DIYbanter - View Single Post

Martin Brown

On 16/12/2014 18:48, wrote:
Interesting feedback thanks. Obviously not a good idea is the general consensus.

Just as a matter if interest I thought Id run these figures by and see if I can get a
clearer picture... My electric storage heater uses 12kwh a night on cheap rate
night tariff. (Thats 85 p/night) That gets it nice and hot.
Im trying to calculate how many panels would give me the equivelent of
12kwh mains a day...so...

How does one calculate average annual UK output for each 100W panel?
Assuming more in summer less in winter etc.
And how can that be converted into 12kwh/day? In other words
in the UK, if one wanted 12kwh for *general use heating* during the day,
how many 100w panels, on average, would be needed to get the
the equivelent of a mains 12kwh/day input?
Thanks ahead.

Average 12kWh/day over the year would be doable if you had very deep
pockets but there is a 5:1 ratio of average summer output to winter.

Ballpark figures for annual isolation at UK latitudes are

1800 MJ/m^2 direct sunshine
1700 MJ/m^2 diffuse light

I think that was for Brighton (worse the further north you go).

A commercial solar cell is no more than 25% efficient.

Being generous you can harvest 900MJ/m^2 = 100kWh/m^2 per year

Area needed therefore = 365*12/100 = 45m^2

But 5x that size if you want 12kWh of daily heating available in winter.
Minor corrections due to solar PV working better in cold conditions and
possible errors on my part converting units here and there. There are
calculators about online for any latitude & climate.

If you are serious you would do well to borrow a copy of the now ancient
book Sun Power An Introduction to the Applications of Solar Energy by J
C McVeigh, Brighton Poly, Pergammon Press 1977.

It dates from the first OPEC induced oil shock but the engineering
content is still just as valid today as it was then and it will avoid
you falling for the spiel from snake oil and green energy salesmen.

--
Regards,
Martin Brown