On Fri, 25 May 2012 00:35:38 +0100, F news@nowhere wrote:

On 24/05/2012 23:42 the_constructor wrote:

In brilliant sunshine, such as we have had over the past couple of days,
what prevents solar panels producing the full permitted 4KW. Down to 2KW
now.

Temperature. The hotter they get, the less efficient they are.

Is the right answer. Panels have power TC of about -0.4 to 0.5% per
degree C, and the panels will get hot, maybe 45-50 C hence, on a hot
day, peak output can be 20-25% below peak output on a cool day wit
sunny spells.

There's even a specification for it, NOCT - nominal operating cell
temperature.

I got peaks of over 4kW out of our 3.99kW system a few weeks ago when
the sky was clear but the temperature low. On similarly bright but warm
days now, the peaks are around 3kW.

Overall, with the longer days, I'm getting in excess of 25kWh each day.