The Power Factor used to declare power ratings on three phase generators is 0.8 (so 100kVA = 80kWs etc.). Unfortunately the difficulty for installers is understanding the effect of power factors within an electrical installation is a changeable situation dependent on equipment within a load. This is something that the installation engineer needs to understand when specifying a generator for a load and in my experience the best method of evaluating what happens in a load is to test it for a lengthy period with a power clamp meter or other load analyzing instrument suitable for the power node you are dealing with.
A power factor can drastically change the amount of power available from a generator and this is what customers struggle to understand in my experience (if you have a Power Factor of 0.3 the amount of power available from the 100kVA machine discussed earlier is less than 30kW's).
The installer should carry out a full tour around the installation and check what physical items of equipment are in the load. Motors are normally something which needs careful consideration (what type of motor is it Star/delta, Soft start or the dreaded D.O.L) other items such as UPS's are they present (and if so how many pulses) as they create a non linear load condition effecting the kVAr and may entail a requirement to oversize the generator.
If the installer/customer analyses the load (PF. kVA, kVAr, kW's) and obtains the correct data a good engineer can normally calculate and advise on the product specification required to deal with the variations within the load condition. Often however people assume too much about an installation install a generator and find they have issues they had not considered and blame the generator. Optional equipment from most manufacturers is often required after the initial sale then to accommodate the issues found, this can be expensive for a customer as it normally entails retrofitting equipment.
Some manufacturers such as the company I work for recognize this and fit some items of equipment as standard to their generator products to lessen the costs and inconvenience customers experience. The issue then is that our product can seem to be more expensive than competitive products where equipment we fit as standard is an optional extra for their machines.