Save the planet, switch to eco-light bulbs. So goes the refrain. But are these as bright, long-lasting and energy efficient as is often claimed?

The traditional incandescent bulb is on the way out. European law means people will be encouraged to use longer-lasting, energy-efficient lights instead.

Old-style bulbs are an eco-enemy

But many remain unconvinced that the common alternative - compact fluorescent lamps (CFLs) - are up to the job.

European legislation has already banned the manufacture and import of 100-watt incandescent bulbs. In 2011, 60-watt bulbs will go, and 40- and 25-watt bulbs will be banned by 2012.

But are these bulbs quite as good as is claimed?

Think those compact fluorescent bulbs are not as bright as the old-style lights they replaced? You are probably not imagining it.

A guide to the amount of light given by a CFL bulb is given on its box as a comparison to the wattage of an incandescent bulb. But the European Commission says in a FAQ document this can be misleading.

"Currently, exaggerated claims are often made on the packaging about the light output of compact fluorescent lamps - for example that an 11-12-watt compact fluorescent lamp would be the equivalent of a 60-watt incandescent, which is not true."

What wattage will be as bright?

The Lighting Industry Federation says the claims on the packaging are the nearest equivalent to the wattage of a soft white light bulb.

Liz Peck, of the Society of Light and Lighting, says this is because CFLs have a phosphor coating. "They compare like for like, but the trouble is people tend to use the clear bulbs at home and it's not equivalent to those."

The European Commission's advice is to divide the wattage of a traditional light bulb by four to get the equivalent brightness. So, to get the brightness of a traditional 60-watt bulb, choose a 15-watt CFL bulb.

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But the Lighting Research Center in the United States goes further.

"We believe in the divide by three rule," says associate director Russell Leslie, who recommends a 20-watt CFL to match a 60-watt incandescent bulb. "The equivalent ratings you see on the box are usually got by testing in a laboratory environment."

At home, brightness varies as conditions change. "A compact fluorescent light is designed to provide maximum light output at 25C, and when it gets hotter or colder than that, its brightness can be reduced.

"If your bulb is in a recessed fixture in the ceiling, and it gets warm, you might see a 10-20% reduction in its light output."

And studies show CFL bulbs can get 20% dimmer over time.

New European regulations expected next year mean manufacturers will have to display lumens - a measure of light output - more prominently than wattage on bulb packaging.

Another complaint is eco bulbs - supposed to last for years - frequently conk out early.

"Unfortunately you get what you pay for," says Ms Peck. While a branded bulb from a well-known manufacturer may indeed last the promised 10 years, one from a supermarket budget line may not.

Won't need to change this for 10 years? Perhaps not

But even branded bulbs don't always last as long as expected - this is because the lifespan given is an average.

When a batch of bulbs is tested, they are turned on for three hours, then off for 20 minutes over and over again until half the batch fails. This point in time is then decreed the average life.

It is often 10,000 hours. As no-one adds up the hours a light is on over its lifespan, this is translated as 10 years, on the assumption that the bulb will be on for an average of three hours a day.

But as half the bulbs will fail before 10,000 hours, a shopper may be unlucky enough to pick a dud that will fail after just 2,000 hours. However, the main manufacturers do their best to make bulbs that cluster around the average life mark, says the Lighting Industry Federation.

And what you do with a bulb can affect its lifespan, says Mr Leslie. Continuously turning it off and on every 15 minutes, for example, will more than halve its expected lifespan.

Just how energy efficient are these lights? The European Commission, the Energy Savings Trust and manufacturers say CFLs use up to 80% less electricity than traditional bulbs.

How is this number calculated? It's worked out by comparing the best compact fluorescent lamp's wattage with the wattage of an equivalent incandescent bulb, says a spokeswoman for the European Commission.

Do the energy saving claims stack up?

But that results in a 5:1 energy ratio between the two - a claim it says is an exaggeration when manufacturers use it.

And it's the "up to" in this 80% claim that is important. The EC spokeswoman says the saving can be as low as 60%.

John Henderson, an energy-use expert from the consultancy Building Research Establishment, says although CFLs are better than traditional bulbs, policy-makers should not draw simple conclusions from simple sums about their energy saving potential.

"When you see an 80% savings figure on the side of a low-energy light bulb, it doesn't actually mean that you're going to save 80% lighting energy, 80% carbon emissions, and 80% costs."

Traditional bulbs expend about 95% of their energy producing heat. The European Commission considers this to be heat loss. But Mr Henderson disagrees.

"Let's say your house uses 1,000kWh a year to produce the light you use. If you were to replace all the old-fashioned light bulbs with the modern low energy lamps, you might expect an 80% reduction - 800kWh.

Do old-style bulbs help heat the house?

"However you'd find about 60% of that 800kWh would get automatically chucked back in by your thermostat-controlled heating system. A typical heating system is only about 75% efficient. So the actual figure you end up with is more like 240kWh a year, rather than the 800kWh you expected."

That number is only a rough guide, as most homes have gas central heating which is cheaper and less carbon intensive than an electric heating system.

Meanwhile, the Institute of Lighting Engineers is considering changing its estimate of the energy savings represented by CFLs from 80% to 70%.

This is because the power factor of CFLs is low, which means a utility company needs to use more energy to get these lights to work, which can also cause disruptions in the power network.

The future's bright... or bright-ish

Ms Peck, of the Society of Light and Lighting, says CFLs have improved in recent years - they flicker less, and warm up faster. Nor should people worry that they contain mercury, as it is a very small amount.

There are other energy-saving options, she says, such as halogen tungsten lights which are about 30% more efficient than incandescent bulbs.

And technology is developing fast, so it could be only a few years before people are lighting their homes with LED lights, which experts say have the potential to be more efficient than CFLs.

INCANDESCENT LIGHT BULB Traditional 100W light bulb Glass mount carries electrical connection from base to the filament Electric current passes through filament which heats up and emits light Inert gas inside bulb protects filament and improves luminescence

COMPACT FLUORESCENT LAMP 'Energy saving light bulb' Electrical current emitted from electronic 'ballast' Current flows into gas filled tube causing it to emit invisible ultraviolet [UV] light UV light causes phospor coating inside tube to emit visible light Requires 20-23 watts to generate same light as traditional 100W bulb