A few people giving advice against a 50L trolley, I strongly disagree. Yes if it is a fixed tank, the pouring of water is hard work, but why on earth would you want a 25L when you can have a 50L? With a 50L trolley you can have one container on it for your domestic work, and two containers on it when you need that extra water, or your doing a larger commercial job. When you have 25L of water on a 50L trolley they weigh around the same as a 25L trolley.
My new trolley for instance will house a water heating system, you will have 2 containers on the trolley when you use cold water, if you want to use the heater you take one container out and the HotWash goes in it's place. Yes it will be heavier than a 25L trolley, but it will allow you to work in freezing weather. Much the same as a wheeling 25L might be a little easier than wheeling 50L, but it gets lighter after every window, and saves running back and forth for more water.
I wish people would think a bit more before giving advice.
Peter