> ...based on a contract with Ontario Hydro
> to buy the generated juice at an obscenely high price

As long as your solar system makes LESS than your minimum energy usage
rate during PV operating hours then the least it pays you is the  rate
per unit, as every unit made by PV is a unit not paid for at retail.

This even applies to hot water heating if energy storage is efficient.
While a thermal solar panel should be more cost effective, if you have
spare solar PV capacity then thermal energy storage in water is better
than not using it.

Then

> Denny Esterline
Said:


> For some time now I've been a bit of an anti-PV solar person. In my mind =
I
> just don't see the math working as a net gain.
> It gets a bit messier when you start adding in various governments tax
> manipulations,

It's still VERY marginal.
There is a major wrong but easily made assumption below.

> but based on this alone...
> $2NZ / W ~ $1.70 USD / W and $0.15 kWh electricity...
>
> 1kW of panel =3D $1700
> Avg sunny 12 hrs / day

Sunny hours and equivalent-full-sun hours are very different.
I use the term "Sunshine hours" (SSH) for nominal full solar output of
"1 sun" =3D 1 kW/m^2.
Some areas can get as high as 1.3 or so kW/m^2 in peak oeriods but 1
sun =3D 1 kW/m^2 is the std.

SSH by month by geographic area can be seen at the superb www.gaisma.com si=
te.
You'll see that a GOOD summer day gives 5 - 7 kWh/m^2/da. 7 would be
exceptional. Many places get < 5.
In winter this drops to typically 2 kWh/m^2/day and is under 1
kWh/m^2/day in New York NY, and about 20 minutes (all diffuse) in
Moscow, Russia.

So the 12 hour/day figures is perhaps high by a factor of 2.

For very long payback periods (decade plus) a proper discounted cash
flow calculation shows very little benefit from the later years  as
the effect of interest rate is crippling (aka if you invested your
money elsewhere at typical going rates .the compounding of interest
starts to make saving more attractive than a scheme that gives inly
modest benefit 20 years fromnow0.

> Panel offsets 12 * $0.15 of electrical cost per day =3D $1.80 $1700/$1.80=
 =3D
> 944 days
>
> So basically three years for payback, not including mounting, wiring,
> battery systems, chargers, maintenance, lost interest on the invested
> capital, etc.

So probably double that.
Then start allowing for reduction of value of return in later years
due to interest effects.

> I would guess actual installed cost of complete system to run 3 to 5 time=
s
> the cost of panels alone (please correct me if you have better informatio=
n),

I am much less familiar with this part of the system BUT I understand
that in high volume installations PC costs used to dominate when
prices were higher. But  probably still a noticeable part of system
cost.

> So effective payoff in 10-15 years - assuming nothing interrupts their
> functional life span. (baseball playing neighbor kids come to mind)

So that becomes 20-30 years and subsidies start to be needed.

Current industry std panels should exceed 20 years *BUT* some that
should won't, and some are still running OK at 30 years.

> This is considerably better than the last time I did the math a few years
> ago. At that time I was seeing 50 year plus payoff time making widely
> optimistic assumptions about maintenance costs. I may need to change my
> opinion...

Maybe :-).
However, there is another factor above that needs adjusting. Current
cost at factory door in China is around $US0.65/ Wmp for say 50W -
200W  panels.
Add freight, tax, insurance, handling and it is still under  $US1.70 /
Wmp. How much less is tbd. Shipping can be about $US0.10 / Wmp to NZ
I'm told. We'll see.


                  Russell
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