A Allan of Milton asks :-

How does a garden solar-powered light work?

John Bahr, a physicist who directs the electronics courses at the University of Otago, responded.

Earth receives approximately 1000 W per square metre of solar energy during daylight hours (say 10 hours as an average over the year). A garden-powered solar light contains a solar panel, rechargeable batteries, an electronic switch and a light source (usually solid state light-emitting-diodes (LEDs), which are very efficient in turning electrical energy into light energy).

During the day the battery charges up with free solar energy via the solar panel, collecting power from both direct sunlight and daylight. If we take a typical garden light with a solar panel of 125 mm diameter, this will intercept about 10-12 W of the incident solar energy over the ten hour period (about 350,000 J). This energy is stored in the batteries which are rechargeable, usually the nickel-cadmium (NiCd) type.

At dusk a separate detector (which detects the ambient light level) electronically switches the circuitry from the `charge' position to the `light-on' position so that the LEDs operate until the ambient light reaches a low enough level that the switch returns to the 'charge' position, usually at dawn. The solar panel and batteries are chosen so that enough energy is provided from the solar panel to keep the LEDs alight for the hours of darkness.

These lights are ideal for garden or patio use. Apart from the initial cost and a battery replacement every 2-3 years, they use free solar energy and cost nothing to run. However they are not really bright enough for good lighting. That would require larger solar cells, batteries and an array of LEDs.