Here's one map of solar insolation for Japan: http://solargis.info/doc/_pics/freemaps/1000px/ghi/SolarGIS-Solar-map-Japan-en.png http://solargis.info/doc/_pics

Here's one map of solar insolation for Japan: http://solargis.info/doc/_pics/freemaps/1000px/ghi/SolarGIS-Solar-map-Japan-en.png http://solargis.info/doc/_pics/freemaps/1000px/ghi/SolarGIS-... ...and for the world: http://solargis.info/doc/_pics/freemaps/1000px/ghi/SolarGIS-Solar-map-World-map-en.png http://solargis.info/doc/_pics/freemaps/1000px/ghi/SolarGIS-...