Thanks to all to answer my question but I want to know more why GPM? Tells discuss in detail why we choose GPM?
PSI is not nearly as important as you may think. Typically residential concrete is rated at or around 2500 PSi so anything over that is just going to cause damage. So even with a 3000-4000 PSI machine, you're going to be lowering the PSI anyways with different nozzles.
Commercial concrete can stand higher, but there's really no need for anything over 3500 PSi ever.
If you're doing Residential work and only ever dealing with small oil stains, you can absolutely get by with a cold water unit and the right chemistry. Does a hot water machine help? Of course it does. But is it necessary? no.
For commerical work though, especially if you're doing lots of parking garages and big oil/tire stains, Hot water is probably a must.
But in the end...all of this is driven by the GPM of the machine. Higher GPM allows you to use a larger surface cleaner, and walk faster.
For Residential work, the sweet spot generally falls between 5 and 8 Gpm.
For Commercial work, the higher the better (10GPM being considered the minimum by a lot of contractors)