machines provide single-phase auxiliary power. Some, such as the Lincoln Vantage range, also offer three-phase auxiliary output, which is useful for running three-phase plant on site.

Specialist Features

Depending on the work, it’s also worth checking:

• Arc gouging capacity: the largest carbon diameter the machine can run.
• Downhill pipe mode: for cellulosic electrodes on pipeline work.
• Remote control: such as Miller’s ArcReach, which lets the welder adjust settings from the workpiece instead of walking back to the machine.
• Wire feeder compatibility: suitcase feeders that draw power from the welding leads, such as Miller’s suitcase feeders and Lincoln’s LN25 Pro.

Single Phase or Three Phase Output? 110V, 240V or 415V?

Match the generator’s output to the equipment it will run before comparing anything else.

• Single-phase (230/240V) runs standard hand tools, lighting, chargers and most small site equipment. Most generators up to around 15kVA are single-phase.
• Three-phase (400/415V) is needed for larger motors, three-phase compressors, workshop machinery and site distribution. Some units provide both, such as the Mosa GE 8000 HBT-L (1 x 400V and 2 x 230V outlets) and the GE12000 HZDT (400V/230V).
• 110V is the standard for portable tools on UK construction sites. 110V centre-tapped earth (CTE) supplies limit the voltage to earth to 55V, which reduces the risk of a serious shock. You can either buy a generator with a built-in 110V outlet, such as the dual-voltage Mosa GE6900 or GE SX-9000 KDM (110/230V), or run 110V tools through a separate site transformer.

Petrol or Diesel Generator?

For the welder-generator version of this question, see the FAQ in our Site Power Guide. For standalone generators, the choice usually comes down to size and how often the machine will run.

In short, a petrol generator suits light, occasional or portable work where weight matters. For daily use, larger outputs or long running hours, a diesel generator usually costs less to run and lasts longer.

Portable, On Wheels, Trailer or Skid?

How the machine moves is as important as its output:

• Portable (hand-carried): compact petrol units up to around 3kVA, such as the Mosa GE 2000 MI, suit mobile tradespeople and small repair jobs, weighing in at just 22kg.
• On wheels: wheel kits and trolley frames make mid-size generators (roughly 4–12kVA) easy to move around site by one or two people.
• Trailer-mounted (towable): common for larger welder-generators and site generators that move between jobs. Check the trailer’s braking and plating if you plan to tow regularly.
• Skid-mounted: a rigid base frame for bolting to a vehicle bed, lifting by crane or forklift, or permanent placement. This is how many diesel welders are run from the back of a service vehicle.

Features Worth Paying For

Inverter output, silenced canopies and AMF auto-start each have their own section below. Beyond those, these are the features that make the biggest difference day to day:

1. AVR (Automatic Voltage Regulation): keeps output voltage steady as the load changes. This is essential for inverter welders, chargers and anything with electronics.
2. Electric start: easier than recoil starting on larger engines, and needed for remote or automatic starting. Many mid-size units offer both.
3. Fuel tank and runtime: check runtime at your typical load, not at a quarter load. A full shift without refuelling is the usual target for site machines.
4. Engine type: air-cooled engines are lighter and simpler; water-cooled engines run cooler and quieter and last longer under continuous use.
5. Engine make: the engine sets reliability and parts availability. Well-supported makes include Honda (on many Mosa petrol units), Yanmar, Kubota, Kohler and Perkins.

Inverter Generators

An inverter generator converts the alternator’s raw output to DC, then back to a clean, tightly controlled AC supply. This is often described as “pure sine wave” power. The result is low harmonic distortion, which matters for laptops, tool battery chargers, test equipment and inverter welders. Most inverter generators also vary their engine speed to match the load, so they use less fuel and run quieter at light loads than a conventional generator running at full speed.

The trade-offs are cost and size. Inverter generators cost more per kVA than conventional units, and most are smaller portable machines. For larger loads, a conventional generator with good AVR is the usual choice. In our range, examples of inverter models include the Mosa GE 4500 MI and the Mosa GE 3900 AVR.

Silent Generators

“Silent” and “super-silent” generators have acoustic canopies that cut noise substantially, typically to a level where normal conversation is possible nearby. At the industry standard reference distance of 7 meters, a silenced model typically measures between 60 and 75 dB(A), while an open-frame unit typically operates between 85 and 105+ dB(A).

This 20 to 30 dB(A) difference is actually a huge reduction in perceived loudness, with the open-frame units sounding roughly 4 to 8 times louder to the human ear than a fully enclosed, silenced site unit. They’re worth paying for when:

• the site has noise limits, often set in planning conditions or the contract
• you’re working next to occupied buildings, offices or public areas
• the work runs early, late or overnight
• the generator will be near the crew all day

Standby and Emergency Generators

For workshops, yards and business premises, a standby generator keeps critical equipment running during a power cut. An AMF (Automatic Mains Failure) generator monitors the mains supply, starts automatically when it fails, and takes over the load through a changeover panel. Our range includes AMF models such as the Mosa GE SX-6000 YDM AMF and GE SX-9000 KDM AMF.

Size a standby generator for the critical loads only, such as lighting, IT, alarms, refrigeration and essential machinery, not the whole site. This keeps the generator smaller, cheaper to run and better loaded when needed. The changeover equipment must be installed by a qualified electrician.

Diesel Welder and Generator Brands Compared

Engine quality and parts support matter more than the badge alone, but these are the ranges we see most often and would always recommend.

Matching the Spec to the Job

Use this as a starting point, then refine with the sizing steps above.

Buying for a fleet? We regularly source machines in volume, including package deals across mixed petrol and diesel generators. Explore our current stock, where bulk quantities will be stated if available.

Running Costs and Compliance

Fuel

Fuel is usually the biggest running cost after purchase. When comparing machines, look at fuel consumption at 50% and 75% load, not just full load, and work out runtime from the tank size. Diesel generally gives more power per litre than petrol, and the gap widens on larger machines working long hours.

Red Diesel

Since 1 April 2022, most sectors, including construction, are no longer allowed to use red (rebated) diesel, and using it in construction plant and generators is illegal. Agriculture remains one of the main exceptions. Businesses can use up rebated fuel already held in storage for emergency back-up power generation. If you’re unsure whether your use is entitled, check HMRC’s current guidance before filling up, or the equivalent regulation guidance in your locality if overseas.

Noise

Site noise limits are commonly set through planning conditions, local authority agreements or the main contractor’s site rules. If you work near homes, schools or offices, a silenced canopy may be required, not a nice-to-have.

Emissions

Engine emissions standards increasingly feature as part of contract requirements. In Greater London, the NRMM Low Emission Zone covers construction site machinery with engines from 37kW to 560kW. Generators in that range must meet Stage V across all of London, and all machinery must meet Stage V from 2030. Smaller generators fall below the threshold. Older used machines in the covered range may not be accepted on these sites, so check the engine’s emissions stage before buying for London work. Again, if overseas, check regulations locally to ensure your kit is compliant.

Buying Used or Refurbished: What to Check

A well-maintained used diesel welder or generator from a strong engine and manufacturer brand can be excellent value, even for daily site work. The case for new is stronger if you need the longest warranty, the latest emissions stage, or a specific current model. For a broader comparison, see our guide to new vs used welding equipment.

When looking at a used machine, check:

• Engine hours. Hours matter more than age. A unit with low hours would be considered anything from 0 - 500; mid-range can be from 1,000 - 3,000 (with condition starting to account for some of the buying decision). High hours are considered anything from 3,000 +
• Service history. Regular oil and filter changes at the manufacturer’s intervals.
• Cold start and smoke. The engine should start cleanly from cold. Persistent blue or black smoke can indicate wear or fuelling problems.
• Wet stacking. Diesel engines running for long periods at light load can build up unburnt fuel and carbon, seen as oily residue around the exhaust. It’s usually cleared by running under heavy load, but it can point to a machine that’s been badly matched to its work.
• Output under load. A loaded or load-bank test confirms the generator holds voltage and frequency at its rated output. On welder-generators, check welding output across the range as well as auxiliary power.
• Enclosure, panel and sockets. Look for corrosion, damaged sockets, missing covers and signs of water ingress.
• AVR and controls. Check that voltage stays stable as loads switch on and off.

At Westermans, used machines are inspected and tested before sale, and many come with a warranty; see our warranty details and how we refurbish equipment for more information.

Should You Hire or Buy?

Hiring makes sense for one-off jobs, short contracts, or for trying a size or type before committing. Buying, especially used, usually works out cheaper for regular use, and it means the machine is on-site when the job is ready to start.

Frequently Asked Questions

How much does a site welder or generator cost?

Size, engine make, features such as AVR, inverter output or AMF, and condition all affect the price. New petrol generators start from around £300-400; large, heavy-duty diesel units can cost up to £50,000.00 +. Industrial welder generators range from around £2,000 for a new 200 amp petrol model, through to £45,000.00 + for a new 800 amp model. If a used unit is acceptable, you can save up to 50% or more on the price of a brand new machine.

Can you run a MIG or stick welder off a generator?

Yes, as long as the generator is large enough and has AVR or inverter output to protect the welder's electronics. As a guide, a 160A inverter welder usually needs around 6–8kVA and a 200A welder around 8–10kVA, less if the welder has power factor correction. Avoid cheap unregulated generators, because their "dirty" power can damage inverter welder circuit boards. See size section above.

What does kVA mean on a generator?

kVA (kilovolt-amperes) is the generator’s apparent power rating. Usable power in kW is lower. For typical site loads, kW is roughly 0.8 × kVA, so a 10kVA generator gives around 8kW.

How do you connect a generator to a workshop or building?

Through a changeover switch installed by a qualified electrician, which isolates the building from the mains before the generator supply is connected. Never back-feed a building through a socket. It’s dangerous for anyone working on the supply and can damage the generator.

Does a site generator need earthing or RCD protection?

Earthing and protection depend on how the generator is used and what it supplies, so follow the manufacturer’s instructions and have site distribution set up by a competent electrician. On UK sites, 110V CTE supplies are standard for portable tools, and 230V supplies should be protected by an RCD.

Can you run a generator in the rain?

Only if it’s designed for outdoor use or under a suitable cover that keeps rain off the panel and sockets while allowing free airflow. Canopied site generators are built for outdoor use; open-frame units need protection. Never run a generator in an enclosed or poorly ventilated space because of carbon monoxide.

Can you run two generators in parallel?

Some can. Many inverter generators have parallel outputs or kits that combine two units for more power. The Mosa GE 2000 MI, or Miller Big Blue Duo Pro diesel welder, for example, have parallel outputs. Larger generators need dedicated synchronising controls. Generators without a parallel function should never be connected together.

How often should a diesel generator be serviced?

Follow the manufacturer’s service intervals, which are usually set in running hours, not months. Generators used for standby also need regular test runs under load, so they’re ready when needed.

How many hours is too many on a used diesel generator?

It depends on the engine. Water-cooled diesels typically last far longer than small air-cooled or petrol engines, and service history matters as much as the hour meter. See our rough guide to hours above.

Can I use red diesel in a generator?

Only for entitled uses, such as agriculture. Since April 2022, construction and most other sectors must use white diesel, including in generators. Check HMRC/local guidance if you’re unsure.

Need help choosing? Speak to the Westermans team for practical advice on choosing a machine based on your power needs, workload and budget. Contact the team today.