Wednesday, October 07, 2009

Finally up to 7.5 Kw of solar

In spite of wind, rain, and visiting friends, I finally finished our solar panel installation. We now have 36 panels on the roof. Each of them is rated for 208 watts, so the total production capacity of the array under ideal conditions is 7488 watts. Today, when the sun appeared between the clouds, I saw the system reach its theoretical maximum of 7.5Kw! The best number I could capture today after getting my camera out was 7.2Kw.

Speaking of clouds, it is possible (and I saw it happen when I had fewer panels on the roof), for the array to put out even more than its theoretical maximum when the sun is peeking around the edge of a cloud. I won't pretend to understand the optics, but the edge of the cloud focuses the sun on the array. The extra power is trivial since the condition only occurs for a brief period of time (clouds move!). But it is important to consider this "peaking" effect when sizing fuses, wires, and battery chargers.

Here's a picture of the cloud obstructed sun earlier today, trying to shine through the timbers and our as-yet-untrimmed windows on the south side of the house.


Blogger brad_bb said...

If I remember, you store power with batteries, correct? What happens when your batteries are charged and you are making more power than is needed?

October 8, 2009 at 1:00 AM  
Blogger oldmilwaukee said...


When the batteries are full and we don't need to turn on the geothermal unit, I go crazy watching all of the wasted opportunity! Our next car is going to be electric... even if I have to build/convert it myself. I could be charging a car in the garage every time the house batteries are full. I would prefer that to selling it back to the grid.

October 8, 2009 at 9:11 AM  
Blogger oldmilwaukee said...

Wow, this article probably doesn't deserve its own blog posting, so I'll stuff it here in the comments. Can y'all believe this?

October 8, 2009 at 10:19 AM  
Blogger Jacob said...

This is like comcast saying,"here's your new tv" and charging you for it the next month.

October 8, 2009 at 12:36 PM  
Blogger oldmilwaukee said...

I think that's a great analogy Jacob.

Imagine 60 cents a month (in perpetuity) for a $3.50 light bulb! "Green Gouging" is still gouging.

October 8, 2009 at 12:47 PM  
Anonymous Anonymous said...

If you have a moment I would love to see a schematic of your PV system. Nothing fancy, similar to your hot water schematic maybe.

October 9, 2009 at 2:48 PM  
Blogger Shawn said...

Thomas, do you have any recommendations as far as what type of solar panel(s) are best? There are tax credits available that one might be able to offset some of the upfront costs. BTW, what can one expect to pay for enough solar panels to cover approximately 11 to 12 squares of roof surface area?

Nice blog - always interesting to see what you're up to!


October 15, 2009 at 4:19 PM  
Blogger oldmilwaukee said...

Affordable-solar is a good place for the DIYer to shop. I chose these Sharp panels because their dimensions worked out to give me complete coverage of my south roof, with only 1" gaps between the panels. The Sharp panels are assembled in TN. Kyocera makes good panels too.

My panels cover 20x30 feet presently - 600 sq ft, 6 square and are rated for 7.5 Kw. If I bought them today at the going rate of $3.50/watt, that would be about $26,000. MMPT's and inverters and wires and connectors are going to bring the total price to $40,000. Then batteries are another $10,000 and you are at $50,000. Not exactly cheap. This is for 6 square, so double for 12 square.

October 16, 2009 at 6:10 AM  
Anonymous Anonymous said...

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October 22, 2009 at 8:12 AM  
Blogger oldmilwaukee said...

aahhh - I would so like to have one of those toys. Thanks for the link! I love the concept of a vertical rotor. Whatever the technology though, I've heard it all comes down to "swept area" and "wind speed" (which is almost always a function of tower height). My wind resource here is crap, but that's just an anecdotal observation. I should get a wind speed monitor and try it in different places up here on this knoll.

Whether hydro or wind, I would love to have another resource that produces something (anything!) at night. If I could get 1.5Khr/night my batteries would be much happier. (my freezer and fridge don't stop at night!)

October 22, 2009 at 9:04 AM  
Blogger pete said...

love your blog. it appears you have hills, have you considered pumping water up hill to a holding tank when your batteries are fully juiced and the sun is out? then you could run it down through a small turbine as emergency back up. you would lose power to pump and turbine inefficiency but it could be a good source of back up 'cloudy' power. some hydro utilities out here in the northwest do this on a large scale to time shift their production for peak power.

October 30, 2009 at 6:01 PM  
Blogger Thomas Massie said...


You're absolutely right - our hills are perfectly suited for something like that. We have about 400 ft of elevation difference from "bottoms" to "ridges" here at my house site, with my house being about halfway between the two. The big commercial hydro storage facilities claim 80% or better, but I just don't see how I could do much better than 25% with off the shelf micro-hydro stuff. Pumps look to be about 50% efficient and micro-turbines have similar efficiency. So round trip is about 25% efficient? Still might be worth doing. My batteries were plumb full today and the solar chargers more or less shut down to avoid overcharging the batteries.

October 30, 2009 at 9:16 PM  
Blogger pete said...

another idea. use your extra solar power to heat your manure digester to produce methane!

November 19, 2009 at 11:47 PM  
Blogger solarben said...

I just read this blog and I love it. Anyone installing solar really gets the "big picture" in my opinion, and it's really gutsy to take it on yourself! I myself just left the engineering world (designed rotating parts at GE Aviation) to pursue a career in solar energy. So I'm now the Cincinnati Operating Manager for Solar Energy Solutions, and I'm so glad I did it! My work is making a better future for my kids, as is the solar on your home, and especially your blog for keeping solar in the public eye! Thanks for taking the time to make this blog. Let me know if I can ever help with anything solar.
Best regards,

January 12, 2012 at 10:51 AM  

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