Businessman standing by solar panelsThere are plenty of solar panel calculators out there for understanding your solar energy potential and cost savings, as we discussed in a previous article. The criteria for selecting the best calculator can be condensed into two for simplicity. First, does the solar calculator cover your region? As noted, PVGIS developed by the Joint Research Center in Italy and funded by the European Commission is excellent for Europe and North Africa but no good for North America since its database is regional. Second, is the tool simple and intuitive to use? Many tools such as Solar Prospector or System Advisor Model (SAM) are too complicated for most homeowners to use and make sense of the data. Both Solar Prospector and SAM are developed by the trusted US-based National Renewable Energy Laboratory (NREL). RETScreen is another example of a powerful but complex tool. Developed by National Resources Canada, it can compare between many renewable energy sources and generate both power and financial charts. However, it requires the user to download a desktop software which then requires the user to input a variety of data before generating any results.

So for a primarily North American audience, let's compare the two main contenders: PVWatts vs. Sunmetrix Discover.

Presenting PVWatts: Free, Simple and Intuitive

For the United States, one of the best available tools is PVWatts. Developed by NREL, PVWatts is a free, simple and intuitive tool which allows you to put in a home or business address. The tool finds the closest location to you which has a weather data file in its database. If there are multiple options, you can choose which data source to go with. Then, it gives you several boxes to change the solar panel system info, as well as the utility cost.  Economic incentives for solar power such as tax credits and utility cost reductions are also shown. Under the solar panel system, you can click on "Draw Your System" to draw a bounded figure within an interactive satellite map to designate the size of your solar panels.

This leads to a chart detailing the amount of incoming solar radiation, the solar electrical output, and the value of this output in dollars for every month, and overall during the year. In addition to the chart, a dollar per kilowatt-hour (USD / kWh) comparison between the grid system cost and the solar panel system cost is available. This gives you a baseline to decide whether solar panels will be worth it. Data tables can be downloaded for each month or even each hour.

Sunmetrix Discover: An even simpler alternative with some added benefits

Another tool that is useful for both Canadian and American residents is our very own Sunmetrix Discover. The user interface for Sunmetrix Discover is even simpler than that of PVWatts. Simply, go to Sunmetrix Discover and input an address or postal code. Unlike PVWatts which uses localized ground stations in the United States and its territories, limiting its analysis to the United States, Sunmetrix bases its data analysis on a global data set allowing the tool to be used anywhere in the world.

After you select your site, Sunmetrix Discover proceeds to the Resource page and gives you a solar score from 0 to 100 and provides the monthly average solar radiation based on the last 20 years of data. Next, users can check out the Energy page which shows the total energy produced in kWh for each corresponding month. You can also change the specifications of your solar PV system, such as the slope, power capacity, orientation, PV efficiency, and system losses. The energy graph updates automatically as you change each parameter. At the bottom of the screen is an interactive satellite map where you can draw a bounded figure to calculate the size of your solar panels. The power capacity is updated automatically to reflect the size of the solar panels you draw.

Next, the Finance page yields a graph of revenues and expenses for the next 20 years based on financial data including the cost of electricity in USD / kWh, system cost in USD, project lifetime, and inflation. This tool gives you a comprehensive look at the cost of your solar panel project and how much money it will generate for you over the years. Once again, the graph updates automatically with each adjustment.

A bonus feature of Sunmetrix Discover is the ability to save your session with the key information on your site, the solar PV system you configured and the resulting financial analysis.

We should also note that Sunmetrix has another interesting tool called Sunmetrix GO which provides near real-time data on solar radiation and electricity generation. Unlike PVWatts and Sunmetrix Discover which uses data from the past 20 years to predict the future, Sunmetrix GO uses live weather data. Although historical data can be particularly useful to get a good idea of your solar energy potential, access to daily estimates that show you exactly what happened on a particular date can increase your confidence in your solar investment.

Battle of the solar panel calculators: Lancaster, CA

Now let's look at a specific example to compare Sunmetrix Discover and PVWatts data for a home in Lancaster, California. Lancaster has recently passed a law requiring that starting from January 1, 2014, all single-family homes must have a solar PV system generating 1 - 1.5 kilowatts (kWs). Presently, one in five newly-built homes in California is solar powered and this trend is projected to grow.

First, we will set up the same site using both online solar panel calculators to estimate solar radiation and electricity output using identical solar panel configurations.

Let's start with Sunmetrix Discover, where we learn that this site has a solar score of 82 which is excellent.

Clicking the right arrow goes to the Energy page where we'll tweak a few parameters to better compare with PVWatts:

  • Slope: 35 degrees
  • PV Efficiency: 15%
  • Power Capacity: 4 kW
  • System Losses: 0%
  • Orientation: 0 degrees (south)

Sunmetrix automatically changes the monthly average energy in kWh and the annual energy produced as these values are changed. For the parameters selected, the annual energy produced is 8457 kWh. Sunmetrix also has a cool feature showing a few statistics about what this amount of energy can be used for, such as charging a smartphone for 241 years, powering a refrigerator for 1 year, driving a Tesla Model S for 25,371 miles, and powering the International Space Station for 4 days.

Clicking the right arrow again leads to the Finance page where the levelized cost is given as 0.14 USD / kWh with a capacity factor of 24%.  Levelized cost refers to the cost of building and operating this system over a period of time. The capacity factor is the percentage of time that the system is operational at full capacity, based on the availability of the sun.

Now, let's use PVWatts for the same location. PVWatts  first informs us that the recommended weather data source for this location is the Lancaster General William J Fox Airfield located 3.8 miles away. After clicking on the right arrow, we are given a series of options for the solar panel system info, initial economics, and available economic incentives. To compare, set up the analysis as used in Sunmetrix, including assuming the system losses to be zero (i.e. set the DC-to-AC derate factor to 1). Unlike Sunmetrix, PVWatts is limited to only four options for choosing the panel efficiency, which includes the 15%, rated as typical. However, there are several parameters which Sunmetrix does not offer, notably the array type or economic incentives. Since the location chosen is residential, setting the array type to be a Fixed (roof mount) is most appropriate. For economic incentives, select both since there is no reason not to take the government incentives given.

PVWatts versus Sunmetrix Discover ScreenshotNote that unlike Sunmetrix Discover, changing the parameters in PVWatts does not show an instantaneous corresponding change in the results for the energy production. The results page is on a separate page from the input page, which we reach by clicking the right arrow. The results page shows a monthly average for the solar radiation, energy, and energy value. The total values for each parameter in a year are also displayed. Here, you see that 8,568 kWh with an energy value of $1,351 is generated by the solar panels by the end of the year. The average cost of electricity purchased from the utility is 0.16 USD / kWh while the cost of electricity generated by the system is 0.14 USD / kWh. PVWatts unfortunately does not have a more detailed financial analysis tool like Sunmetrix and presents no information on levelized cost, capacity factor, annual revenues and expenses, etc. Furthermore, only the data tables can be downloaded.

A side-by-side screenshot comparison of the results is shown above (please note that there are two screenshots for Sunmetrix since unlike PVWatts, Sunmetrix combines the inputs and outputs on one page but separates each section).

Our verdict: Both are free, both are fun to use - but Sunmetrix Discover offers some added benefits

Sunmetrix Discover has two different pages for energy and finance to allow the user to play around with either the energy parameters or the finance parameters for instantaneous feedback on either page. The final results show that Sunmetrix predicts 8,457 kWh generated in one year, whereas PVWatts predicts 8,568 kWh generated. These two energy values are close enough to warrant a look at both tools. However, those who are interested in a more detailed look at the revenues and expenses for their solar investment may want to choose Sunmetrix Discover.