Power
By this characteristic BSU can be successfully divided into four main groups:
- up to 1 kW;
- 3-4 KW
- 5-10 KWT
- more than 10 KW
You can not choose a gasoline generator in this parameter, otherwise there is a risk of overpaying or getting a low-power car. There is a simple calculation formula: the sum of the power of all household appliances, the power of which is supposed from the BSU + 20%( power reserve).It is optimal to add a little power for reliability in this calculation. 
Cooling type
Choosing a gasoline generator autonomous for this aspect should pay special attention. It is the type of cooling that determines the dimensions and weight of the device, but the duration of continuous operation also depends on it.
Air cooling makes BSU easy and compact, a low-power gasoline generator of this type can safely take with you on a hike, placing on your shoulders in the form of a backpack. However, it is worth remembering that such a generator can not work for a long time, unless, of course, the air temperature is low enough.
Water cooling makes the gasoline generator larger and many times heavier, but also gives more chances for a long work. 
Continuous operation time
Operating time without shutdowns, as it was said, directly depends on the type of cooling. If the determining factor is exactly the duration of uninterrupted power supply, the water-cooled generator leaves forward with a detachment - it can work without interruptions. But the air-cooled will need a break after 8-10 hours.
The way to start
There are only two options: automatically using a key or a button or manually. This parameter will also affect the price that will have to be paid for the gasoline generator stand-alone.
And, it would seem, why overpay? However, automation is very useful, if the generator is planned to start in winter( fuel, oil thicken and manually launch will become very difficult).
Noise isolation
The maximum start-up volume of the BSU is approximately the same as when starting a moped. If this fact causes significant inconvenience, you can pay attention to the units in the casing - they are much quieter. However, the casing will reduce the time of the device's continuous operation almost twice. Is it worth it?
Speed of revolutions
Low-speed gasoline generator autonomous( 1500 rpm) - quiet, has a large resource and is more expensive. High-speed( 3000 rpm) can be purchased in the event that it is planned to use it no more than 500 m / h per year - this will significantly save.
Phase
When planning the use of a gasoline generator with retrofitted electrical appliances and wiring, everything is simple - you need a single-phase generator. However, for three-phase wiring, the same apparatus is needed, and it will be more difficult to connect it. Distribute the load across all phases will have to be so that no more than 20-30% of the skewing is obtained.
An important aspect in the installation of BSU is also the protection against sudden voltage. In order not to risk wiring and household appliances it is better to install a small device at once - the voltage regulator - and the network inside the house will be safe. 
Price issue
Gasoline generator stand-alone will always be cheaper than its diesel equivalent( difference of about 30%).A good low-power import device( 1.5-2 kW) will cost around 800-1500 euros, a powerful, capable house with a high level of electricity, the installation will have to spend 4,000 euros.
Conversion of gasoline generator to gas
If the issue of saving costs so much that there is a desire to purchase an inexpensive gasoline unit, and then convert it to gas power, it is simply to calculate the cost of the process. It will be much less profitable than buying a ready-made gas appliance that uses natural gas and propane-butane mixtures. The gasoline generator, even if it is correctly converted, will still not be as economical. Therefore, even if the gasoline generator is self-contained, it is worthwhile to calculate all the pros and cons. Maybe it's better to sell gasoline and buy gas, than spend money on reworking?
OKOF
According to the All-Russian Classifier of Fixed Assets, the gasoline generator belongs to Article 143149010 "Mobile power stations, gasoline generators, diesel generators".