For your small generators there are 2 main camps out there as far as the design goes. Inverter generators. These vary the speed of the engine and use a 3 phase alternator to generate power, take the 3 phase alternator, turn it to DC then use a power inverter to turn the DC into 60Hz AC power. The power says a constant 60Hz. The inverter generators use less fuel a low load. Anything over half loaded the inverter is pretty much even with fuel consumption with the direct driven counterpart. You might as well just get a big deep cycle battery and power inverter at that point for powering real small items. Direct drive or fixed speed where the generator is directly coupled to the engine via tapered shaft or drive belt. The generator output frequency is directly related drive speed. Direct drive are normally your gas guzzling noisy generators, but they don't have to be. Around 95% of all the generators you find under 10kw will be gasoline powered direct drive 2-pole units that run 3,600RPM. The 3,600RPM part is why they are so loud and suck down so much fuel. But there are also 4-pole 1,800RPM generators and even more exotic 6 pole 1,200RPM. These can be direct drive or belt driven. The direct drive 4-pole generators tend to burn much less fuel, are a lot more quiet but cost a lot more. 6kw 4-pole Kubota generators I work on use between .25 and .33 gallons per hour of fuel with a 2kw load. Belt driven generators will take an engine that cant quite turn a 4-pole generator at full out put when limited to 1,800RPM and allow the engine to turn faster, reducing the engine crank speed down from some higher speed (usually around 2500RPM) down to 1,800RPM to get the proper 60Hz out of a 4-pole generator. There are other generators out there besides whats at the hardware store or harbor freight but they cost a lot more.