[Brian Graham]

I'd like to check the basics of the calculations with you folks.

Discounting inefficiencies for the moment, how does the following sound?

Say I have 2 x 60 watt bulbs that I want to run on pv. That's a 120 Watt load (1 amp ac).

If I had a hypothetical 120 watt pv panel, with an average of 2.5 hrs of winter sunshine, then I should be able to power those lights for 2.5 hrs a day.

To run the load at night or during inclement weather, we go to batteries. 1 am ac would result in a 10 amp dc flow at 12V. A 150 ah battery should only be discharged to 50%, yielding an available 75 ah. Divided by 10 amps dc, the battery would be good for 7.5 hrs of operation.

Now if I had 2 batteries, in parallel, there would now be 15 hrs available. If the batteries were in series instead, there would still be 15 hrs available but the current flow would be halved, and the wiring size could be reduced. Of course the inverter would have to be for the correct voltage.

Back to the panel. The 120 watt panel wouldn't produce 12v x 10 amps, it would actually ring in around 17V with the current reduced accordingly. That's where an mppt controller comes in, essentially bringing the voltage back to 12 and the current up to 10.

Ok. How did I do?

As for system sizing, the tendancy is to want to be sized for 100% pv at all times, but if I size for winter sun, I'll have tons of excess in the summer. It would seem to make more sense to size for summer yields and supplement with the gen in the winter.

A question about inefficiencies. I've seen some places use 85% for inverters, batteries, wiring, and I'm not sure what all else. Resulting in almost doubling the panel size. Good practice or old data?

Ok. Fire away. Be nice!

 

[William P.N. Smith]

Discounting inefficiencies for the moment, how does the following sound?

Sounds like you've got the basics down, and have discovered our dirty
little secret - This Isn't Rocket Science!

Inefficiencies will of course change your numbers, and I reccomend
properly characterizing your load, but it sounds like you've got a
pretty good handle on the basic calculations.

If this really is such a small system, you won't lose a lot by
expanding, if your system wants to start out pretty big you'll want to
do a more careful engineering analysis of course...

 

[George Ghio]

I'd like to check the basics of the calculations with you folks.

Discounting inefficiencies for the moment, how does the following sound?

First fatal flaw.

Say I have 2 x 60 watt bulbs that I want to run on pv. That's a 120 Watt load (1 amp ac).

Why AC?

If I had a hypothetical 120 watt pv panel, with an average of 2.5 hrs of winter sunshine, then I should be able to power those lights for 2.5 hrs a day.

Not likely

To run the load at night or during inclement weather, we go to batteries. 1 am ac would result in a 10 amp dc flow at 12V. A 150 ah battery should only be discharged to 50%, yielding an available 75 ah. Divided by 10 amps dc, the battery would be good for 7.5 hrs of operation.

2 - 20Watt halogen lamps would only require 3.3Amps.

Now if I had 2 batteries, in parallel, there would now be 15 hrs available. If the batteries were in series instead, there would still be 15 hrs available but the current flow would be halved, and the wiring size could be reduced. Of course the inverter would have to be for the correct voltage.

Incorrect, if you have two 7.5Ah batteries in parallel then you would
have 15Ah. In series you would only have 7.5Ah but twice the voltage.

Back to the panel. The 120 watt panel wouldn't produce 12v x 10 amps, it would actually ring in around 17V with the current reduced accordingly. That's where an mppt controller comes in, essentially bringing the voltage back to 12 and the current up to 10.

Still not enough PSH to maintain battery charge

Ok. How did I do?

Take off your rose coloured glasses.

 

[Alan Combellack]

I ran through a similar set of calculations for a bigger system for my
house in Canada some months ago. One (obvious I suppose) thing I forgot at
first was that you have to make the array of PV cells big enough to be able
to charge your batteries as well as run whatever you need during the
sunlight hours. When I got into making allowances for days without sun as
well as limited sun hours in winter the damned thing became too expensive
for me. I dropped the idea for now and may revisit it when (actually that
should probably be if) panel costs come down from the present $4 to $5 per
Watt to one dollar or less. This may take some time!! I figured that I
would not get pay back over the 20 to 30 year life time of the array. 5
years seemed like a reasonable objective but I don't know how to get it.
Alan C

 

[Ecnerwal]

The way to get 20 year payback down to 5 years would be to get the cost
down by 1/4th. Evidently, everyone that can make silicon cells can sell
as much as they can make at $5 a watt. I heard there was only one
outfit that made the silicon ingots, and they sold everything to the IC
makers, and the PV outfits had to take the scrap and seconds. Recently
I read that there is now a silicon foundary in China, so looks like
they have broken the raw material monopoly. I suppose cheap pv will
come from China first. So somebody like Warren Buffet needs to get
interested in building a vertically integrated fab that goes from sand
to pv and bypasses the middlemen. Whoever breaks the $5 a watt barrier
with big manufacturing capacity can really start cranking.

A: Your information is not correct. You don't read much, and you don't
bother to read accurate sources, so you're probably a troll. There is no
monopoly, several solar outfits already make their own silicon, and you
can buy assembled modules for less than $5/watt if you bother to shop a
bit.

B: The only benefit arising from "someone in China" (or anywhere else)
making cheaper cells will be more profit for them. If they can make it
for a penny a watt, then they can make a heck of a profit selling it for
(say) $4.99 a watt, and they will. They won't be selling for 2 cents or
5 cents a watt. This can be observed in the present market, by noting
that various outfits which claim to have more efficient manufacturing
processes are not, in fact, the low-price leaders on delivered product.

 

[Ecnerwal]

Ecnerwal:
This can be observed in the present market, by noting
that various outfits which claim to have more efficient manufacturing
processes are not, in fact, the low-price leaders on delivered product.

======================================================
Does this seem to 'violate' the economic sense of supply and demand?

Cost of manufacture has no impact (other than the profitablity of the
maker) as long as the market will bear a higher price for the product.
If the maker does not choose to invest those profits in much more PV
capacity, there's no reason to expect the price to come down. If some
other company gets into the business (and many have) they are going to
take a long time to have any impact on the market as a whole, and they
are likely to start out charging pretty much what the others do for just
as long as possible, in the interest of making as much profit for
themselves as possible. And, as a startup, they will have higher costs
than established companies, and be more likely to go back out of
business before having any noticeable impact on overall prices.

At
what level will market forces start driving competition? When
PV becomes a 'commodity' like pork bellies and sugar??

When manufacturing capacity exceeds market demand, and someone is asking
"how do we get these things to move out of the warehouse?". There was a
low a few years back down into the $3/watt region. Demand has grown
faster than manufacturing since then, so the market is working, just not
in the direction you want it to. Also, energy prices as a whole are up,
and refining silicon takes a lot of energy, so the cost of manufacture
is up. If you happened to believe incorrect pricing models that said
that the price of PV would continuously drop (as it had from 1950-2001
or so), well, they were wrong. Likewise the predictions of clean nuclear
power that would be too cheap to meter...

 

[Brian Graham]

Well that's it exactly. Sizing the PV for the 2.5 hrs winter average is bad enough. Then size it 25% bigger to be able to recharge 1 days depletion in 4 days. Ouch!

Ideally I'd be wanting the fridge & freezer on PV. $$$$

I've decided to do things differently. I'm going to start by running my bedroom/bathroom lights on PV. That and a wee bit more for a slow cooker or limited microwave usage. Then during an outage, the battery bank is available for cooking meals. Who cares about lights then?

And then slowly over time, the system can be expanded to do the fridge/freezer.

Oh. I'll be sizing it based on summer values (+25%). I'll have the battery bank sized so that it'll get me through the week. I'll recharge on the weekend from a gen or the mains.

In this way I'm achieving my goal - Reducing my dependancy on hydro and reducing impact on me when it suddenly isn't there.
--
Brian

I ran through a similar set of calculations for a bigger system for my
house in Canada some months ago. One (obvious I suppose) thing I forgot at
first was that you have to make the array of PV cells big enough to be able
to charge your batteries as well as run whatever you need during the
sunlight hours. When I got into making allowances for days without sun as
well as limited sun hours in winter the damned thing became too expensive
for me. I dropped the idea for now and may revisit it when (actually that
should probably be if) panel costs come down from the present $4 to $5 per
Watt to one dollar or less. This may take some time!! I figured that I
would not get pay back over the 20 to 30 year life time of the array. 5
years seemed like a reasonable objective but I don't know how to get it.
Alan C

 

[Anthony Matonak]

What no one has mentioned is that you should consider using Compact
Fluorescent Lights (CFLs) instead of 60W incandescent bulbs. That would
probably bring your load down to something like 2 * 15 watt = 30 watts
total.

....

They make CFL's that run on 12V.

Anthony

 

[Brian Graham]

Say I have 2 x 60 watt bulbs that I want to run on pv. That's a 120 Watt load (1 amp ac).

Fluorescent Lights (CFLs) instead of 60W incandescent bulbs. <<

Absolutely. I've already got my household lighting converted to CF's. The 2x60 was there to make some easy calulations. 120W/120V=1amp etc.

 

[Brian Graham]

They make CFL's that run on 12V. <<<

Now that's something I hadn't heard before. I'll have to check that one out!

 

[Ecnerwal]

They make CFL's that run on 12V. <<<

Now that's something I hadn't heard before. I'll have to check that one out!

Essentially, it's a bulb with a built-in inverter, and rather than go
through 120V, it just inverts to whatever the bulb itself needs. Compare
prices and consider use - if you need a lot and they cost more, an
inverter and regular CFLs might be cheaper to buy and run. For a light
that you want to use while you have your inverter shut down, they make a
lot of sense, and also do if you don't have an inverter yet.