How Much Pure Water Do I Need to Clean Windows?

A window cleaner arrives at a building, sets up the pole, connects the system and starts work. By mid-morning, the tank runs dry before the planned section is complete. The problem is not usually just the tank capacity: it normally starts with failing to calculate how much pure water I need based on the type of work, the actual flow rate and the quality of the incoming water.

When professionally cleaning windows with pure water, an accurate estimate helps prevent stoppages, unnecessary journeys and oversized equipment. It also helps you decide whether it is best to work with a tank, a direct mains connection, several backup containers or a reverse osmosis and deionisation system suited to the workload. There is no universal figure: a low-level shopfront round does not use the same amount of water as a day spent cleaning building façades with a telescopic pole.

How much pure water do I need for the job?

The quantity required depends on four variables that should be assessed together: the square metres of glass, the level of dirt, the working flow rate and the cleaning method. Looking only at the surface area can lead to mistakes, because two façades of the same size may require very different amounts of water.

As an operational guideline, maintenance cleaning of exterior windows with normal levels of dirt may use approximately 0.3 to 0.8 litres per square metre. On highly accessible surfaces, with refined technique and controlled brushing, consumption may be lower. However, if there is accumulated dust, rain residue, pollution, very dirty frames or ingrained dirt, longer rinsing will be needed and consumption will increase significantly.

For an initial estimate, you can use this formula:

Estimated litres = square metres of glass × expected consumption per m² + safety margin

The safety margin should be at least 15–25%. For new services, routes with no consumption history or jobs involving several operatives, it is sensible to use the higher end of this range. Running short affects the productivity of the entire day; carrying a reasonable reserve helps maintain continuity.

Example calculation for periodic maintenance

A residential development has 180 m² of accessible exterior glass that can be cleaned with a water-fed pole. Maintenance is carried out regularly and the dirt level is moderate. If consumption is estimated at 0.45 litres per m², the initial calculation is 81 litres. With a 20% margin, it is advisable to plan for around 100 litres of pure water.

This volume may be sufficient for a specific job, but it should not automatically be applied to another building. If the frames are heavily coated with dust, the glass has deteriorated solar-control coatings or areas need to be repeated because of stubborn marks, consumption will be higher. Estimating helps you work intelligently, but it does not replace the operative’s experience.

Flow rate determines actual runtime

A system can produce pure water of the correct quality and still provide insufficient runtime if it is operated at an excessive flow rate. The pump flow rate, measured in litres per minute, determines how quickly the tank is emptied. That is why it is useful to convert the available litres into actual working minutes.

The formula is simple:

Runtime in minutes = available litres ÷ litres per minute

With a 250-litre tank and a flow rate set to 1 litre per minute, the theoretical runtime is 250 minutes. At 1.5 litres per minute, it falls to around 166 minutes. This difference may determine whether a system can cover a complete route or needs to be refilled halfway through the service.

The flow rate should not always be set to maximum. A small window, a sheltered glazed section or a touch-up clean does not need the same water supply as a large façade. Adjusting it according to the brush, height, dirt and wind reduces consumption without compromising rinsing. The key is to maintain enough water to carry away the dirt and leave the surface free of residue as it dries.

Height and technique also affect consumption

The longer the pole, the more likely the operative is to increase the flow rate to compensate for the feeling of reduced control or to make rinsing easier. This is understandable, but it is not always necessary. A lightweight pole, a properly guided hose and a suitable brush help you work accurately even at height.

Technique is just as important as equipment. Brushing in an organised manner, starting with the frames when they are dirty and carrying out a thorough rinse from the top down avoids having to repeat the work. Repeating sections because of inadequate rinsing or because dirt has been moved from one frame to another is one of the less visible causes of high water consumption.

Incoming water quality changes system capacity

When asking how much pure water is needed, it is important to distinguish between two concepts: the volume required for cleaning and the volume the system can produce or treat before maintenance is needed. The quality of the mains water, particularly its dissolved solids content, directly affects the performance of consumables and the planning of the working day.

In deionisation systems, incoming water with a higher mineral load will exhaust the resin sooner. In reverse osmosis systems, production also depends on pressure, temperature, available flow rate and the condition of the filters and membranes. As a result, the same system will not have the same operational runtime in every area.

The TDS reading is the practical reference for checking that the final water maintains the purity required to dry without leaving mineral deposits. If the outlet reading rises, simply adding more water to the tank is not enough: the condition of the system must be checked. Working with water that no longer provides the required quality can cause marks, repeat work and complaints, cancelling out the advantages of the method.

Tank, direct production or backup containers

The right configuration depends on the route and the working environment. An integrated tank makes the system mobile and provides a clear reserve for jobs without convenient access to a water supply. It is a common option for companies serving different customers throughout the day, especially when the historical consumption for each service is known.

Direct production may be suitable for fixed installations or jobs with reliable mains access and conditions appropriate for the system. It reduces the need to transport large volumes, although the available flow rate and actual production capacity should be checked in advance. There is little point in planning a day around a theoretical production rate that cannot be maintained under working conditions.

Backup containers are useful as a reserve, but they must form part of a realistic calculation. They do not replace a suitable configuration if every day requires transporting more water than necessary or if the system regularly runs short. When this happens, review consumption per operative, the flow rate and system capacity before adding containers on an improvised basis.

How to reduce consumption without compromising the finish

Reducing consumption does not mean restricting the water to the point where the result suffers. It means eliminating waste and adapting the work to the surface. A clean brush, with filaments suited to the type of glass and level of dirt, removes dirt more effectively with fewer passes. Keeping the hose, connections and valves in good condition also prevents losses which, accumulated over a route, can amount to several litres.

It is advisable to record the litres used per service, approximate square metres, incoming and outgoing TDS readings, and any issues for several weeks. With this data, a company can establish its own ranges, which will be far more reliable than any general estimate. It can also identify whether an operative needs a flow-rate adjustment or whether a route has become more demanding due to seasonal changes, nearby building work or greater dust accumulation.

Before choosing a system, consider the average surface area per day, the number of operatives, access to water, local water hardness and expected growth. At Limpieza Cristales Unger, technical advice and support from SAT Unger in Spain help configure systems, consumables and spare parts based on professional use, rather than nominal capacity alone.

The best estimate is not to fill the tank to the maximum out of habit. It is to understand the consumption of each route, keep the quality of the final water under control and carry enough reserve to complete the job to the standard the customer expects.