Consider a 6V, 6W bulb connected to an a.c. supply by cables having a total resistance, R.

To operate at 6W, the bulb needs a current of 1A. This current must, of course, flow through the resistance of the cables.

The power converted to heat (power wasted) in the cables is given by

So, to have as little power wasted as possible, we should aim to decrease the current, I (assuming that we have already made R as low as possible).

This can be done as shown below.

If the transformers are 100% efficient, and the current flowing through the bulb is 1A then the current flowing through R is only 1/8A. This means that the power wasted in the cables is 64 times (8² times) less than in the previous case.

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