Good Morning! East is Best!

28 May : I met someone today who got a quote for his house... but the supplier would not quote as the roof faces East. Actually, the orientation is the same as for my house, its ridge is north south, but tilted 10 degrees to give favour for the sloping pitch to the East.
   My observation after nearly a year of this is that MORNINGs are less cloudy statistically , and more to the point, the air temperature is lower in the morning after the night. PV efficiency is closely linked to temperature. Cool panels produce more in similar sun angles etc. and East seems to work acceptably. Afternoons seem to be marred by broken cloud blowing in, even when the morning started by looking very clear skied.
  The Peveril solar house will meet all its annual energy costs from this east roof, plus a healthy surplus, despite the East angle.
  The other question is Low sun angles from trees. Well the harvest in Dec and Jan is so small that the very low angle is not worth worrying about (also deciduous trees are not usually in leaf in winter). Two good days in May/June can be equivalent to the whole month of December or January, and the chances of these coming are random, but good. If you have a good April or May, the difference between that and the expectation can be more than the entire harvest in December.
You must be careful not to have anything shading the active surface during the summer months, such as a tall chimney or tree that will grow higher and more foliated in summer.

PV example in the author's house

1 May : If I can encourage you to think more about PV, I could add that it's not only about saving your energy bills and making some revenue from feed in tariff. It becomes an interesting hobby, checking the weather, recording the statistics, matching the lifestyle to the performance (eg use appliances more when sun is shining) etc.
The last two months of Photovoltaic generation have been very good, both months exceeding the JRC expected amount by about 40%. (Helps us to forget the poor days of Jan and Feb.)

• March should have been 207 kWh but was 281. 
• April should have been 306 kWh but turned in a score of 429.

The amount by which these exceeded expectation were more than the entire months of Jan and Feb combined, which rather makes me think that you should not worry if sun angles or long shadows in the months of Dec and Jan are not going to be favourable. It seems better to optimise for the spring to autumn months.
Looking at the charts here, the final weeks of April were astonishing, we had a run of nine days in a row, all touching or above 20 kWh, apart from one freakishly cold dark day on 15 April. The lower illustration is a typical run of days, from April 15 to 21. Even though our roof faces east, the early morning sun gives us good performance. I notice a pattern whereby days often end with cloudier afternoons after a good start - enough to put me off recommending a west roof where an east one is available.