What is a static system?
A static system is essentially a tank system that is stored inside a business premises or garage. Most static systems are plumbed into the water mains with pre filters screwed to a wall or other permanent structure. Because a static system is plumbed into the water mains this type of system will generate pure water 24 hours per day, 7 days per week. It is important to mention that a solenoid switch must be installed inline so that when the tank is full, it cuts the water. With out a solenoid you will run the risk of chucking money laterally down the drain as it bypasses the filters when the static tank is full. Solenoids are cheap to buy and in my opinion are an essential part of a static set up. Alternatively, you can fill and monitor the tank manually. Multiple van users will find this difficult.
So what are the advantages of installing a static system for your window cleaning business?
The advantages are water on demand and not having to wait 6 hours for filling as is the norm on a van mounted system, one point TDS control if you run more than one vehicle in your business. Static systems work out way cheaper to buy using IBC tanks and marginally cheaper on consumable items such as filters and resin should you be running multiple vehicles.
Other advantages are that you only need a cheaper delivery only tank in your vehicle or even better a trolley system. A static system is great if you have your eye on expansion. You can build a fleet of window cleaning vehicles very quickly in conjunction with lease van options as advertised by GrippaTank for example only installing more cost effective delivery only tanks.
Are there any disadvantages?
You will need space. If you are a self employed window cleaner and run your business from home then you will defiantly need free space in an out building, shed or garage. Be sure to protect your system from freezing. This can be done by insulating or using heaters during the winter months that drop below zero temperatures.
Those with business premises, usually space is not an issue, however, when you install your system make sure that it is near to the point where your company vehicles will fill up from.
If you get your static size wrong for your use or business projection and run multiple vehicles, you may find a situation where all the water has been used with vans still empty. Example, if you have 3 vans with 600L delivery tanks on board then you would be better off having a 2000L static system. Think about your business plan. Do you plan to expand?
What if I want to expand my window cleaning business?
Having the space to expand is essential here. If you run your business from home a garage might be great for growing your 1000L to a 2000L but to increase further you may have to consider renting a unit or other business premises and lets face it, if you expanding from 2000+ 
the money will be coming in thick and fast.
IBC Vs Purpose Manufactured
IBC tanks are usually cheaper than manufactured tanks and sometimes easier to daisy chain to additional units.
Silo style static tanks as sold by Ionic is perfect for the larger window cleaning companies.