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What's in a Solar Production Estimate?

A potential customer contacted me a couple of weeks ago. She lives in a town with a municipal utility. She asked me about a solar array on her house. The first thing I did was pull up the satellite image.

From the satellite image, you can see that the roof is nearly east/west facing. I know, from using PVWatts, that east/west facing roofs are low solar energy capture roofs.

Based on PVWatts, east/west roofs with a 6 pitch (26.6 degrees) or lower produce between 80 and 84% of an ideal roof. Roofs of 7 pitch (30.3 degrees) and above produce at below 80% of an ideal roof. It's important to note, that these PVWatts production readings DO NOT factor in the production loss due to shade. (The partial table below was built using PVWatts and shows the easterly solar production relative to the ideal roof in the Worcester MA area.)

To understand the impact of shade requires a site visit and use of a shade evaluation tool such as Solar Pathfinder (~$300 + camera), Wiley ASSET (~$500, includes camera), or Solmetric SunEye (~$2,000).

Each of the devices listed above works slightly differently but all will give you roughly the same answer.  (The main difference between the SunEye and the other tools is speed of result. The Pathfinder and ASSET require you to analyze your digital photos on a computer. The SunEye does the analysis on board.)

In all cases the devices start with roof pitch (angle) and heading (azimuth) information. Why? Because they will all apply production adjustments from PVWatts then subtract out the shade impact.

Ultimately we are looking for the site's relative production when compared to an ideal site. An ideal Massachusetts site faces true south (azimuth 180) and has a tilt of around 40 degrees (42.5 if you ignore weather, 37 if you don't). In Massachusetts, the CEC (solar rebate organization for customers of National Grid, NStar, Unitil, and Western Mass Electric) will not approve a rebate unless the site performs at 80% or better of an ideal site. This makes sense because they don't want to fund a site that will perform poorly. (Unfortunately, the municipal utilities do not have the same requirements or oversight.)

Now, back to my customer above.... After I told her I thought the roof would perform poorly, she forward me an email from another solar installer (I don't know which one).

Here is what that installer said (un-edited by me except removal of some potential identifying information)

First to start with your shading numbers I got from the Solmetric Suneye.  The day I was there, the shading readings I got were between 90-94% on either side of the roof.  In the quote I sent you, I used the lower reading of 90% just to be conservative.  Now to understand how we came up with the estimated output I'm providing what factors go into it and the industry standard for calculating it.

1.  We input the Azimuth of each roof as well as the pitch angle into the quote model

2.  We take a conservative shading percentage based on Solmetric suneye (in your case 90%)

3.  We plug in the modules and inverters being used in estimate

4.  The quote model the uses PVWatts to estimate your output.  PVWatts is an industry wide tool used to estimate solar output.  It takes 30 years of weather history to make the calculations.  

While the process described above appears sound, by my standards, something is missing. The solar installer never told the customer how well the roof would actually perform. They say that they put the angle and azimuth into their quoting model along with the 90% shade and from that information, they calculate an annual production (which they then use to estimate electricity savings and economic return). But they never say how good or bad the roof is. In fact, they leave the impression that the roof is a 90% roof.

So how good is her roof? Since the back of the house has a heading of about 97.5 degrees and the roof pitch is 35 degrees (I visited the site and measured the roof), we can expect the site, AT BEST, to perform at 80 to 81% of an ideal roof. If the shade reading was 94% (the installer's best estimate), then the production is 76% of an ideal roof. If the shade is 90%, than the production is 73% of an ideal roof. In either case, the site is well below the standard set by the Mass CEC.

While the customer is free to do what they want, solar is a big investment. I think they should be told that their roof is far less than ideal. They should also know that the low production level will dramatically extend the break-even time for the array.

Sanyo Panel Performance: Real World Data

5,125 watt Sanyo array (25 x 205-watt panels)

Satellite Image

We've been installing Sanyo solar panels for some time. I've always known they were good performers. Now I have data to prove it. Here are the results for a site we have in Eastern Massachusetts. Later I'll publish more results from other sites.

The first site is pictured above. Just under three years ago we installed 25 Sanyo HIT power 205-watt modules for a total of 5,125 watts. The inverter for this system was a Solectria PVI-5000 string inverter. The system was wired with five strings of five modules. The combiner box was located in the home's attic. The combined DC was run from the attic to the inverter in the basement. We also installed Solectria's remote monitoring system "Solrenview."

The home's orientation is within a few degrees of 180 and the array tilt is approximately 35 degrees. Using the Solar Pathfinder, I estimated that the site would perform at 98% of an ideal site because the only shade on the roof was from the roof itself (late day shade).

If you use PVWatts (for Worcester, MA), a 185 degree heading and a 35 degree tilt, the annual production is estimated for a 5.125kw array is 6,263 kilowatt-hours.  Since this system went live on April 23, 2009 (within a few days of 35 months) by PVWatts estimates, this system should have produced about 17,937 kilowatt-hours (35 months at an average production of 512 kWh per month). When discounting for shade (at 98%), the total production should be 17,578 kWh.

What we've actually seen however, is considerably better. When we pull up the production on Solrenview, we see that the site has produced 18,655 kWh. This is approximately a 4% per year improvement over my initial estimate. (FYI, the Solrenview tends to record fewer kWh than the revenue grade meter we install next to the inverter).

The customer probably paid a 20% premium for the Sanyo system over standard mono or poly panels (I'd have to go back and dig through my contracts to verify). That 20% spread out over 20 years is a 1% per year increase in cost. However, he's realizing a 4% or more annual improvement. I'd say he's getting his money's worth.

Here's an interesting article by Kurt Cobb over at the The Energy Bulletin. It's about an article by Nassim Nicholas Taleb and Mark Blyth. While I haven't yet read Taleb's article, Cobb gives some insight.

I've read one of Taleb's books (Fooled by Randomness: The Hidden Role of Chance in Life and the Markets) and found it quit interesting (if not a bit scattered).

The apparent thesis of his recent article is that suppressing volatility creates a worse situation than not suppressing it. As a (recovering) engineer, I can agree and I can cite a number of examples.

The first one that came to mind was the airplane wing. The wing is designed to flex. If it cannot flex, then it will likely break. On a related note, the Wright brothers realized that the wing needed to be flexible for the plane to fly. I have not looked into the physics of this but it appears that the Wright Brothers were right! (pardon the pun).

The more obvious one is in parenting. Imagine you are at a public event (like church) with a toddler and that toddler starts to act up. Sometimes shushing will get him or her to quiet down for a while, but eventually, the restless cherub lets loose.

Lowering my Energy Use Step 1: Where is it going now

I recently had an energy audit done by a company who shall remain nameless (their service after the audit has been embarrassing). Despite their post-visit incompetence, I did learn a few things, and I started a project to evaluate where my energy goes. Here's a quick summary*:

First: Yes I am ashamed of our electricity usage. I have been trying to change the family behavior but it is a losing battle. I'll not go into detail because I value the relationships with my wife and children. I suspect that in the fall, when two go off to college, our electricity usage will drop dramatically and we'll see a bit of a drop in gas usage.

Second: Notice that our heating energy usage is three times our electricity energy usage (in BTUs) yet the cost is about the same. That's right, I purchased more than three times the energy from the gas company but it cost me about the same as my electricity. In other words, electricity per unit energy is more than three times the cost of natural gas. (If there were an efficient way to generate electricity with gas, I would consider doing it.)

Knowing these numbers, I can now begin to evaluate options for energy reduction because I can determine the costs and benefits of many different actions and pick the most effective ones. For example, I now know that every kilowatt-hour I save is worth about 11% of a therm (11,000 BTUs). Or every 100,000 BTUs of gas (1 therm) I save is worth just over 87 kilowatt-hours.

What direction will I go?

Electricity: After switching all of our light bulbs to compact fluorescent and upgrading to Energy Star appliances, the next major leap on the electricity use reduction is behavior change or kicking out some of my family. Since I have two starting college in the fall, kicking out is easy (though expensive). As I've found over the years however, getting people to change behavior is not.

Gas Usage: Cooking is a relatively small portion of our gas usage. The bulk is heating. I've put in the auto-setback thermostats and added insulation to the attic. I'm just getting started on tightening up the house. I believe that the biggest bang for the buck will be reducing the passive loss of heat. Here are some steps I'm considering:
1. Remove fiberglass insulation in attic, spray foam on the ceiling, seal all electrical and other penetrations, replace the fiberglass. This will reduce air leakage to the attic.
2. Seal around all electrical boxes (outlets, switches)
3. Spray foam insulation at sill plate (transition between foundation and house)
4. Seal around all windows (remove moldings, and foam between window and framing.
5. Remove siding and put up rigid foam, tape joints, reattach siding
6. Put 2" of rigid foam insulation on exposed concrete that abuts living spaces
7. Insulate walls between garage and house (garage is unheated).

Granted some of these projects are "small" (#2) and some are huge  (#5). I'll be starting small. First and foremost, I'll be addressing air leaks. (#1 and #2)

-Mark


*Electricity supplied by Hudson Light and Power. Gas supplied by NStar. I divided the total bill by the total usage. Therefore delivery charges etc. are averaged over each kilowatt-hour or therm.

Thermo Bio-pile or heat from compost - Part 1 - Project Background

Back in the fall of 2010, my son Russell was looking for a science fair project. He wanted to enter the regional HS science fair competition. After discussing several ideas, he remembered a web page we discussed about getting heat from decomposing material.

I first learned of the idea from one of my solar customers. She was a long time reader of MotherEarth News and sent me this link right after we installed her solar electric system. (FYI, this customer has a 1,500-watt solar electric array which zeros out here electric bill each month and she grows about 50% of her food on an 8,000 square foot lot (including the house) in Jamaica Plain (Boston).

The link takes you to an article about Jean Pain. In the 1980s Pain built piles of compost and threaded them with piping to capture the heat of decomposition and the methane. It was an early example of someone attempting to live sustainably off renewable energy. Pain's heat capture method was based on water, and, in effect, worked the same as a solar hot water (solar thermal) system.

After getting approval from his Science teacher, Russell began his plans. The key difference between his system and Pain's system was that he wanted to heat air rather than water. He felt that an air system would be easier to build, less costly, and less complicated. I agreed.

NOTE: This was a purely empirical experiment. Russell does not yet have the background to go deeply into the thermodynamics of a system like this and determine beforehand what to expect. Instead he relied on common sense and a bit of  input from me (Dad, a recovering nuclear engineer). I did play a relatively small roll - as the rules of the science fair dictate. (Though after going to the science fair, I'm doubtful that everyone followed that rule.) I also had to play "reminder of the budget limitations" and reminder of the stated objective - "To learn if useful heat could be captured from decomposing material using an compost-to-air-based heat exchanger."

Next Post - Part 2 - Experiment Design

Today's Theme - Limits to Economic growth

I don't pretend to understand all of this but ...

Here is a research paper that treats energy limits on organism growth as an analogy to energy limits on economic growth. The parallels are enlightening. http://www.aibs.org/bioscience-press-releases/resources/Davidson.pdf

Here's an article on "Tainters Law." His thesis is that social structures generate negative returns when they become too complex. http://ourfiniteworld.com/2011/03/31/tainters-law-where-is-the-physics/