What You See Is Not What You Get
Standby generator output is alternating current (AC), and manufacturers normally quote the power of the generator either in Watts or VoltAmperes. Watts (W) is the true power whilst VoltAmperes (VA) is the apparent power. The true power is determined by something called the Power Factor. The power factor is a dimensionless number between 0 and 1. The value of the Power Factor is the ratio of the real power over apparent power. Were the true and apparent power to be equal, the power factor would equal 1; but the equipment being powered causes reactive power (VAr), which as its name suggests works in opposition, so the real power will be less than the apparent power and so the power factor will be less than 1. This is why the Watt value is smaller than the VA value quoted. The closer the values are to each other the more efficient the generator:
To get 1 kW of real power at 0.2 power factor, 5 kVA of apparent power is needed.
To get 4 kW of real power at 0.8 power factor, 5 kVA of apparent power is needed.
This shows why the power factor is so important.

Specs to be aware of:
Power Rating kVA/KW
Output Voltage
Fuel Tank Capacity in Litres
Noise Level (dba@?m)
Physical Size (LxWxH mm)
Weight (KG)

Remember: The bigger the load the more fuel used

Safety Considerations
Some makes of generator can be noisy and they can also pose serious safety hazards to you and to others, so it is extremely important to follow all safety instructions provided by the manufacturer.

Be sure that the total electric load on your generator won't exceed the manufacturer's rating and always locate your generator where its exhaust will vent safely.

Use correctly rated and correctly fused plugs and cables. Ensure that each lead or cable used can safely carry the current expected of it. Overloaded leads can overheat and cause fires or damage to equipment. Beware of leads rated at 5 amps that have a 13 amp fuse in the plug! Take nothing for granted...

Keep leads & cables out of the way so they don't present a tripping hazard—especially in dimly lit doorways or halls. Never run cords under rugs or carpets where heat might build up or damage to a cord may go unnoticed.

Generators have their own safety features so do not use a Residual Current Device (RCD) as generators are configured in a different way to the normal mains supply as they have what is termed as a "floating earth". RCD's are designed to work on the mains supply, which has an earthed neutral. A generator would need to be modified by an approved electrician to use a RCD and would require the use of an additional earth spike. Unless you have consulted a qualified electrician, you should not have the generator modified in any way and strictly comply to the manufacturer's user instruction manual.

Never connect a generator to the mains system during a power cut as you might end up electrocuting someone who is working on the problem or cause damage to equipment connected to the system. A standby generator must only be connected to the mains system by a qualified electrician using approved switching components.