Now to light the lamp to its full 100watts, it needs 500 volts across it, 0.2 amps flowing through it. Looking into the line at the generator end we see the resistance of the lamp and the resistance of the transmission line once again, this time it is 2500 + 100 = 2600 ohms. To get 0.2 amps flowing in the circuit, the generator then needs to supply 0.2amps x 2600 ohms = 520 volts. So in this case, only 20 volts will be lost along the transmission line. Since its resistance is still the same, the power lost in the line is now smaller, only 20 volts * 0.2 amps = 4 watts. so now to light the 100 wat lamp, we need to pump in 104 watts at the generator, so we are now about 96% efficient! Thats a big difference!
In general, to be as efficient as possible, we want the resistance of the load to be much higher than the resistance of the transmission line. We can achieve this by designing our load to work from a higher voltage. The power lost in the resistance of the line is proportional to the square of the current. Also, we would like the voltage at the end of the line to be as stable as possible: in the first example, if we were to disconnect the lamp, the voltage at the end of the line would increase to 200 volts, twice what it should be! Also connecting a smaller lamp would also increase the voltage! This is a problem. In the second example disconnecting the lamp would only increase the votlage at the end from 500 to 520 volts. So by making sure the load resistance is much larger thant the lines resistance, we can have higher efficiency and better voltage regulation.
This is why we use AC mostly for transmission. With AC we can easily change the voltages with transformers. We can step it up to a high voltage for transmission, then step it down again at the users end. Then we can achieve high efficiency with smaller conductors (higher resistance) for the transmission line.