Monday, June 16, 2014
The system was initially sized with the intention of completely canceling out our electric bill. Of course, there are a number of factors that can cause actual generation in any given year to vary from projected generation, of which variations in weather is probably the largest.
Overall, we came pretty close to hitting our target; our net electric bill for the year was $100, only about 5% of our actual electrical usage. I'll call that a success.
I knew that we'd get a lot more power generated in the summer than the winter -- longer days, better sun angle (power generation is proportional to the cosine of angle of incidence), clearer skies, and no snow on the roof. But I was surprised by just how big the swing was -- almost a 10x difference between generation in June and generation in December.
Below is a graph of cumulative power generation by month over the past twelve months; the Y axis is kWH.
Saturday, May 11, 2013
We chose to lease rather than buy. A number of companies now offer leases on solar power systems, usually for a 20 year period, where they own and maintain the system, act as general contractor for procurement and installation, and handle the tax/permit/utility paperwork. Leasing was cost-competitive with buying -- and far less hassle and paperwork. The leasing company offered a choice of a monthly fee or a one-time payment. The one-time payment is a far better deal; the monthly payment basically represents financing at 7%. The install was completely hassle-free. The system uploads data to a monitoring site, so you can get graphs and reports of your production.
In Vermont, there are three sources of subsidy for solar: a 30% federal tax credit, a state tax credit based on generating capacity (which came out to about another 10%), and utilities buy your generated power at a premium. (Some utilities will only give you a credit; ours (Green Mountain Power) will cut a check for any credit balance at year end.) With a lease, the leasing company gets the tax rebates (which reduces the cost of the system) and the homeowner keeps any payments for the power generated, and the leasing company handled all the tax paperwork and associated risk.
I was apprehensive about whether to believe the projected generation capacity; with a month of data, I am gaining some confidence that they were reasonable. Based on these projections (and assuming that power rates stay the same), the system should offer a ten-year payback and a 8% return on investment. In hindsight, I would have gone with a slightly bigger system (there's still plenty of room on the roof); the standard approach seems to be to size the system to net out your power bill to zero, but this seems more of a psychological than financial target.
The key risk items are:
- Generation. The system may generate less than projected, though the first month looks promising. (To meet their targets, I need to average generating 26 kWh/day through the year. In May, we averaged 38 kWh/day; I would expect to generate even more in July/Aug and much less in Jan/Feb, but it is believable that we will hit this average.) Even if the projections are accurate, we are of course still dependent on weather.
- Changes in utility policy. Green Mountain Power offers an effective 7c/kWh subsidy on top of the regular tariff for any power we generate. However, the company could change this policy, and probably will sometime in the next twenty years.
- Change in power rates. If power rates go up, the return is better; if power rates go down, the return is worse. I have to assume over 20 years electric rates will go up.
Monday, June 7, 2010
Thursday, May 20, 2010
About a week ago, the system started behaving badly -- IE crashing, Thunderbird crashing, and starting yesterday, the whole thing blue-screening. After wasting a lot of time trying to figure out "what software was updated recently", I started to suspect memory errors. So I ran the Windows memory test program that shows up on the boot screen -- nothing.
After more dorking around, I downloaded and ran MemTest86+ (www.memtest.org), burned it to a USB drive, and ran it. It immediately found thousands of memory errors; by trying various combinations and moving modules from slot to slot, I was able to identify the bad modules. I had bought Crucial's top of the line (Ballistix Tracer LED) from Newegg; the Crucial folks immediately shipped out a replacement.
Given how many errors MemTest found, its amazing that the Windows test found nothing.
Thumbs up for MemTest86+ and Crucial customer service. Thumbs down for Windows Memory Test.
Tuesday, May 18, 2010
Sunday, January 31, 2010
This is a nice little book about the history of mathematics and the 350-year quest for the proof to Fermat's Last Theorem. It was written by the fellow who wrote the BBC / Nova TV special on Andrew Wiles, but includes a lot more information than a one-hour show could. It does a nice job at hitting many of the high points of mathematical development from Pythagoras to modern day, including the "discovery" of zero, then negative numbers, then imaginary numbers, techniques for grappling with infinity, Turing-computability, and Godel's incompleteness theorem. It doesn't attack any of these in great depth, but it does provide a nice historical perspective while remaining about as accurate as a lay book can do. It also does a nice job of illustrating the near-hubris required for Wiles to lock himself in a closet for eight years in order to solve a problem that had eluded mathematicians for centuries. Mathematicians will enjoy the panorama; non-mathematicians will likely find the introduction to some of these obscure concepts accessible and enjoyable. Also by this author: The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography .
(Recommended to me by: Stuart Marks.)
We've recently gone on an "energy efficiency" rampage at the house, replacing bulbs with CFLs, identifying devices that are unnecessarily left on all the time, wrestling with Windows to stay asleep during periods of inactivity, etc. We also recently just installed a "continuous" or "on demand" hot water heater, replacing the 50G direct-vent tank heater we had (it was getting to the end of its lifetime and it was easier to replace it preemptively.)
Unfortunately, the state requires all newly install water heaters to have a thermostatic mixing valve that limits the water temperature to 120 degrees. (For tank systems, it is recommended to keep the tank water at 140, to prevent the bacteria that causes Legionnaire's disease, but 140 is hot enough to scald. But continuous systems have a control system for the output temperature, so can be safely kept at whatever temperature you program in.) And its probably not even working right, since the output temperature is even less than 120. The valve adds cost to the system and to the installation (probably a dozen additional welds in addition to the valve), and while we now have an infinite supply of hot water, generated more efficiently, its not as hot as we like it.
Reputable plumbers are not able to remove or bypass the valve, which means we need to either find a disreputable plumber or I need to do it myself (read: find an incompetent plumber.)
Note to lawmakers: in my many years of successful shower use, I've learned a secret trick to avoid getting scalded: put your hand under the water first -- if its too hot, turn down the water temperature before getting in!
Thanks, elected officials, for making my house systems both more expensive and less useful.