Solar-Powered Lights Keep Getting Better

Solar-powered lights have improved as the technology they use has advanced -- today's models give off more light than their ancestors did. Plus, they don't go dark when the electrical lines go down.

Need to know: Solar lights store power from the sun during the day, then release it at night. But to get the brightest and most consistent light, placement is important. Put them in full sunlight, most manufacturers promise, and you can have illumination within 15 minutes. Put them in shaded areas, and it takes longer for the lights' solar cells to charge -- and the illumination will not be as strong.

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 a 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 cell stops producing power and the sensor or timer turns on the LED lamp.

Power up: Remove the cover of a light fixture, and you're likely to 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 a diode, which keeps current from the battery from returning to the cells at night.

Battery pack: Older solar lamps and some models made today use a standard AA NiCd (nickel-cadmium) 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 NiCd battery of comparable size.

Thanks for the memory: The difference between NiCd and NiMH is how the battery retains its power over time. In a NiCd 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 much 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 much. As one expert noted, 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 not burn out over their lifetime, usually expected to be about 25 years, which means they are not 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 higher. That's because solar cells are still expensive to produce -- they're manufactured from silicon crystals in clean-room conditions -- and NiCd and NiMH batteries also are expensive.

If you have a long driveway or garden path that will need a couple of dozen lamps, going solar will be pricey upfront. But once the lights are installed, they'll cost nothing to operate.