Lets have a think. Oil heating costs about 1.4 pence / kwh.

Are you sure? I dont know, its just that gas is more than that.

Not certain but FWIW, this is my source:
http://www.chic-info.org.uk/electric.html

I havent done the maths here, but ave of 1/8 kW per sqm sounds much
too low.
I dont know what kind of collector youre considering, all I can say is
big trackers can pay back according to previous calcs. But it does
depend how you design them. Somewhere I've got detailed figures on
this one: want me to mail them? Might be of interest.

Yes please.


Right. I'd be tempted to just run it slower, by using a series
impedance or transformer. Sounds pretty lousy tho if its crapping out
already.

It was making a nasty whining noise, obviously too much back pressure,
I have a 1/4 hp one on order though.


I reckon if you get more than 50% through it should gain you. Youre
really losing a lot with an open collector. It wont matter that theres
light scattering.

I did another calc on the uncovered collector at the weekend. Now that
the coil is divided into 4 circuits, the flow rate is much higher but
the temp gain seems the same. Perhaps it's just better conditions now
(mid summer, high noon, no cloud at all). I got 10kw! If there is 800W
/sqm available then from my 25sqm collector, that's an efficiency of
50%. Given this, do you think the polythene will still benefit?

Nick