“I think A to B should do a review of existing options for solar powered bicycle lights. I found one at www.energycapture.co.uk and the Alternative Energy Centre in Wales [www.cat.org.uk] sells a solar-powered 6 LED red emergency light which is sold with a bracket to attach to a bicycle.”

Jonathan Pattison
Leeds

Clearly, in this case, we’re looking at a very much smaller power requirement, from a correspondingly small solar panel. First let’s look a bit more closely at the power demands of a bicycle front light. A typical conventional or halogen filament bulb for a bicycle front lamp consumes two watts.That’s manageable, but realistically, few people would want to carry around a solar panel large enough to provide that sort of power, so we need slightly cleverer technology to reduce the energy demand. Fortunately, as regular readers will be aware, Light Emitting Diodes are advancing very rapidly and can now provide an adequate light output with much lower power consumption.

The only front LED lamp currently available to German (and by default, British) standards is the Cateye EL300G, although the situation is changing rapidly, so this may already be out of date.When we tested its predecessor, the EL300 (A to B 33), we found power consumption of 0.58 – 0.89 watts, depending on battery voltage. Consumption is claimed to have fallen since, so we’ll generously base our calculations on the lower figure.

“…the solid-state LED solar torch is perfectly viable and will no doubt be available soon…”

To provide 0.58 watts for, let us say, two hours each evening would require 0.58 watts x 2 hours = 1.16 watt/hours. If we reckon on twelve hours charging time each day in summer, the charge rate would thus need to be 1.16 watt/hours divided by 12 = 0.097 watts.With peak power of 65 watts per square metre of solar panel (see above), each square centimetre will provide 0.0065 watts, so a panel of 15 square centimetres would more than cover our requirement of 0.097 watts.

This raw data suggests that a panel measuring just four square centimetres would provide two hours of light output per day – if only it were thus! The problem, of course, is that our panels provide a peak output of 65 watts per square metre. In reality, the mean figure over the course of a bright sunny day is about half this, increasing the panel requirement to 30 square centimetres.

The Cateye EL300 is one of the few cycle lights efficient enough to run from solar power

Unfortunately, even in June, the sun is often partially or completely overcast, and during my experiments last summer I recorded an even lower mean figure over a 16-day period of 20 watts per square metre, giving a panel size of nearly 50 square centimetres to work our low-consumption front light.

Nevertheless, the figures suggest that a square panel measuring just 7cms across would provide up to two hours light each night throughout a typical English summer, and a panel of double that size would be physically possible, making a light of this kind practical for most of the spring and autumn too. One proviso – the panel must be angled towards direct sunlight throughout most of the day, something that’s hard to arrange in practice, as we discovered during our experiments.

But what of the Energy Capture cycle light Jonathan found on the internet? Armed with our raw data, we can tell quite a lot:The solar panel measures a reasonable 88 square centimetres, and is rather optimistically claimed to have a 1.1 watt output.That may be attainable in a test lab, but working on our 20 watts/square metre average figure, I would suggest 0.17 watts in practice.That would certainly power an LED front light, but unfortunately the Solar Light is fitted with a filament bulb drawing four times as much power as the Cateye EL300!

For occasional use (say 30 minutes per day) a lamp of this kind might suffice, but for nearly £50 you really would be better off buying a conventional light with rechargeable batteries, and recharging a spare set each day by mounting a small solar panel on a south- facing window sill. However, the solid-state LED solar torch is perfectly viable and will no doubt be available soon. Regrettably, it seems Energy Capture has ceased trading, demonstrating that this example might have been an idea ahead of its time.