What Size Generator for a Pressure Washer?

A 13-amp pressure washer may appear straightforward to power from a portable generator. Get the sizing wrong, however, and the motor may struggle to start, the generator may trip, or the washer can run hot and suffer avoidable damage. The answer to what size generator for pressure washer work is not based on pressure or water flow alone. It comes down to the machine’s electrical input, its start-up demand and the quality of the generator supplying it.

For a professional user working away from mains power, generator capacity is not an area to guess at. A little spare output costs less than downtime on a job.

Start with the pressure washer nameplate

The rating plate on the pressure washer is the first place to look. Find the input power in watts or kilowatts, or the voltage and current in amps. Do not size a generator from the motor horsepower, the pump’s litres per minute, or a plug rating alone.

Most portable electric pressure washers used in the UK are 230V single-phase machines. If the plate gives an amp rating, use this calculation:

Volts x amps = watts

A machine rated at 230V and 13A has a running demand of roughly 2,990W, or 3kW. That is its expected running figure, not necessarily what it needs during motor start-up.

The generator’s continuous rating matters more than its headline maximum or peak figure. Some generators are advertised with an impressive peak output, but can only sustain a lower load. Your pressure washer must sit comfortably within the continuous rating, with room for its starting surge and any other equipment connected at the same time.

What size generator for a pressure washer: the practical rule

As a working rule, choose a generator with a continuous output at least 25 to 50 per cent above the pressure washer’s stated running wattage. For washers with a conventional induction motor, or machines that are hard to start under load, leaning towards the larger margin is sensible.

A 3kW washer should not be paired with a 3kW generator. It may run briefly on paper, but there is no useful capacity left for the motor start, voltage stability or normal site conditions. In most cases, a quality 4.5kW to 5kW continuous-rated generator is a better fit.

This guide gives sensible starting points for one pressure washer with no other major electrical load:

| Pressure washer running input | Typical generator continuous output | Suitable use case |
|---|---:|---|
| Up to 2kW | 2.5kW to 3kW | Light-duty portable cold-water washer |
| 2kW to 3kW | 3.5kW to 5kW | Mobile valeting and general trade cleaning |
| 3kW to 4kW | 5kW to 6.5kW | Regular commercial cleaning work |
| 4kW to 6kW | 7.5kW to 10kW | Heavy-duty equipment and site cleaning |

These figures are not a substitute for checking the specific washer and generator specifications. A generator with a generous surge rating, proper voltage regulation and a proven ability to start motor loads can outperform a cheaper unit carrying the same headline kW figure.

Why motor start-up changes the calculation

An electric pressure washer motor draws more current when it first starts than it does once it is running. Induction motors can create a substantial short-duration inrush demand. If the generator cannot respond, the voltage drops sharply. You may hear the generator labour, see lights flicker, or find the washer trips out before it gets going.

Repeated low-voltage starts are poor practice. They put strain on the motor winding, capacitor and generator alternator. They also waste time when an operator is trying to clean a fleet, a yard or plant machinery to a deadline.

The unloader valve condition matters too. A pressure washer that restarts against trapped pressure can be harder to turn over than one that has fully unloaded. If a machine starts reliably in a workshop but not on a generator, check the washer’s service condition as well as the power supply.

Inverter generators can be a good option for smaller machines because they tend to provide stable output and are often quieter. For heavier commercial pressure washers, a well-built conventional generator with adequate continuous capacity is often the more practical choice. The important point is not the style of generator alone, but whether it is designed to handle the motor’s starting demand.

Do not confuse kW and kVA

Generators are often rated in kVA, while pressure washers are usually marked in kW or watts. They are related, but they are not always identical because motors have a power factor.

For straightforward sizing, use the generator manufacturer’s stated kW continuous output where available. If only kVA is shown, do not assume that a 5kVA generator supplies 5kW of usable power. Many portable sets are rated at a power factor of 0.8, meaning 5kVA may equate to around 4kW.

That difference is enough to turn a supposedly suitable generator into an overloaded one. Check both ratings before buying or hiring a set.

Single-phase, three-phase and hot-water machines

A standard 230V single-phase generator cannot run a 400V three-phase pressure washer. Three-phase machines require a correctly specified three-phase generator, with the right voltage, frequency and phase balance. Connecting a three-phase machine through an improvised adaptor is not an answer and can damage equipment.

Hot-water pressure washers need closer attention. A diesel-fired hot-water washer uses electrical power for the pump motor, controls and usually the burner fan or fuel system, but the heat itself comes from diesel. Generator sizing is therefore often based mainly on the motor load, subject to checking the machine plate.

An electrically heated hot-water pressure washer is a different proposition. Its heating elements draw significant power, commonly far beyond what a practical portable generator can supply. These machines usually need an appropriately sized fixed electrical installation or a substantial site generator. If hot water is essential for mobile cleaning, a diesel-heated washer or a petrol-driven machine may be the more workable option.

Allow for everything connected to the generator

The generator rarely powers only the washer for long. A wet vac, work lights, battery charger, water transfer pump or van-mounted equipment may be added during the day. Add up the running load of every item that could operate at once, then size the generator around the combined demand and the largest motor starting load.

Avoid working permanently at the generator’s limit. Apart from poor starting performance, an overloaded set uses more fuel, runs hotter and tends to have a shorter service life. A generator operating with sensible headroom is quieter, more stable and less likely to interrupt the job.

Long extension leads add another complication. An undersized cable creates voltage drop, especially with a high-current pressure washer. Use a fully unwound, heavy-duty extension lead of suitable length and conductor size. A generator large enough at its sockets can still deliver poor results if the cable is inadequate.

Safe set-up matters as much as capacity

Run the generator outdoors in a dry, ventilated position, well away from doors, windows and air intakes. Carbon monoxide from a petrol or diesel generator is dangerous, and a generator must never be operated inside a van, workshop or enclosed yard space.

Keep electrical connections off wet ground and protect them from spray. Use the correct RCD protection and follow the generator manufacturer’s earthing instructions. Earthing arrangements vary between generator types, so this is not something to assume. For commercial work, the condition of plugs, cables and sockets should be checked as routinely as the washer hose and lance.

If the pressure washer trips only when used from a generator, do not bypass safety devices to keep working. The cause may be undersized supply, a faulty capacitor, cable damage, an RCD compatibility issue or a machine fault. Find the cause before it becomes a repair bill.

Choose capacity for the job you actually do

For occasional light cleaning, a correctly rated 3kW generator may be enough for a small washer. For a commercial 13-amp machine used daily by a mobile valeter, contractor or facilities team, a quality 5kW continuous generator gives a much more dependable margin. Once the washer moves into higher-output commercial territory, petrol-powered or engine-driven pressure washers may be more economical and simpler to run than carrying a large generator.

The right generator should start the washer cleanly, hold its voltage under load and leave enough capacity for a real working day, not just a quick test in the yard. If you are matching equipment for regular site work, RealKleen can help identify a pressure washer and power arrangement built for the load, the location and the service demands behind it.