Using NREL’s PVWatts calculator for solar

If you’re starting to think about installing solar at your home or business, you may have come across PVWatts, a calculator developed by the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL).

PVWatts estimates the energy production and cost of energy for solar photovoltaic (PV) systems around the world, making easy for homeowners, small building owners, installers and manufacturers who want to estimate the performance of a potential PV installation. 

Using PVWatts to design a solar panel system

PVWatts is super-customizable – you can use it to can get solar production estimates based on your specific location, the type of equipment you’re using, and your installation site. Here’s how it works.

Step 1: Enter your location

When you land on the home page for PVWatts, you’re prompted to “get started” by entering an address. You can choose to be as specific as you want – enter a city or town, zip code, or the exact address where you’re thinking about installing solar.

Entering your location on PVWatts

This location information is the foundation of the calculator, and a big part of what makes it so customized. Instead of looking at national averages, PVWatts uses your location to tap into local weather data and sunlight data from NREL’s National Solar Radiation Database.

google maps image of boston
PVWatts will locate your property on Google Maps to get accurate weather and sunlight data

Step 2: Enter your system information

PVWatts comes preloaded with assumptions about the size of your system, the equipment you’re using, and what you currently pay for electricity for your utility. You can use the assumptions in the calculator, or input your own information. The specific inputs for the simulation are:

DC system size: This is the power rating for your system. The default assumption is 4 kilowatts (kW), equivalent to 16 standard-sized 250 watt panels or 13 high-efficiency 300 watt panels.

Many homeowners will need a larger system to cover all of their electricity use – the average system size for EnergySage Solar Marketplace users is 7.8 kW. If you’ve input your full address, you can also use the calculator’s “Draw Your System” tool to see what size system will fit on your roof.

Module type: Some solar panels are more efficient than others. If you have limited space, or are prepared to pay a little more, you can change the default setting from “standard” (which is about 15 percent efficient) to “premium” (19 percent efficient).

Array type: Most homeowners will install a fixed-mount system on their roof. However, you also have the option of installing a ground-mounted system that has tracking capabilities to follow the sun across the sky throughout the day. A system with trackers will cost more, but will improve your electricity production.

System losses: No system performs at 100 percent efficiency. PVWatts will make standardized assumptions about the performance losses you can expect in real-life conditions. If you have more specific information for your location, you can use the “loss calculator” to adjust for things like dirt, shading, and snow on your panels.

Tilt: Most solar panels aren’t installed perfectly flat – there is some degree of tilt to the system. If you are installing a rooftop system and know the pitch of the roof, you can input that here. If you’re installing a ground mounted system, you’ll want to install the panels at the latitude of your location to maximize production.

Azimuth: The azimuth refers to the direction that the panels face. For maximum electricity production, your panels would face perfectly south – an azimuth angle of 180°. You can click the info button on the calculator to get angles for other compass headings.

Retail electricity rate: This field is used to tell you how much the electricity you produce is worth. It’s auto-populated based on a dataset from 2012, but electricity rates have changed significantly over the past few years. If you know what you’re paying for electricity now, you can input it here to get more accurate savings information on the results page.

There are also some advanced parameters that you can adjust if you want to get really technical. For all of these inputs, you can leave the defaults in place, or adjust based on information that you have for your specific property.

Step 3: Results

The results page shows you monthly and annual solar radiation, electricity production, and the dollar value for electricity produced by the system you just designed. It also offers a system output range that factors in year-to-year variations in weather.

The example results below show that the system will produce just over 5,000 kWh annually, with an annual value of slightly under $1,000.

PV watts calculator results in boston
Example results for a 4 kW system located in Boston, MA

Also on this page, you can download monthly or hourly results, which give detailed electricity production information that is based on a “typical year weather file.”

What about the cost of installing solar, or long-term savings associated with it?  

PVWatts is a flexible tool that can give you customized estimates of electricity production based on your specific weather patterns and system design, but it doesn’t have everything you need to get a complete picture of your solar options.

If you want to understand the cost of installing solar where you live, as well as a detailed estimate of your long-term savings with solar, you can use EnergySage’s Solar Calculator.

JEnergySage’s calculator uses PVWatts weather and sunlight data to make accurate estimates of solar production where you live, but it takes things a step further to calculate your savings. Unlike PVWatts, the Solar Calculator has real-time pricing data from installers who are submitting quotes on the EnergySage Solar Marketplace in your area. Prices vary from state to state, and this is the most accurate way to get an estimate of what you’ll pay.

In addition, EnergySage’s calculator factors in current electricity rates, tax credits and local rebates to tell you what you can expect in terms of net savings over 20 years when you install solar. Costs and savings will depend on whether you pay for your system in cash, take out a solar loan, or sign a solar lease or power purchase agreement with an installer. All of these options are presented in the calculator.

energysage solar calculator results page
Results from the EnergySage Solar Calculator for an example property in Boston, MA

Calculators and other methods of estimating solar production are helpful, but they will never be as accurate as a custom-designed quote. If you’re ready to take it a step further, you can use the EnergySage Solar Marketplace to get actual quotes from local pre-screened installers that are specifically designed for your home at no cost to you. Installers pay EnergySage for the opportunity to work with Solar Marketplace shoppers, so you never have to pay to use the Marketplace to compare quotes for your home.