A couple of days ago, a search on Google would use around 0.0003 kWh of energy and produced around 0.2 grams of CO2 according to Urs Hölzle, Senior Vice President, Operations at Google who presented those figures back in January 2009.
This figure, which is more than 18 months old, is likely to have decreased as Google swaps its servers for more efficient ones (between 2005 and 2009, Google used SEVEN generations of servers to power Google Search).
Because Google Instant now introduces a number of new parameters, namely that it automatically updates SERPs on the fly and has a number of yet-to-be identified server-side optimisation, actually trying to find out how much power per search is consumed will be tricky.
Indeed, finding out what is considered to be a search query becomes awfully more difficult; it is not tied to a URL (in the traditional sense), the URL structure becomes dynamic and users may end up exploring other SERPS (and clicking elsewhere) while typing in the initial query.
Therefore, one can argue that the power consumption of one Google Instant search is less than for a traditional search engine but because of the serendipity factor (see here), actually FINDING what you want will have a bigger carbon footprint.
In a post on Google's website, by Ben Gomes, Distinguished Engineer, stated that an average query for Google Instant will generate up to seven times more results for each one.
In absolute terms, the quantitative increase will be ridiculously low but multiplied by billions of searches and it becomes frighteningly big.
However Google will try to offset it by investing in newer technologies, like RE