http://en.wikipedia.org/wiki/Solar_variation\nThere are two common meanings:\n\nthe radiation reaching the upper atmosphere\nthe radiation reaching some point within the atmosphere, including the surface.\nVarious gases within the atmosphere absorb some solar radiation at different wavelengths, and clouds and dust also affect it. Measurements above the atmosphere are needed to determine variations in solar output, to avoid the confounding effects of changes within the atmosphere. There is some evidence that sunshine at the Earth's surface has been decreasing in the last 50 years (see global dimming) possibly caused by increased atmospheric pollution, whilst over roughly the same timespan solar output has been nearly constant.\n
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For an Irish house, the perfect angles for your panels would be around 63º in the Winter and 43º in the Summer. However, as most sun arrives in Summer, a roof of 45º or so, makes sense. (An angle of 60º will get more benefit from panels during the Winter) The annual energy collection only varies by a maximum of 10% for rooves between 20º and 40º, if between 30 degrees east of south and southwest.\n