On 19/08/2019 19:59, Chris Green wrote:
I know only too well that a (for example) 250 watt solar panel doesn't
produce a continuous 250 watts for 24 hours a day every day of the
year. There's those things call nights, not to mention dull winter
days, and even dull summer days.

I've searched diligently but I can't find a site providing what seem
to be realistic 'ball park' figures for UK regions. I suspect there's
a special word for this percentage which I haven't found.

One book which stems from the OPEC induced oil crisis of the 1970's is
called Sun power, by J.C. Mc Veigh, Pergamon Press 1977

https://www.amazon.co.uk/Sun-Power-I.../dp/0080208622

"Save It" campaign was way more active than anything of recent times.

I'd recommend avoiding the Kindle edition which is overpriced by about
an order of magnitude. I have a copy that was withdrawn from the library
system and sold off as scrap. You want table 2.4 which has experimental
monthly figures based on Kew data 1958-1968. It is in MJ/m^2.

There have been a few improvements since it was written but it isn't a
bad guide to DIY solar collectors 4 decades on from when it was written.

So, can someone either provide a figure for the average output of
(say) a 250 watt panel over the year in eastern England or point me at
a site that will tell me this. (I seem to remember the figure is
something like 20 to 25 percent, so I'll get an average of 60 watts or
so from a 250 watt panel).

Better than that in summer and much much worse in winter. Peak summer to
worst winter ratio is about a factor of 9. Week by week is more extreme.

Solar powered devices where I live in North Yorkshire wreck their
batteries every winter. This is a shame since they work perfectly in
midsummer but fail to remind people that on frosty mornings entering a
tight bend too fast results in hitting a tree.

--
Regards,
Martin Brown