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Solar Energy Per Square Foot In Temperate Zone? - Page 3

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Posted by Morris Dovey on September 21, 2009, 12:47 pm

Ken Maltby wrote:

Interesting - that scales to 890 W/m^2, but the "maximum" warns me that
the value may only apply on the ecliptic at solar noon with ideal
atmospheric conditions. I assume that Soo's value applies at sea level.

I would expect that, all other things being equal, an increase in
altitude might increase the actual value, while increases in either
displacement from the ecliptic or time displacement from solar noon
would decrease the actual value.

Hmm - it might make an interesting leisure time play activity to try
putting together a model to approximate maximum insolation at any given
altitude, latitude, and date-time...

Well, it might matter if we're trying to work from total insolation to
calculate usable input to a cell, so that there's a meaningful value to
which the rated efficiency can be applied, yes?

That makes sense.

I'll take your word for this (it seems intuitively right) and recognize
a need for me to learn more about this.

Agreed, although I suspect that in the foreseeable future the huge
majority of panels will be in fixed or semi-fixed installations.

Yuppers. One of my "back burner" projects has been a solar
(non-electric) pump for water-cooling PV panels. My focus has been
irrigation and remote-area water supply, but I understand that PV panel
performance can be improved by cooling the cells.


Morris Dovey
DeSoto Solar
DeSoto, Iowa USA

Posted by (PeteCresswell) on September 21, 2009, 2:23 pm

Per Martin Riddle:

Seems like my 'joker-in-the-deck' is at least possible then -
unless some basic law(s) of physics preclude higher efficiencies.

Double that efficiency and you've got 4kw possible....

Seems like I read about a project somewhere (in Europe?)
dedicated to exploring something like "The two-kilowatt
lifestyle" where everybody on the planet lives a reasonable life
on a total (sustainable?) energy consumption of two

Posted by bw on September 22, 2009, 6:06 am

You don't specifiy your current consumption. Common value for that is over
10 kWh per day.
Shows you will get about 3 kWh/m2 per day of solar in December and January.
You will need 10 square meters of panels to convert solar input to the same
value electrical.
You will never recover the cost of the panels if you live outside the
southwest USA, unless the cost of your electrical power from the grid starts
to get close to one dollar per kWh.

Posted by (PeteCresswell) on September 22, 2009, 11:38 pm

Per bw:


I was thinking in terms of what hope there might be globally for
the future - and it sounds to me like if somebody comes up with
really-cheap, really-efficient photovoltaics there's some hope
there of making a significant dent in the energy situation.

The operative word, of course is "If".... but at least it sounds
like the energy per square meter is there.

Posted by Roberto Deboni on September 23, 2009, 12:04 pm

(PeteCresswell) wrote:

When looking about solar energy, power (Watt's) is not very meaningful.
It's better to look the available daily energy (kWh's or BTU's).

For Philadelphia:


that is, at optimal tilt, the following solar energy availability:

    kWh/m2/day  kWh/sf/day  BTU/sf/day
Jan    3.5        0.325        1.11
Feb    4.1        0.381        1.30
Mar    4.6        0.427        1.46
Apr    4.7        0.437        1.49
May    4.7        0.437        1.49
Jun    4.8        0.446        1.52
Jul    4.8        0.446        1.52
Aug    5.0        0.465        1.58
Sep    4.8        0.446        1.52
Oct    4.5        0.418        1.43
Nov    3.5        0.325        1.11
Dec    3.1        0.288        0.98
Year   4.3        0.399        1.36

giving your roof a solar energy availability from 2100 BTU/day (winter)
to 3100 BTU/day (summer).

At 15% electrical conversion efficiency, you may gather from 90 kWh/day
(winter) to 130 kWh/day (summer). That's more than 35'000 kWh/yearly
But PV it's a very costly solution. And storing electricity is also costly.
Heat capture and storage is better.
In winter, concentrating at useful temperatures, you get at least 50%
efficiency, that's about 1050 BTU/day at say 170 F
And solar thermal collectors are cheaper:


and vacuum tube solar collectors give hot water also in cold climates


Heat storage is more affordable (you store hot water), and if you have
space problems,  Glauber's Salt tanks can reduce your needs:


I would start covering your household heating budget.
For example 600-700 BTU/day ?
About 1400 sf of roof would be covered with thermal collectors.
And 600 sf of roof with PV collectors (about 8 kWp) producing about
10'000 kWh/year

Summer air conditioning could be made with an absorption heat pump


Remember: you have a lot of heat (1500 BTU/day in summer)

The answer is Yes.
But an only PV solution is not the best one.
A mix of 2/3 to 3/4 of thermal collectors and the remaining as PV is a
better choice, returns more useful energy. And cheaper (or less costly
... :-)

Roberto Deboni

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