What is the power input and power output in generator and where does excess power go when we use less power.?

The power input is the mechanical power supplied to the generator by the engine. This will be more than the actual output always. Typical efficiency of the generator may be under 90% depending how it is measured. The losses are caused by the energy to operate the field, the friction and windage, the losses in the conductors, and the losses in the magnetic cores. Most of this lost energy is transformed to heat energy, and a bit of noise. The generator only produces as much energy as the load drawn from it. The efficiency may change at different loads, but the mechanical power input is proportional to the electrical load. The fuel usage drops as the load drops. Engines can be rated for specific fuel consumption as pounds per HP per hour or similar. I was looking up a larger diesel generator a few days back that used 201g of heavy fuel oil per KW per hour at full output. So if this could all be scaled down linearly... Power to drive the generator for 100W output = 120W for 83% efficient generator. Thus we expect 201g x 0.12KW = 24.1g of fuel per hour. Convert to liters using the specific gravity for that fuel oil, which may be 0.96, that would be 25.1ml of fuel per hour for 100W load. Repeating these figures for 10KW, we see 2410 grams per hour or 2.510 liters/hour (for that imaginary engine and fuel) and 10KW load. In practice the engine uses a certain amount of fuel just to keep running. At full power about 1/3 of the heat in the fuel is mechanical output, and the other 2/3 go as heat in the cooling system and the exhaust. The generator also uses energy for the fan and field excitation even when the load is very small. That would be more than our tiny load of 100W for the generator alone. The whole machine may be 500ml of fuel per hour with no load (guessing for this imaginary setup). Running at very light loads like 100W is therefore inefficient as we expect only 25 ml per hour for an efficient setup (as the efficiency would be near full load). Maybe it uses 500ml per hour to power a 100W lamp. This is one reason it is best to try to organise at least 1/3 of the maximum output with diesel generators. It does depend on the machine. Some smaller power plants have a smaller engine for the times the load is lighter. Incidentally most engines are not suitable for running at very low loads for long periods. It leads to more maintenance. The change in efficiency with low loads is worth understanding, as it means wasted fuel. If you are using 1 liter per hour at virtually no load for a 10KW machine, it may be something like a dirty air cleaner or a restricted exhaust or other maintenance issue. Look for black smoke too. Consult the manufacturer for realistic figures, though they may just say it is not intended to be run like this. It is useful to have exhaust temperature and fuel consumption figures for specific loads to help with maintenance.

First, with one 100-watt bulb, your generator will NOT produce 10 kW of power. It will produce only the power needed to light the bulb, overcome friction and other mechanical losses, etc.. Most generator have a throttle sensor which actually controls the amount of RPM the generator has to turn to produce the power being required at the time. As the load increases, the generator will speed up somewhat. Ron

If your generator is designed well enough, the generator will operate at a reduced rate of fuel consumption when loaded with a smaller output load. If not reduced in capacity, the excess power production will be dissipated as heat somewhere within the circuit. As for the power input, I don't know what flavor of fuel you use, so I cannot answer. If the power input is greater than the power output, which is the only thermodynamically valid case, then the excess power is rejected as heat in the exhaust stream.

it appears to be diesel generator. the volts output and RPM is same for all power , but the current made available at the armature winding is made to vary by the fuel intake so as you up/down the load, the fuel intake goes up/down.But, at low ends of the FL as in your case the system losses will be much higher, that's why you have to buy generator of adequate FL. In normal running the difference between input and output is due to system losses comprising of friction, magnetic and elecrical circuit losses.

The electrical loading on the generator is proportional to the mechanical power it takes to turn. This is Lenz's Law at work. So if you lighten the load on the generator< it becomes easier to turn and the driving engine uses less fuel.

I can only answer the last part of your question without draging out my slide rule. Excess eneregy is always without exception expended as heat that is not used in the processs and excess heat needs to be transfered to the atmosphere.