I recently gave a talk in Brunswick (“Clean Energy Independence”) at which I handed out a document that I had prepared for the 2013 solar tour. Here is a copy for those who missed my talk and have requested a copy:
Fred & Hadley Horch, Green Buildings Open House 2013
Our house was built in 1828 and has been falling down ever since. Ever the optimists, in 2012 we had a new metal roof put on over our existing shingles. This gave us a nice platform which should last 50 years on which to mount solar collectors. For 184 years our home’s roof was simply heating up all day and cooling off all night without any of that energy being harvested; we’re eager to make up for lost time.
Our goal was to maximize the amount of energy we collect from our roof, so we installed both a hot water system and a photovoltaic electric system. Trees owned by the Town of Brunswick and our neighbors partially shade our roof some of the year, but otherwise our roof is ideal for solar. Our roof slope allows an unusual configuration: our hot water collectors are on the north side, while the PV panels are on the south.
Our solar hot water system provides domestic hot water with an electric back up; it heats a large tank of water that we use for sinks, showers, dish washing and clothes washing. This system is not tied into our space heating system, nor is it powered directly by our solar electricity panels. Instead, the pumps and the electric back up heating element are powered by the grid. We chose evacuated tube hot water collectors for two reasons: first, wind blows around the tubes so we can tip them up on our roof (and keep them up out of the snow in winter) with less risk of problems, and second, they perform better in cold temperatures than flat plate collectors.
Our solar electricity system is grid connected with no batteries. However, unlike older systems which are unusable in the case of a grid failure, our inverter does have a plug that allows us to use our solar electric power even if the grid goes down. In case of an extended power outage, this “solar plug” might come in handy to keep a freezer going or to charge our electric car. Recently we had the chance to test this plug when the grid went down in Brunswick. It worked!
You may notice that we have two service entrances and four electricity meters on our house. When we bought the house, we had two meters: the first for the main house and the second for an in-law apartment. One 200-amp service entrance provided enough power for the entire structure.
We upgraded from heating oil to electric heat as part of our commitment to stop directly buying and burning fossil fuel. We replaced our oil boiler with an electric boiler, allowing us to keep all of our existing distribution plumbing and control systems. Switching to electric heat did require adding a second 200-amp service and a third meter for that new service, which is used to heat our house and charge our electric car. CMP decided to replace the transformer on the pole across the street, and the wires from our house to the pole. Once the utility work was done, swapping out the oil boiler for an electric boiler was quick and easy. Everything was done in the space of a few hours.
When we added the solar PV, CMP came out to add a fourth meter to measure the electricity our panels produce and send to the grid. Our panels are wired as DC strings which feed a central inverter in our basement. From our inverter, an AC wire feeds into a panel that supplies our boiler and our electric car. This panel is connected to two meters to allow us to participate in the net energy billing program. One meter measures how much power comes from the grid to our panel; the other measures how much power goes from our panel back out to the grid. Every month, our electricity bill shows these amounts and we pay the difference (if we use more electricity than we generate in a month).
We lower our cost of electricity by participating in CMP’s time of use billing [note: as of January 2014 we no longer participate in this program due to price increases; instead we purchase green electricity through Union Atlantic Electricity]. We generate solar electricity mostly during peak hours when rates are higher, whereas we use grid electricity for space heating and car charging during off-peak hours when rates are lower. As a consequence, we expect to net out all of our high on-peak usage and pay only for low off-peak power. In the summer, when our heating system is off, we have generated more electricity than we have consumed. This extra electricity can be banked for up to year. We expect that in September we will begin using more electricity than we produce. Our PV panels were installed in July 2013 so we do not have much data on their production, but so far they are tracking expectations per the online PV Watt estimator.
Due to the meter placements, electricity that we use directly does not get counted at all. We can tell how much electricity is being used “behind the meter” by comparing the output of our inverter with our meters. For example, if we are charging our car during a sunny day, power is produced on our roof, converted to AC through our inverter, and distributed to our car through our panel, but neither the incoming nor outgoing meter detects it. The kWh log on our inverter increases, but neither utility meter increases.
We expect the pumps on our hot water system will be the first components to fail. Our inverter should last for at least ten years, our hot water collectors at least 15, and our PV panels at least 25.
The upgrade to all electric heat was an opportunity to remove a chimney and eliminate a combustion system that drew cold air into our house whenever our oil boiler fired. Our house is now safer (no exhaust gases are being produced inside) and more snug. We are tracking our total energy consumption to quantify our efficiency gains from upgrading to an electric car and heating system. Anticipating that we will need purchase grid electricity once heating system gets underway, we have signed up for the green electricity program through the PUC. This program promises that all grid electricity we purchase is generated from clean sources in Maine.
People sometimes wonder how our grid can handle the extra power that solar panels are generating and feeding into it. In comparison to other loads that come and go, the amount of power that our rooftop solar panels produce is relatively modest. In full sun, our roof produces about 3,400 watts. That’s about twice as much power as our toaster requires. (On the other hand, that’s also about ten times as much power as all of the light bulbs in our house require, since we use super efficient, high quality LEDs.) So if neighbors on either side of us turned on their toasters, they’d suck up all the solar power we are providing to the grid. Over the course of the day, our panels ramp up from 0 to 3,400 watts, depending on sun angle, temperature, cloud cover, and shading.
We have a lot more roof space that could be covered with solar. Unfortunately, being in a small in-town lot, we are surrounded by trees that we do not own. Assuming a mysterious disease does not suddenly infect our neighboring trees, any additional collectors will need to be very carefully placed for us to get the maximum return on investment. Still, we expect the investment we have already made in solar will more than pay for itself financially. The peace of mind and sense of doing the right thing that you get with solar are a nice bonus!
Fred Horch owns Spark Applied Efficiency, which helps businesses become more valuable and sustainable through efficiency. If you’d like to take personal action to save energy or prevent pollution, he’s happy to provide free advice and compare notes!