PVWattsTM is a calculator developed by the National Renewable Energy Laboratory (NREL) that aids in the design and evaluation of solar PV systems. 

To use PVWattsTM, begin by selecting your location from the map or the dropdown menus.  The program will get you geographic coordinates and solar irradiance data from databases that are later used in calculating PV system performance.

You will be asked to enter data about your system on the second screen. 

  • Enter the DC Rating of your panels or array in kilowatts.
  • The DC to AC Derate Factor accounts for all the efficiencies of the wiring, controllers, inverters, etc. that deliver the power to the gird or point of use.  The default value is 0.77, but system designers today recommend a value in the range of 0.84 due to improved device efficiencies.
    • You can refine the Derate Factor if you know actual efficiencies of your controllers, inverters, etc.  For example, if your components have losses or tolerance such as 10% temperature, 5% output tolerance, and 3% loss at the inverter, the result is a "Derate Factor" of 0.82.
    • If you are evaluating an off-grid system, the Derate Factor may be much lower due to the inefficiencies of battery charging and discharging systems.
  • Use the Fixed Tilt array type.  You can experiment with tracking system results, but you will find that the increased installation and maintenance costs of tracking systems will be much greater than the increased output.  If you need more output, it will be more cost effective to add more panels.
  • Array Tilt is the angle from horizontal that your panels will be mounted.  The default value is your latitude, which is the angle that will provide good average performance.  However, if you will be mounting the panels on a roof, adjust the mounting angle to the pitch of your roof (see table blow).
  • The Azimuth Array is normally 180, or facing due south.  If your roof or other mounting position faces a different direction, adjust the azimuth (see table below).
    • If you are in an area with Time Of Day metering, you might desire to mount the array facing southwest to maximize power output during the afternoon when the utility system demand and your electrical rates are highest.
  • Adjust the Cost of Electricity to the cents per kilowatt hour that your PV-generated electricity is worth.  If you are evaluating a grid-tied system with a contract that pays you a premium rate, such as 10 cents/kWh, and your purchased power rate is 9 cents/kWh, then enter 19 cents/kWh. 
  • Click the 'Calculate' button to get your results.
    • The results page will display a table that estimates the kWh of energy produced by the system each month and the value of that energy.  You can use this information as the basis for estimating the economics of your proposed system.
    • Some locations have hourly solar insolation data and PVWatts can estimate the output of the array on an hourly basis.  The hourly results can be analyzed in a spreadsheet to find the maximum output of the array, which is needed to size the DC-to-AC inverter.  You may discover that the highest outputs are in winter when the panels are operating at much lower temperatures than in the summer, even though the solar radiation is stronger in summer.

PV Array Tilt Angle by Roof Pitch

Roof Pitch Tilt Angle (°)
4/12 18.4
5/12 22.6
6/12 26.6
7/12 30.3
8/12 33.7
9/12 36.9
10/12 39.8
11/12 42.5
12/12 45.0

Azimuth Angles by Heading

Heading Azimuth Angle (°)
N 0 or 360
NE 45
E 90
SE 135
S 180
SW 225
W 270
NW 315

Refer to the PVWatts How to Change Parameters for additional information.

Click here to launch PVWattsTM