Understanding your solar energy potential and exploring the various options for putting solar panels on your roof can be daunting at first. Many homeowners start with some basic questions such as "Are solar panels worth it?" or "Can my roof support solar panels?". If the initial research looks promising, then they embark on a more detailed look and inevitably more questions emerge - "What is going to be the cost of installing solar panels?" or "How much electricity can I generate at my location?".

What to look for in an ideal solar panel calculator

Fortunately there are many free options available to help you answer some of these tough questions, starting with our very own Sunmetrix Discover, which is easy to use and full of great location-specific information to help you better understand how much solar energy you can produce, what you can save, whether you should buy or lease, as well as a list of highly-qualified installers in your area. Ideally, you want the calculator to help you with all the three key ingredients that make the best solar investment decision for your home: resource, technology and financing.

  • Resource simply refers to the level of solar radiation available at your location. There's really not much you can do to change this (you are at the mercy of Mother Nature), except for preparing your roof to minimize shading on the panels (e.g., by pruning some trees). If you want to dig deeper into the importance of your location, take a look at a previous article we wrote on the Top Five Factors Determining Solar Energy Potential.
  • Technology is the combination of hardware and software that you will choose for your rooftop. The main components to choose are the solar panels, the inverter, a mounting system, wiring and a performance monitoring system. In today's solar marketplace, there are thousands of products competing for your business. In most cases however, your choice will be limited to what your solar installer is offering. Thus, it will probably come down to comparing a few quotes with different hardware and software components.
  • Financing will make or break the profitability of your solar investment. A location with a poor solar resource can yield a profitable investment if you can manage to get a great deal and take full advantage of all the incentives and grants in your area. Conversely, your home can be located at a great solar location, but overpaying for your solar panels can result in a very poor investment. From lease or buy options to power purchase agreements, there are many financial decisions that need to be made before solar electricity starts flowing in.

A quick review of five contenders, other than Sunmetrix Discover

Now that we know what to look for in a solar panel calculator, we're ready to review our list. We should note that all of the calculators presented below are freely available.

PVGIS JRC1. PVGIS: PVGIS (Photovoltaic Geographical Information System) is an online tool funded by the European Commission and developed by the Joint Research Centre in Italy. PVGIS can be used to determine the expected performance of grid-connected or off-grid photovoltaic systems. A map-driven user interface enables the users to select the location of interest. Once the location is determined, various system specifications can be modeled using drop-down menus and tick boxes. Data coverage is limited to Europe, Africa and parts of Asia. PVGIS has very good solar resource information, and some solar technology choices, but little, if any, financial information.


pvwatts solar panel calculator2 . PVWatts: PVWatts is another online tool for estimating the output of your solar energy system. It is developed and operated by the US-based National Renewable Energy Laboratory (NREL). One of the interesting features of PVWatts is the ability to include all the three key elements of solar investments. You can take a look at your solar resource, play around with some basic technology choices and get some ideas about the financials. You can choose the location of your project site and build a basic model of your system by specifying system size, slope of the panels, cost of electricity and other parameters. The outputs include monthly and annual energy production estimates.


retscreen  3. RETScreen: RETScreen is one of the most powerful clean energy calculators. This comprehensive software tool is developed by Natural Resources Canada and it covers many renewable energy and energy efficiency technologies including solar, wind and biomass. Its solar energy module covers both photovoltaic systems and solar thermal systems. Furthermore, all three key elements of a solar investment are included: you can take a detailed look at your solar resource, review various solar technology options and run detailed financial analyses. The accompanying website is full of useful examples. One significant drawback of RETScreen is its user interface: it is not an online calculator. You will need Microsoft Excel to install and use it.


homer_logo4. HOMER: Just like RETScreen, NREL's HOMER energy modeling software is a desktop application for Windows. It covers many green technologies including wind, solar, fuel cells, hydro and biomass. On the solar front, both solar thermal and solar photovoltaic applications are included. HOMER is particularly good if you are interested in combining solar energy with a conventional energy technology. For example, a solar panel - diesel generator combined system for an off-grid location. HOMER comes with a database full of solar resource information, and different system configurations can be selected to explore solar technology options. Cost calculations and financial estimates can also be obtained.


SolarProspector5. Solar Prospector: Yet another online tool by NREL, Solar Prospector, combines a map with various analysis tools to determine the suitability of a solar investment at a particular location. Since it is geared towards larger solar investments, some of the analysis tools could seem overly complicated for residential users. If your home is in the United States, you are in luck, since Solar Prospector is very rich when it comes to solar resource data. Unfortunately, users located outside the US cannot use most of the functionality of the Solar Prospector.

How to pick the best solar panel calculator to meet your needs

As we've just seen, there are many free online solar panel calculators that you can use in order to get a better understanding of your solar energy potential. In order to find the best match for your particular case, take a closer look at these three factors: data, models and user interface.

Solar radiation data is at the heart of all the calculators. As we have explained in a previous article, when it comes to solar energy, it's all about location, location, location. You need to make sure that the calculator has data coverage for your area. For example, PVGIS has coverage for Europe, Asia and Africa, but not for other parts of the world. Solar Prospector, meanwhile, provides excellent coverage of the US, but there is no data for other parts of the world.

Models are the algorithms that are used to compute many different phenomena that has an impact on the output of your solar panels. From solar angle to cloud cover, from solar panel prices to the inclination of your panels, many different variables are included in these models. It is very difficult for a homeowner to know which model would perform better for a specific solar panel configuration. Thus, we highly recommend running the numbers in more than one calculator, in order to gain more confidence in the results. Doing so may be time consuming, but it's definitely worth the effort.

The user interface is also an essential element of a solar panel calculator. Whether it is map-driven or table-based, a simple and intuitive user interface is always desirable. Adequate documentation and user tips with examples are also very useful features. Finally, while some calculators allow users to download a pdf file with their results,some of them only provide summary information on a web page. This may not seem very important at first, but when you start comparing numbers, it's very useful to have summary results readily accessible.

Our verdict: Start slow, but don't be afraid to dig deeper into the numbers

If you are just starting to learn about solar energy, we recommend checking out PVWatts, PVGIS or our very own Sunmetrix Discover first. All three of these calculators are relatively easy to use.  Once you get your bearings and you are ready for some more information, installing RETScreen or HOMER would be definitely a good idea. As a third step, if you are based in the US, and don't mind dealing with larger data sets, Solar Prospector is definitely worth your while. Even a cursory look at some of these calculators can give you the edge when you are negotiating with your solar installer.