RK:

I've seen no evidence that renewables can be geared up at a feasible cost to provide 25% of Japan's electricity in just ten years or even 20 years. A 2010 report of the National Academy of Sciences said that, in the US, non-hydro renewables could conceivably reach 10% of the nation's electricity generation by 2020, if there are sufficient cost reductions that match and exceed cost reductions in other sources, e.g. natural gas. This could conceivably rise to 20% by 2035. Going to 50% or more would require new scientific advances.
The report also notes that past prediction of the penetration of renewables have over-estimated their share. (Executive Summary, Electricity from Renewable
Resources: Status, Prospects, and Impediments, 2010 from, National Research Council/National Academy of Sciences). I don't see why this would be different in Japan. So, I'd like to hear why you think that is the case.

It seems to depend on who's doing the study. The International Energy Agency, for example, was not supportive of renewables until a few years ago, when they started hiring specialists. The agency's new study "Energy Technology Perspectives 2012" includes a 2 degree celsius warming scenario wherein there is 50% renewable penetration in US power by 2050 (p 627).
Moreover, the most sophisticated study on the US appears to be the NREL's Renewable Electricity Futures Study, "a collaboration with more than 110 contributors from 35 organizations including national laboratories, industry, universities, and non-governmental organizations," concludes that 80% by
2050 is doable with advances in grids, storage and etc:

http://www.nrel.gov/analysis/re_futures/

Some studies write off the potential for renewables by invoking the need for technical advances and then throwing up their hands. Some studies see the technical challenges as opportunities to gain leadership in what they depict as the melding of energy, IT and biotech (such as in the algal biofuels that 200 firms in the US are busy with, along with help from NASA, the military, Boeing, the airlines, etc).

What we do know, as empirical fact, is that the Germans went from 7.8% renewables in electricity in 2002 to 20% in 2011 (and are now over 25%):

see slide 4
http://irena.org/DocumentDownloads/events/MaltaSeptembe
r2012/4_Ellen_von_Zitzewitz.pdf

and for 2012:
http://cleantechnica.com/2012/07/26/germany-26-of-elect
ricity-from-renewable-energy-in-1st-half-of-2012/

In 2005 Angela Merkel deemed Germany's 20% by 2020 target "hardly realistic":
http://www.windpowermonthly.com/news/963739/Change-air-
energy-policy-general-election-returns-coalition-govern
ment/?DCMP=ILC-SEARCH

And Germany did that without - it would seem - impairing the performance of their economy. For example, the World Economic Forum's Competitiveness Index ranks them 6th in the world:
http://reports.weforum.org/global-competitiveness-repor
t-2012-2013/

In the Japanese case, much of the METI and all of the nuclear village certainly have not been keen on renewables. Their target (now rescinded) for renewables in power generation was 1.63% by 2014, which the relevant committee's members declared to be "bold."
These interests were ready to push serious work on smart grids out to the 2020s, and countered the IEA's encouragement to build a national grid with the insistence that it was cheaper to build more nuclear.

Perhaps someone ought to calculate the opportunity cost of protecting these kinds of monopolies when you're in the middle of a distributed energy revolution.

Anyway, Japan's energy basic plan of June 2010 projected 21% renewables by 2030:
http://www.npu.go.jp/policy/policy04/pdf/20110607/siryo
u3.pdf

Now they're talking in the ballpark of 30% by 2030.

Meanwhile, the feed-in tariff that former PM Kan got put in last summer took effect on July 1 and has in 2 months led to 72,680 projects totaling 1.3 gigawatts:
http://www.enecho.meti.go.jp/saiene/kaitori/dl/setsubi/
201208setsubi.pdf

The FIT spreads around a significant pecuniary incentive for big capital, SMEs, farmers, households, local governments and the like to cooperate. We see this eg in the Mitsubishi-JA Zennoh deal this month to do a tie-up that will put 400-600 solar arrays on farms from Hokkaido to Okinawa:
http://www.mitsubishicorp.com/jp/en/pr/archive/2012/htm
l/0000015277.html

Banks and credit unions are organizing special packages to feed finance into this expanding demand.

The utilities have also enhanced the above incentives to deploy renewables (and efficiency) via their price increases and talk of more to come in order to maintain their businesses. Their customer pool would appear to be shrinking through inroads from renewables and efficiency as well as because local governments and other actors are bailing out on them as soon as they can contract with alternative sources of supply. Keep in mind that Tokyo Metro is installing its own gas and renewable generating capacity, explicitly aimed at "blowing a hole" in Tepco's business model (vice-gov Inose Naoki's phrase:
http://www.zakzak.co.jp/society/politics/news/20120703/
plt1207030745005-n1.htm).

Can Japan outdo the Germans on deploying renewables?
One wonders. But there seem to be a lot of incentives and organizing overlooked by Yomiuri etc commentary that suggests the ambition to get out of nuclear is mere emotionalism.