First, we need to find the energy consumption of the lights before and after the change. To do this, we multiply the power by the time and by the number of days. The unit of energy is kilowatt-hour (kW h).

Before the change, the energy consumption is 600 W x 7.0 h/day x 200 days/year = 840 kW h/year.

After the change, the energy consumption is 600 W x 4.5 h/day x 200 days/year = 540 kW h/year.

Next, we need to find the difference between the two energy consumptions. This is the amount of energy saved by switching off the lights when not needed. To do this, we subtract the smaller value from the larger value.

The energy saved is 840 kW h/year - 540 kW h/year = 300 kW h/year.

Finally, we need to find the cost saving by multiplying the energy saved by the price per unit of energy. To do this, we multiply by 7.0 cents/kW h.

The cost saving is 300 kW h/year x 7.0 cents/kW h = 2100 cents/year or $21/year.

Therefore, the saving is $21 per year by switching off the lights when not needed.