Sound good doesn’t it? A torch and/or bicycle light that recharges itself for free from the sun – no more batteries, no more dynamos, no more fuss.You get to save lots of lovely money while saving the planet. Unfortunately, the reality is a little more prosaic, and it’s worth explaining why.

To generate a reasonable amount of power, you need quite a large solar panel – 11cm x 5cm in this case, giving overall dimensions of 17cm x 7cm. Even with a panel of this size, output is quite small.The exact figure depends on the latitude, the season, the time of day, and the degree of cloud cover.We measured a low of 0.3mA at 3pm on an overcast November afternoon in England, and a high of 20mA in the sun-drenched tropics.Well, no, actually we created that one with a 40 watt light bulb on the same afternoon, but the effect is much the same.

With so little power available, the torch bulb needs to be small by bicycle standards – 600mW in this case, or about a quarter of the brightness of a typical front light. Running at 2.4 volts, the bulb draws current at the rate of 250mA, or somewhere between 12.5 and 833 times faster than the solar panel can supply the juice.Thus, the torch will need to sit in the sun for somewhere between 12.5 and 833 hours for every hour of use. In the sun-drenched tropics, you could realistically expect to use the torch for an hour or so each evening, but in an English winter, the same charge would take months…Well, you get the picture.

Should anyone have the patience, the internal rechargeable battery will give a theoretical run time of about 2 hours 20 minutes, but if you get fed up with waiting, there’s room for a couple of conventional AA batteries too.That rather negates the environmental advantages of course, as does the fact that the internal batteries are nickel cadmium, which require careful disposal.

Don’t write off the idea of solar powered lights. Replace the bulb with state-of-the- art light emitting diodes, and the inefficient solar panels with satellite-grade jobbies and you’d have a truly practical device. Unfortunately, this isn’t it.

But we shouldn’t be too negative. If you live somewhere sunny, the Solar Torch will work well, and even in the UK, it could provide self-charging emergency cover, as long as you don’t use it too often.There’s an array of flashing LEDs in the tail (not sure what for, though) and it floats, which yachtsmen will find useful. Particularly in the tropics.

Multi-purpose Solar Torch . £15.99 . Centre for Alternative Technology tel 01654 705959