Electricity Conservation and Solar PV Power

Since I got my grid-tied solar PV system on my roof turned on almost two weeks ago, I have gotten a number of questions about it from friends and neighbors. There seem to be some common misconceptions about how a system like this works, so I will try to dispel a few of them here.

First, a little information about how solar PV systems work. You have one or more arrays of solar PV panels on your roof that convert the light from the Sun into direct current (DC) electricity. These arrays send the DC power to an inverter, which changes the DC power into alternating current (AC) power, and feeds it into the main electrical panel of your house. With a grid-tied system, the inverter also matches the characteristics of its output to what is coming in from the utility company. Most grid-tied systems do not have any storage batteries, since they are expensive to buy and maintain. That also means that you are not storing any power, so you remain dependant on the utility company at night and on cloudy days. With a grid-tied system, you also need a special net-metering electrical meter that allows you to push any excess electrical power you produce during sunny days back into the utility grid (and get credit for it).  The meter keeps track of how much power you deliver to the grid, and how much power the grid sends to you.

In my case, my PV system starts producing a small amount of power as soon as the Sun is visible on the eastern horizon, with the output increasing minute by minute as the day goes on. The peak output is about 2800 watts at about 12:30PM. My baseline electrical consumption with nobody home is between 200-300 watts (for things like the refrigerator, cable modem, wireless router, DVR, etc). That means that by about 8AM, my PV output covers my baseline consumption, and as the day goes on I am putting more and more excess power back into the grid, peaking at about 2500 watts at 12:30PM. After 12:30PM, my production begins decreasing over time, going down to zero a little before sunset. Once my PV production goes below my consumption, I have to get the excess from the utility grid. That means that I get ahead in the kilowatt “war” with the grid during the day, and then lose ground every night. So far, I am about 60 KwH ahead of IREA after 13 days. Cloudy days or any bad weather have a very significant effect on my PV production.

My 3.15Kw system cost about $21.5K. There is a 30% Federal income tax credit through the end of 2010 on solar PV systems. Here in Colorado, the Governor’s Energy Office is offering a $1.50 per watt rebate for solar PV systems, so I am going to get a $4500 rebate check for that. I did not finance the system, so my payback period is probably somewhere between eight and ten years at current electrical rates. My main motivation for getting the system was not really financial. Most electrical utility companies also offer rebates for solar PV systems. Xcel Energy (which serves most of Colorado) pays $3.50 per watt for PV systems. They do this for two reasons. First, they are under a state mandate to get a certain percentage of their power from renewable sources, and second, they realize that encouraging conservation and getting power from grid-tied solar and wind systems can be less expensive than building large new generation plants.  Conservation is similar to optimizing the workload on your database server, while adding generation capacity with distributed solar PV and wind systems is somewhat akin to scaling out instead of scaling up with computer hardware.

Unfortunately, I live in Parker, which is in IREA territory. IREA owns a large stake in the new Comanche 3 coal-fired electric plant that will be going online soon in Pueblo, Colorado. Because of this, they are very much against using renewable energy or encouraging conservation measures. Every month, they include a nasty little newsletter with their electric bill that rails against the “myth” of climate change, and warns about how expensive and impractical renewable energy is. Consequently, they do not offer any rebates for PV systems, although they do (under protest) follow Colorado state law, and allow “net metering”. If you are considering a solar PV system, hopefully your electric utility is more forward thinking than IREA is.

If you are concerned about reducing the amount of money you spend on your utility bills and perhaps even reducing your carbon footprint, you will get the best rate of return by taking conservation measures. There are many things you can do to conserve energy with small changes in behavior (such as turning things off when you are not using them).  You can also do simple and inexpensive things like switching to CFLs, using a setback thermostat, using Sleep mode on your computers, etc. that can have a very substantial effect on your total energy consumption without making you feel like you are depriving yourself of the modern comforts you are accustomed to. In most areas, you can get an energy audit on your house that will tell you exactly what bigger projects you can implement that will have the biggest impact on your consumption. In many cases, it will be sealing air leaks and adding insulation in your attic.

I have been using Microsoft Hohm since it went online about a year ago. My main criticism of Hohm is the amount of information you need to enter into the site about your house to complete your home profile. Not from a privacy perspective, but from a knowledge and tedium perspective, since they want to know things like how many windows you have on each side of your house, how many doors, etc. That would be fine, if their conservation recommendations were really specific to your house, but they seem to be pretty generic. Below are a couple of graphs from Microsoft Hohm showing the energy usage of my house. In the first graph, you can see my average daily electrical usage in kWh by month for 2010.


Average Daily Electrical Usage correlated with Average Temperature


The second graph shows my monthly electrical usage over the last three years. Notice how much lower 2010 is compared to previous years. I got my TED 5000 energy monitor in January 2010, and that has had a huge effect on my usage, since I can track exactly how much electricity I am using in real time. This can really affect your behavior!

Year to Year Comparison of Monthly Electrical Usage

There are lots of financial incentives from the Federal government, state and local governments, and most utility companies for energy conservation measures such as upgrading your insulation or buying energy efficient appliances. You can get details about the upcoming Cash for Caulkers program.  Here is some information about Standby Power use that might surprise you.

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