Posted By: chriskempHow much does output drop with the lack of direct sunlight?

More than I expected! - on a sunny day this month our 3.25kw system delivers 14kWh but a couple of days earlier just managed 1.4. kWh - June with it's longer days was far below April for us here in West Yorkshire.

Look at my stats for generation this year:

http://www.earth.org.uk/saving-electricity.html#meter2012

compared to last:

http://www.earth.org.uk/saving-electricity.html#meter2011

either by the day or by the month.

Looking at June this year, 507kWh generated, 591kWh June last year.

And PVGIS, which is usually slightly conservative, predicts 545kWh for June from my array: http://www.earth.org.uk/note-on-data.html#PVGISpred

(May differences look even more dramatic.)

So maybe we've also been on the lucky side for sunshine the last few years?

Rgds

Damon

Posted By: DamonHDLooking at June this year, 507kWh generated, 591kWh June last year.

And PVGIS, which is usually slightly conservative, predicts 545kWh

(507 + 591) / 2 = 549

Pretty close

Posted By: fostertom I guess that's more critically variable? a mild but miserable wet winter wd prob be worse than a hard cold but less cloudy one?

Yes, but the production over the winter months is considerably less ( hence the bell curve of PVGIS output estimates) and less critical

I'm interested in optimising solar for winter, for Dec/Jan even, let the 'easy' months look after themselves. 'Production over the winter months is considerably less' doesn't say whether weather has a greater of lesser critical effect on it, such as it is.

Posted By: DamonHDSo maybe we've also been on the lucky side for sunshine the last few years?

I understand that up until the last few months the S of the UK has had above average sunshine pretty much over the whole of the period since the FIT scheme was introduced. Aside from anything else the drought orders introduced just before it came wetter are probably an indicator of that.

Sooner or later the other side of the coin was likley to turn up...

I've wondered before now (on Navitron) if the take up of the FIT scheme might have been different if it had been introduced during a different weather trend, such as the one just experienced in the S, i.e. if people were saying to their friends "production is no where near what we were told" rather than "it's even better than the sales blurb said!".

PS. In the informal Monthly PV stats topics on Navitron our NW Scotland production has been right up there with installations on the Isle of Wight for the last 3 months. That's not something I'd expected to see!

PPS. Last year while the S was enjoying Sun, Sun, Sun, in May, we an amazingly wet May. Funny thing weather...

There is less variation in output in winter than summer because of the lower overall output.
One thing about solar that we do know and can predict with great accuracy and precision is when it will not produce.
Once radiation levels have reached a certain point, there is less of a variation due to cloud cover because of scattering and diffused light as well as less possibility of lower levels.
So if you have a clear day with direct beam coming in at say 1000 W/m^2 and you get a cloud that knocks it back to 900 W/m^2, some would say that is 10% less, but your inverter may not see that. Would not be the same if it went from 500 W/m^2 down to 450 W/m^2 though.
Life is not linear, nor is PV production.
If you want to 'predict' the short term output, just look at which way the wind is blowing.

Not on cost or CO2 if the hydrogen comes from natural gas via steam reforming, or used natural gas directly.

Posted By: chriskempI assume that FITs assessments & calculations take into account seasonal variation / geographical variation of soalr potential output.

I'm not sure where the geographical variation comes into it? However it is was calculated in the first place, the rate payable is based on the type & size of system and the date of registration. Within those parameters an instalation at Lands End would get the same pence per kWh as one at John O' Groats (or even Lerwick). The one at Lands End is likely to generate significantly more power (& income) over the term of the FIT contract.

Posted By: RobinBBut you've got longer days this time of year than them down south.

But the sun is not so high and roofs are generally at a steeper angle than down here.

Landsend can be a wee bit misty and no one has every seen raw sunlight in St Just.

With the cumulative total not actually tracking that far below predicted:

Reason for the lousy weather?

The Met Office have had to stop using the word "summer", either directly or by inference, for the duration of the Olympics.

Posted By: RobinBBut you've got longer days this time of year than them down south.

Wot ST said, plus at the extreme ends of the day the sun is more behind the array than infront of it (unless it's a very poorly oriented array).

skyewright: "poorly oriented"? Pah! What about east- and west- facing? B^>

Rgds

Damon

Posted By: DamonHDskyewright: "poorly oriented"? Pah! What about east- and west- facing? B^>

Fair enough, but that's taking two bites at the cherry...