We recently had solar panels installed on our home by Real Goods Solar in March of this year. This post describes some of the process and analysis that went into the decision, the installation and tracking the results.
We are a pretty green family and try to minimize our impact on the environment. Chris has been very interested in energy use and the environment for a long time (including majoring in this subject in college and grad school and is currently working in the field).
Solar electricity produces no air pollutants or greenhouse gas emissions during operation (there is some energy and pollution generated when the panels are made, but that is typically offset in a year or two). It also produces electricity during times when demand is highest which helps to alleviate stress on the electric grid. Peak electricity use in California occurs in summer afternoons, when air conditioning use is highest. Since we don’t have air conditioning, the excess electricity that we generate during these hours helps to reduce demand during peak hours (which offsets electricity use from inefficient and polluting peaking power plants) and reduces strain on the transmission and distribution system.
There were a couple of ways to get the solar panels: buying or leasing. With leasing, you don’t own the system and you pay the solar company instead of the utility for the electricity that the system generates, but the big benefit is that you don’t have to pay much (or anything) upfront to go solar. With buying, which is what we did, you pay for the system upfront, but then you own the system and we won’t have to pay PG&E or anyone else for electricity ever again.
The net system cost after federal and utility rebates was $9500. A big question that many people have is how long will it take for the system to “pay” for itself through savings on our electric bill. The answer is, it depends. It depends because we don’t know what PG&E rates will be in the future. If we continue to use the same amount of electricity and if electricity rates stay the same in the future, then it’ll take about 20 years before we save enough money to pay for the system. If rates rise at 2-3%/yr (the historical average), then our system will pay off slightly faster in 16 years. Basically, we have pre-paid our electricity bill for the next 16 to 20 years and our electricity will be free after that (hopefully another 20+ years). Solar panels typically lose about 0.5% efficiency each year, so after 40 years, the maximum output will still be (hopefully) around 80% of the rated power. 16 year payback is equivalent to a 6% return on investment.
One of the reasons why it takes so long for our system to pay for itself is that our system is relatively small, and labor is a big part of the installation cost. Another reason is that we are very efficient with our electricity usage (lots of Energy Star appliances, fluorescent bulbs, etc. . . ). PG&E has tiered rates and as you use more electricity, the price of electricity rises. If you are in the top tier, each additional kWh of electricity you purchase costs 33c/kWh, whereas we only pay 13c/kWh. If your solar panels can offset your usage of electricity that costs 33c/kWh rather than 13c/kWh, your system will pay back much faster.
It is unclear whether having the solar panels increases the value of our home (we recently had an appraisal for our refinance and the appraiser didn’t add any value due to the solar panels). However, it would be logical for a rational buyer to value the panels based on the present value of the expected electricity bill savings. (Sandia National Labs created a tool to help estimate an appraisal value for a solar system and after plugging in the relevant stats on our system, it came out with an average value of $7837 for our brand new system).
Gathering data and analysis
The previous owners of the house had solar panels on the roof. You can see from this satellite image on Google maps that there were 22 panels on the roof. When we bought the house, the panels were no longer there, though we are not entirely sure of the circumstances (it was a foreclosure, so the bank didn’t disclose much). Anyway, the panels were gone but the mounting brackets was still largely on the roof and in good shape. So when we bought the place, the idea that we could install some panels on the roof was fairly obvious.
We contacted Real Goods Solar, who looked at our past energy usage and checked out the roof in order to determine the size and design of a system. (We went with Real Goods because we’ve been to their store/solar living center a few times and their sales rep Justin was super helpful and knowledgable).
Our electricity meter from PG&E is a smart meter so we have access to hourly data on our energy usage (both natural gas and electricity) since we’ve lived in the house.
Chris used a web-based program called PVWatts from the National Renewable Energy Lab to download representative hourly solar data for San Francisco in order to estimate the amount of solar generation we might expect on an hourly basis. Our climate isn’t exactly like San Francisco – we’re 15 miles away across the bay – but it is similar enough to get a decent estimate. If anything the estimates will be conservative since San Francisco is a bit foggier/overcast in the summer.
Chris put all the data into excel and ran a few calculations to aggregate the data in to monthly averages. This figure compares our average hourly electricity usage for the month of July 2011 (red line) to the estimated solar generation in July for a system like ours in San Francisco (black line). You can see the electricity demand is relatively flat while solar generation peaks in the middle of the day and is much higher. Our system does not have batteries because we are still connected to our utility (PG&E). In essense, we can use the electricity grid as our storage system. Once we installed the solar panels and our system was approved and connected to PG&E, our system became “net-metered” meaning we would draw power from the electricity grid when demand was greater than supply (before 7am and after 6pm) and we would be feeding power to the electricity grid for others to use when our generation was greater than our demand (between 7am and 6pm). The meter keeps track of the net flows of power and keeps track of whether we had excess demand or supply in the month and that is the amount they will charge us for.
This next figure shows the same data but for the month of January. Solar generation is lower because the sun is at a lower angle and is out for fewer hours. Demand is a bit higher than July, mainly due to more lights and our furnace (which runs on natural gas to produce heat, but uses electricity to power the blower to move the heat around the duct system).
We typically use about 3300 kWh/year (around 200 kWh/month in the summer and up to 300-400 kWh/month in the winter). This is about half of the average household electricity usage for PG&E (6500 kWh/yr). Our system is estimated to produce a little bit of excess generation for the year (3600 kWh/year, ranging from 160 kWh/month in the winter to over 400 kWh/month in the summer).
Here are two of the Real Goods installers on the roof. You can see the existing roof mounts, little metal brackets that attach through the roof to the roof rafters, that were left behind by the previous owners.
They are setting up some rails to the brackets which are then used to attach the panels.
Here are all the rails after they have been installed.
The crew had a cool method of getting solar panels onto the roof. We had 10 panels installed (225 watts each for a total of 2250 watts). Talking with the crew, they used to strap panels to their backs and climb the ladder. These panels weigh about 46 lbs each and are about 5′ tall x 3′ wide. But as you might expect, this is pretty dangerous since the panel will catch any wind and these guys are 30 feet off the ground. So the had a small electric motor and a pulley system at the top of the ladder to pull the panel up to the top of the ladder where one of the installers could grab it.
Placing the panels onto the mounting rails.
The finished installation (10 panels, 8 on top and two below). Of course it’s cloudy when this picture is taken (you can see the BART tracks and the San Francisco Bay in the background). Chris climbed the ladder to get a closer view of the solar panels. It’s bit scary being up 30 feet on a ladder.
Another view of our roof and some of the unused brackets. Here we have a view of the Berkeley hills in the back. We can always add additional panels in the future, which would be useful if we decide to get an electric vehicle (someday).
System Details and Performance
The system in detail
- 10 Kyocera KD240GX-LFB photovoltaic panels (240 watts each)
- 10 Enphase m215 microinverters (225 watts each)
One Enphase inverter (the device that converts the DC power of the solar panels into the AC power that comes out of the wall plugs) is connected to each panel. The cool thing is that they are all internet connected so it uploads the power generation data (in five minute increments) to the Enphase website where you can remotely monitor your system, including information about each individual panel. We actually use another website called PVOutput to monitor our panels because it easily allows us to monitor, share data and compare our system with thousands of other systems.
The first month or so, Chris probably looked at PVOutput dozens of times each day. Here is a graph comparing solar generation for two days: the first (in yellow) is for a very sunny day in June and the second (in green) is for a typical summer day in our area, where the morning is cloudy/overcast and it burns off around noon. If you want to explore our solar generation in more detail, you can check out our PVOutput page.
From March through October 2012, we have already generated about 3 MWh (or 3000 kWh), so we are on pace to exceed the estimated 3600 kWh annual generation. Because we installed our system at the beginning of spring and have operated through summer so far (and generated much more than we have used), we have already built up over $110 in credits with PG&E. This money will be used to offset our net electricity usage during the winter months when we use more electricity than we generate.
One of the interesting things about having the solar panels on the roof is that we have become much more aware of how sunny it is outside and the length of the day. Overall, we are very happy to have the solar panels generating clean electricity and offsetting emissions and money from the utility. It has also inspired more thinking about other efficiency and energy projects around the house.