Big brand retailers, technology giants and -- perhaps more surprisingly -- financial services firms are vying to capitalise on the burgeoning "keen-to-be-green" market.

Under the government-backed "We're in this together" campaign, launched last week, eight leading companies have come together to fight global warming.

They aim to reduce carbon emissions in every household by at least one tonne over three years -- a total 25 million tonnes.

Supermarket giant Tesco is halving the price of energy-saving light bulbs and home improvement chain B&Q is doing the same with its two best-selling insulation products.

BSkyB is introducing technology that saves emissions by automatically switching inactive Sky boxes onto standby, while mobile operator O2 is paying a credit to customers who do not want a new handset when they renew their contract.

Financial firms are in the race for the green pound, too.

HSBC became the first big bank to commit to going "carbon neutral" in 2004. Barclays achieved the same feat in the UK in March, while HBOS aims to be carbon neutral by the end of 2007.

Others, such as Royal & Sun Alliance (R&SA) and Barclaycard -- among the new corporate alliance, which also includes British Gas and Marks & Spencer -- have launched eco-products.

Bridget McIntyre, chief executive of R&SA, says: "By offering consumers positive environmental choices, large companies can make a tangible difference in the fight against climate change."

But, while such initiatives might be kind on the environment, are they pleasing on the pocket?

Competitiveness, it seems, is an issue. Take car insurance, which is rapidly emerging as a key eco-finance battle-ground.

More Than, the direct retail arm of R&SA, gives owners of eco-cars -- hybrids, electric and alternative fuel vehicles -- an insurance discount of up to 15 percent.

For those with non-eco cars, it will pay to carbon offset their first 3,000 miles' driving.

Rival CIS offsets 20 percent of customers' carbon emissions by investing in reforestation and renewable energy projects.

Other insurance policies offer schemes that help eco-minded consumers become greener drivers.

Although not specifically labelled "green", Norwich Union's (NU) "pay-as-you-drive" motor insurance -- launched in October last year -- aims to reward those with limited mileage.

The cost depends on when, where and how often motorists drive, determined by an in-car global positioning system (GPS).

In theory, less usage should mean lower carbon emissions -- and lower insurance premiums.

More Than also plans to help drivers track their habits.

Under a pilot scheme due later this year, it will give 1,000 customers a GPS device to measure anti-environmental behaviour, such as the number of short journeys taken, excessive speed, over-acceleration and idle time leaving the engine running.

Being an eco-conscious motorist will, of course, save money on running costs; the Energy Saving Trust estimates that greener drivers can save an average 120 pounds per year on fuel.

But eco insurance could leave less green in your purse.

Motorists are being stung with premiums of up to 44 percent more than market-leading standard policies, according to research from price comparison service moneysupermarket.com.

And, unless you are an exceptional case -- a motorway-loving, low-mileage, off-peak driver -- working out whether NU's "pay-as-you-drive" policy will save you money -- or help the environment -- can prove tricky.

Richard Mason, director of insurance at moneysupermarket.com, expects the green car insurance market to grow, but says providers must be more competitive.

Until then, drivers should opt for the cheapest deal that meets their needs and give the money saved to green causes.