Operating manual: A solar lamp consists of a solar panel or photovoltaic cells that collect and convert energy from the sun, a rechargeable battery that stores the electricity, a light-emitting diode (LED) lamp, and some sort of sensor or timing device. During the day - except in the winter and on cloudy days - the battery reaches maximum charge. When the sun goes down, the solar cells stop producing power, and the sensor or timer turns on the LED lamp.
Power up: Remove the cover of a light fixture and you'll likely find the solar cells on top. A single solar cell produces a maximum of 0.45 volts and a varying amount of current, depending on its size and the amount of light striking the surface. Most outdoor lights have four cells, which produce 1.8 volts and about 100 milliamps of current in bright, direct sunlight. The cells are wired directly to the battery through the diode, which keeps current from the battery from returning to the cells at night.
Battery pack: Older solar-lamp models and some models made today use a standard AA NiCad (nickel-cadium) battery that produces about 1.2 volts and can store a maximum of 700 milliamp-hours. But many newer models use NiMH (nickel-metal hydride) power packs, which can have two to three times the capacity of a NiCad battery of comparable size.
Thanks for the memory: The difference between NiCad and NiMH is how the battery retains its power over time. In a NiCad battery, the charge begins degrading almost immediately. A NiMH battery is designed to operate from fully charged to fully discharged.
A brighter light: LEDs are five times brighter than the bulbs previously used in solar lamps, drawing about 45 milliamps. And, unlike conventional bulbs, they emit light but not heat when electricity passes through them. One LED emits roughly the same brightness as a two-watt bulb. Still, the light produced is about half that produced by a candle, which isn't that much. As one expert notes, it's enough light for you to differentiate your sidewalk from your lawn, but not enough to keep you from falling over your kid's bicycle.
Some solar lights use both halogen bulbs and LEDs, with the LED providing consistent lighting and the bulb turning on when it detects motion. LEDs are designed to never burn out over the lifetime of the product, usually 25 years, which means they aren't replaceable in the light fixture.
What will it cost: The cheapest solar yard lights are $10 to $20 per lamp, and the better models run even higher. That's because solar cells are still expensive to produce - they're manufactured from silicon crystals in clean-room conditions - and NiCad and NiMH batteries also are expensive.
If you have a long driveway or garden path that will need a couple dozen lamps, going solar will be pricey up front. But once the lights are installed, they'll cost you nothing to operate.
Want Alan J. Heavens' advice on a home-improvement project or purchase? E-mail him at email@example.com or write to him at The Inquirer, Box 8263, Philadelphia 19101.