My least favorite forecast, and we've been having a lot of it lately. Since winter began (Dec 21st), my PV array has produced only 102 amp-hours of power. There have been only 3 days that could be considered sunny at all (even for a few hours). To put this in some perspective for you, Wolfram Alpha tells me that 102 amp-hours is half of a typical car battery capacity. So I've really had to scrimp & save. I disconnected my router to save power, and I plumbed the depths of the battery bank (further than it's really safe to go). And I was completely out of power for several days.

So, I've bought a generator. I had to order it online, because it seems every store in the entire eastern US is completly out of stock, due to hurricane Sandy. This house has a generator shed (complete with a massive old generator that doesn't work), but I put the new, small generator in the furthest outbuilding, from where it can only barely be heard in the house. Later I'll be able to run a cable from its 12v terminal to charge the battery bank (if the sun doesn't get there first); for now it only powers my laptop. Nasty noisy smelly dirty thing, it pains me to run it.

Back to the "overcast" forecast. I've heard meterologists say that one unexpected consequence of climate change seems to be that weather patterns are persisting longer, giving us this summer's weeks of neverending heat, and large, slow-moving storms. I suspect this could also explain the blankets of clouds that have been settling in for weeks at a time, on and off since mid-November. But I only have 2.5 years of not very good data. Before, I paid about as much attention to whether it was sunny as anyone else, which is to say, not really very much. Anyone know of a source of historical cloud cover data?

PVWATTS hourly performance data
The older version 1 PVWATTS calculator has hourly performance data "determined to be 'meteorologically typical' for the site". It could be used as a baseline to compare with current weather. Also, looking at data from PVOutput can be useful to measure the amount of sunny hours in recent days in an area.
Comment by deuxpi