
Developed by the National Renewable Energy Laboratory (NREL), PVWatts is an online calculator for easily estimating the energy output and cost savings of a grid-connected photovoltaic (PV) installation. The calculator is widely used, especially during the design stage of proposed installations.
We are going to use the PVWatts calculator to explore and understand the concepts of power, energy and efficiency. It is an excellent opportunity to apply these concepts to a real world application. It also give us the chance to gain a better understanding of solar photovoltaic basics.
PVWatts Walk Through
Go to PVWatts
1. In the Get Started: field, enter “Harrisburg, PA”.
You’ll see a page describing the recommended Solar Resource Data for use in this estimation. Why is this important? Solar energy is the “fuel” for a photovoltaic system. The more fuel, the more electricity it can produce. The calculator uses historical weather data to estimate how much sunshine is available in a particular area.
2. Using the arrow on the right, “Go to system info”
On this page you provide basic information about the photovoltaic design you are considering.
- DC System Size (kW): From lesson 1, you’ll recognize “kW” is a unit of power. For a photovoltaic system, power indicates the rate at which energy from the sun is transformed into electricity. Individual solar cells and modules all have power ratings. The power rating for a solar module is the sum of the power ratings for each cell. The power rating for an array is the sum of the power rating of all the modules.
Remember our 240W modules from earlier in this lesson? If we build an array of twelve of them, the power rating of the array is (24 modules x 240 W/module) = 5,760 W or 5.76 kW. This “kW” value relates directly to the number of modules and is considered the “system size.” When someone says, how big is your system? You’d say “5.6 kW.”
In PVWatts, you’ll see the default value for system size is 4 (kW). For now, leave it as this value.
A quick word about AC/DC...
Don't get too distracted by the terms AC and DC. They are just two different forms of electricity. In both cases, electrons are flowing through a conductor (typically a wire). Direct Current (DC) means the electrons are moving in one direction all the time. Alternating Current (AC) means the electron flow alternates directions. A solar panel generates electricity in the DC form. The electricity grid (wires from the utility company) provides our homes and businesses with electricity in the AC form. In photovoltaic installations, we use a piece of equipment call an inverter to transform electricity from DC to AC.
- The next three fields are Module Type, Array Type and System Losses (%). Use the info button to the right of each to learn more (and see pix!), if you like. For purposes of exercises in this class please use default settings [Standard, Fixed (open rack) and 14].
- The next two fields are closely related..
Tilt (deg): describes the angle of the array relative to horizontal (ground). A flat array (like on a flat roof) has a tilt of 0 degrees. A vertical array would have a tilt of 90 degrees.
Azimuth (deg): describes which way an array faces. An array that is installed facing South has an azimuth of 180 degrees. Facing North, it would be 0 degrees.
The amount of energy an array gets from the sun depends on the amount solar radiation coming from the sun and how directly the array is facing the sun. PVWatts uses data from the selected weather station for your location to predict the amount of solar radiation available. The unit for this is energy per area per day (kWh/m2/day). PVWatts uses your location, tilt angle and azimuth to determine how directly the sun’s energy is hitting the array how much of the sun’s energy is available to generate electricity. (For example, when the sun is high in the sky, an array with a lower tilt will receive more of the sun’s energy than an array at a steeper tilt. When the sun in lower in the sky, an array at a steeper angle will receive more of the sun’s energy. Similarly, for azimuth, we’d want an array facing the path of the sun. In the Northern Hemisphere, an array would gain the most energy is facing south. In the Southern Hemisphere, facing north would be best.) Not required, but if you’d like more information, here’s a terrific resource, Solar Radiation on a Tilted Surface.
Using the info buttons in PVWatts, read documentation related to the two fields Tilt (deg) and Azimuth (deg). For now, leave these fields at default values (20 and 180).
3. Using the arrow on the right, Go to PVWatts Results.
Ta da! This table gives the PVWatts prediction for how this photovoltaic system will perform. These estimates take into account historical weather data for your location (to determine how much solar radiation is available), the size of your system (kW), the direction your array is facing (tilt and azimuth) and several other factors we left at default values (module type, array type and system losses).
Take a few minutes to consider what this table is telling you.
- The second column is the amount of solar radiation at your location. In photovoltaic terms, this is available fuel! Notice that it varies by month. Makes sense, right? Look at the unit of measure carefully: energy per area per day.
- The next column is “AC energy.” This is the electricity coming out of the photovoltaic system—the DC electricity from the array has passed through an inverter and now is exactly the same as electricity from the utility company. It is measured in kWh. Notice that on months when the solar radiation is highest, the amount of energy generated is highest. And vice versa.
- For this class, please ignore the Energy Value column. The NREL parameters used to generate these values are not good indicators for purposes of this course.
- The bottom row of the table provides Annual results, including estimate of the total AC Energy this system will generate per year (a long term average).
4. Notice at the top left of the screen that you can Print Results. Print your results and then do some experiments…(You don't submit this work. It is to help you build understanding.)
- For the same location (Harrisburg, PA), enter all the same data except double the system size (use 8 instead of 4). Compared to your printed results, how are these results different?
- For the same location (Harrisburg, PA), enter all the same data (back to 4 kW), except change the tilt (use 40). Compared to your printed results, how are these results different?
- For the same location (Harrisburg, PA), enter all the same data (4 kW and 20 degree tilt), except change the azimuth to 135 (southeast). Compared to your printed results, how are these results different?
- Go back to all original data (4 kW, 20 degree tilt and 180 azimuth), but change location to “Anchorage, AK” (Alaska). Compared to your printed results, how are these results different?
- Experiment more if you like!
You will be using PVWatts to complete many questions on the Lesson 2 Activity. Use the demonstration above to be sure you understand the concepts. If you have questions, post them to the Questions about EGEE 401 discussion.