Number crunching

Yes. I am including the main panel upgrade and roof shingle replacement in the bill. Though they are not included in the calculation of cost/W for the PV installation, for not all installers I contacted provide additional services. To compare an apple to apples, I am only including the PV panel installation cost without any incentives applied for my comparison shopping.

The thing is, I don't have enough tax liability to recoup the full tax credit in a single year without doing some creative financing. Even though unused portion of the credit can roll-over, it will take several years for me to get the full credit back. So, my aim is to keep the total upfront cost as low as possible without taking any loan. Tree felling is free for me... in fact it can generate some positive cash flow, if I can sell the logs. But site prep for ground mount frame foundation is not going to be cheaper than the roof top, I imagine.

 

I'm jealous of your winter production at 30% of summer. Over the past four years, my December production has averaged 13% of July. One year was under 10%.

But I'm not jealous of your capacity need. I've been achieving net-zero on annual production of 5,900 - 6,100 kWh and even giving the utility some free surplus energy, though the new PHEV should break that pattern. This all-electric house originally burned 10,600 to 11,000 kWh/year, but the heat pumps and numerous other improvements shrank it 5,000/year. Though conservation laziness and new usage has allowed that to float back up to 6,000/year even before the PHEV arrived.

When open land is available, I do like the ground mount idea for ease of spring cleaning (I just scrubbed off a heavy layer of pollen from nearby trees, not as easy or safe on the roof), and for separating it from the roofing cycle and/or avoiding roof penetrations when you don't want to pay for a metal roof. Though when open land isn't available, for either dense housing or forest, the roof-top system does very significantly cool the attic, saving on AC.

 

That's what the solar guys predicting on the chart. In reality, it could be much less. I am sure they tend to overestimate the production rather than underestimating.

 

With the various estimation tools available, taking into account both local weather history and location-specific tree shadowing, they should not be making being large errors in their predictions.

 

Well, they should not be making large errors, but some of them are. None of them has come out to my house to do site evaluation yet. They are all using AI assisted solar design mostly by Aurora system. The program is great for quickly updating design and making on the fly quote, but there are so much they can predict only from the satellite view of the roofs.

For one thing, it sometimes makes "wrong" assumptions such as how tall the threes are and what types of trees they are, such that parameter changes by operator can affect the solar output predictions greatly.

I now have system quotes from seven different installers. Even though the system size and efficiency are very similar, the lowest winter month production predicted varies widely.

Here are two very similar systems quoted by two different installers showing large difference in winter month production.

Installer A shows more than 30% production in Dec compared to the max production in July.


Installer B on the other hand, shows only ~10% of the winter lowest production compared to summer max.

 

Go to:

PVWatts Calculator

and plug in your location. For other parameters, just start with the defaults, though if you know the roof tilt and azimuth, change those defaults to match. Then go to the data results, and it will show you monthly estimates, which should give a reasonable winter / summer difference or ratio.

Not remembering well where you live, I just plugged in a random northern Maine location, and found it gives a much better winter result for you than for me. That would be the result of different local weather patterns, e.g. we have lots of heavy overcast during that season.

Tree interference is a separate issue not covered by the NREL calculator, and normally requires a site survey. Or an AI program with reasonably accurate mapping of your local tree interference. Google Earth has a somewhat reasonable 3D view of my location, but I don't have much idea how well that translates to improved PV estimations.

 

Thanks for the link. I will try the site. I suspect the "default" setting will give somewhere in between 10-30% production rate in winter compared to the summer high. That's somewhere in between the two extremes I showed you above. You are correct in that our weather tend to be clear sky in winter, not that we have prolonged cloudy rainy season any time of the year.

My take on the variability is due to the tree shading calculation which as you commented, without site survey would be difficult to assess accurately. I have cut down most of tall trees on the south side of the roof within 50-100 feet or so, but there are still some huge trees that would shade the roof in winter, especially early and late ours when the sun is low. Those shading patterns are not visible from the satellite view. Vegetation all shows "blob" of green leafs on satellite without showing canopy hight. One Installer commented on the tree next to our house and mentioned the shading by it may cause the reduction in production. But the tree he was pointing were group of ornamental shrub no higher than 5 feet tall. No way that shrub is going to shade any part of the roof at any time of season.

 

Just did this, using default parameter for everything but using exact location, roof tilt and azimuth. The result shows higher than the highest estimate I received from any installer. Highest in July is 1,984kWh and lowest in Nov is 819kWh which represent 41% production rate in winter. If this number is true, then I would not need the size of system quoted. It will be more than 100% offset. So, is it possible that installers are underestimating the production in order to sell me bigger than necessary system to make more money?


Here is the prediction made by an installer for this14.58kW system. The annual production from the system is predicted to be only 9,874kWh which is only 55% of the Nrel prediction of 17,927kWh... Hummm...Something is fishy.

 

Yeah, I am not worried about the trees. I am not going to clear 12 acre of virgin woods just to gain a few kWh or electricity. What I am now starting to worry is that maybe I have been quoted way larger than necessary systems from installers. Looking at the Nrel prediction, I may only need 10kW system at my location to offset 100% annual usage. That would be almost 30% less installation cost.

 

Yeah, that sound true to almost any manufactures/installers. A few of manufactures/installers I got quote have production warranty. So, if they underestimate the production initially, it would be easy for them to guarantee the level of production for many years. But I don't really care about those 25 years warranty. Who knows how many of those installers or the parts manufactures still exist 25 years from now.

I want to max out the panels on our roof to produced more electricity than we are currently using for the future whole house heat pump operation. I have looked for an installer who would help me DIY kit installation, but could not find single installer who would help me. I was initially focused on getting off-grid system with battery backup... But as long as we live in our current house at this north, retrofitting the house to off-grid is too costly if not impossible. For one thing, even with the best winter production rate, in order to produce sufficient electricity during the winter means the panel has to be over sized more than 3 times of the grid-tied system. Add the battery cost, it was easy to see the off-grid system for this house is not plausible.

 

Yeah, we have a similar company somewhat local in NE who specializes in DIY solar, altE Solar. I inquired them about my project, and they were extremely helpful, but in the end, it was still for a DIY project which I will have to do everything. I needed professional installer help on site. But that was not what they do. There are a few other companies selling DIY solar kits. I may try it someday on a smaller scale using off-grid kit on a small cabin I plan to build on our back woods.

 

Bummer , that your degraded performance is due to an INCORRECTLY made bus cable , hope that situation gets resolved

 

Do note that CA's definition of renewable hydro is much more broad than Washington State's definition. Our large scale hydro production does not count towards our in-state future renewable energy requirements. But when that very same hydro energy is shipped to California, they do count it as renewable. Here, only small scale hydro is being counted.

We even have considerable political pressure to tear down hydro dams and re-wild the rivers. The two Elwha River hydroelectric dams were shut down in 2011, and breached and torn down shortly thereafter. The good news: salmon spawning has returned very well.

Four Snake River dams have long been targeted for the same treatment, but the removal proponents haven't reached critical mass. But along the Oregon - California border, one Klamath River hydroelectric dam was taken out last year, and three more are being taken out right now.

 

Damn.