Marketing clean technology

How does one convince customers to spend more money for the environment?

Following the inconvenient truth, global warming and raising oil prices last year, it seems that developing clean green technology is the thing to do. Finding investors for the field is easy, especially since there are plenty of government incentives to develop more environmentally conscious technologies.

The problems start when the product is ready, and the marketing and sales part begins. Assuming that the technology is more expensive then existing dirty alternatives, how does one convince the customers to spend the extra money on it?

It is difficult enough to convince customers to spend 10% extra on green buildings when one can promise savings in the utilities bills (saving water, gas and electricity), though one can not prove ahead of time what the saving would be, since energy prices change. An even more difficult situation is when the product doesn't even save money in the long run, and the only benefit is less hard to the environment.

Any thoughts or suggestions on how does one market an expensive product that gives unproven, unmeasurable benefits to the customers?

Will you be selling this as a financial benefit, or as something less tangible? If you market it as "save some money on your heating bills", then consumers will want to know how much, when etc. If you market it as "save the planet, feel good about yourself and set an example to others" then the conversation changes.

You may like to consider the following suggestions:

1. There's already a target audience who feels bad about wasting natural resources. They probably don't need a guarantee to believe your claims. They want to believe it to feel better, and they are willing to pay a premium for that feeling. They will also work to your advantage if they repeate your claims. In fact, especially in this field there are lots of prominent claims that were proven false, afterwards.
2. You can also work around the lack of unproven, unmeasurable effects by citing credible authorities on the matter. Hire someone with the relevant reputation to study your technology and confirm your claims about its effects. Increase your own authority by mentioning your eduction, expertise or experience in the field. Or you can increase your credibility by mentioning your disadvantage upfront, before your pitch.
3. Even if you can't promise savings in the utility bills, you can promise savings in the utility consumption (or wastage). Think light bulbs: Modern ones run 5 times as long and consume only a fraction of energy, compared to the old ones.
4. It's also possible to use a different "base" for the calculation to make it less abstract and more "real": Instead of saying "Save 50% on X", you could also say "Save two and a half trees now!", provided 50% savings of X translates to 2.5 trees (or any other less abstract resource).
5. While you may not be able to promise savings, you can still calculate savings. Use a proper (scientific) model to extrapolate and predict future prices, and you can promote these predicted savings (Disclaimer: I know nothing about the advertising laws in your country, so you better consult a lawyer before you do so).

Hope it helps.