Divide tank volume by the machine's flow rate - that gives minutes of continuous trigger time, and it is almost always shorter than people assume.

A machine drawing a typical professional flow will empty a modest tank in well under an hour of continuous work. Because real washing is intermittent, elapsed working time is longer, but the calculation is the right starting point.

Decide from the job. A unit that must complete a site visit without refilling needs a tank sized for the whole visit. A unit working where a hydrant, standpipe or supply is available needs only a buffer, and the tank can double as a break tank protecting the mains.

Water is heavy, and every additional unit of tank capacity is weight on the trailer and on the towing vehicle. This is where tank size collides with the licence question.

Baffles matter in any tank that travels part-full - unbaffled water surges under braking and affects handling significantly.

Total laden weight decides both the towing vehicle and the driver's licence category, and the rules differ by jurisdiction - establish them before buying.

A full water tank is the dominant weight. A trailer that is manageable empty can exceed limits when filled, so the calculation must use the laden figure, not the unladen one.

Most jurisdictions set thresholds above which a trailer requires brakes, and further thresholds that restrict which licence categories may tow it. Drivers who passed their test in different years often hold different entitlements.

The towing vehicle has its own limits: maximum towing mass, noseweight and combined train weight, all of which are set by the manufacturer and enforceable.

Roadworthiness of the trailer - lights, tyres, coupling, safety chain or breakaway cable, and any required registration - is checked at the roadside. Build a pre-tow check into the routine.

Trailer where the unit travels to sites behind a car or van; skid where a truck, container or existing trailer is already going there.

A trailer is self-contained transport. Hook up and go, no lifting equipment, and the unit is ready the moment it stops. The cost is the road-legal burden and the space it takes up when parked.

A skid frame is lifted on and off. It suits fleets running flatbeds or tipper bodies, operations that containerise equipment, and anyone who wants one machine that can move between vehicles.

Skid units also suit fixed deployment - dropped at a compound for the duration of a project and collected at the end.

Lifting is the constraint. Certified lifting points, a suitable forklift or crane, and a safe place to set it down all have to exist, and that is a lot of infrastructure for an operation that only has a van.

Cold for construction, agriculture and general grime; hot where the work is fleet washing, food industry or anything greasy - and hot adds fuel, weight and complexity to an already loaded frame.

A trailer-mounted hot water unit is a substantial machine: engine, pump, burner, fuel for both, and a water tank. Everything competes for weight and space, and the burner fuel tank is easy to forget in the calculation.

Where the work justifies it - mobile fleet washing, agricultural equipment, food processing sites - hot water is transformative and the unit earns its cost.

Where the soil is mud, dust and general weathering, a cold unit with a larger water tank does more useful work per tow.

A middle route used by some operators is a cold trailer with chemical application, reserving hot machines for the sites that need them.

A hose reel, a lockable box, a generator or lighting, and sometimes waste water recovery - the frame is the whole mobile workshop.

A hose reel is close to essential. Long hose is what lets the unit stay parked while the operator works, and unreeled hose on the ground gets damaged and creates trip hazards.

Secure storage matters because these units are left unattended in public places. Lances, nozzles and chemicals disappear, and lockable boxes are cheap insurance.

Lighting and a small generator extend the working day and power ancillary equipment, particularly in winter or for night work in city centres.

Where the job requires containment, a recovery tank and a wet vacuum can be mounted alongside - increasingly a requirement rather than an option for street and forecourt cleaning.

Every addition is weight. Specify the frame with the full fit-out in mind.

Wheel clamps and hitch locks on trailers, chassis-level locking on skids, tracking on both, and never leaving it hitched overnight in a public place.

Unattended trailer units are a well-known target. A hitch lock prevents casual coupling, and a wheel clamp prevents towing away with the coupling forced or cut.

Ground anchors in a compound, and reversing the trailer against a wall so the hitch cannot be reached, both raise the effort required considerably.

Concealed tracking devices are now routine on high-value units and have a good recovery record - though they work best combined with physical measures rather than instead of them.

Mark the machine and record serial numbers of engine, pump and chassis. Insurers usually require it and it is what makes recovered equipment identifiable.

Skid units in a locked vehicle body are inherently better protected, which is a genuine if unglamorous argument for them.

It has to be contained or discharged legally, and 'it ran into the nearest gully' is the answer that generates prosecutions.

Mobile work is exactly where wash-water discharge goes wrong, because the operator has no control over what the drain in front of them connects to. In most places surface water gullies run to a watercourse untreated.

For street, forecourt and vehicle work, containment mats with a wet vacuum recovering into a waste tank are the standard method, with the collected water taken away for proper disposal.

Some units carry a recovery and filtration system that allows water to be reused on site, which reduces both fresh water demand and waste volume.

Where discharge to foul sewer is possible, it usually requires prior consent from the sewerage operator.

Rules vary considerably between jurisdictions. Establish what applies at the location, not at the depot.