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                PV WATTS v.1 - How to Calculate Solar PV Power Performance Output

Without a lot of tech-speak, we are going to take you through a quick tour on how to easily calculate solar power performance output using PVWATTS Version 1. This will take us 3 minutes.

How useful is this tool? …The short answer is that every solar company uses it in some way, shape or form for every solar project. Plus, most solar sales modeling software programs use the PVWATTS database in their software.

This tool takes most of the guesswork and tedious calculations out of estimating the electrical and cost performance of solar PV power systems of any size in any location.

To spit out a basic report, the only data you need is the estimated size of the system, location, tilt angle of the panels and azimuth (direction the panels will be facing when mounted). Let's go…

Step 1: Go to PVWATTS and click on the State where the system will be located. Our example uses New York State.

Example 1

Step 2: After clicking on the US map you will jump to a State map. Click on the weather station closest to the proposed solar power system installation location. For this example, we are using New York, NY.

Example 2

Step 3: After clicking on the weather station location, you will jump to a data entry screen. Since we already know our PV system's location, to keep things easy, we are going to change 2 pieces of information on the default settings.

Our systems will be 5kW DC and the tilt angle of the panels will be 26.57 degrees (the angle of a 6:12 asphalt shingle roof). After entering the changes into the data fields click Calculate.

Here's a glossary/tutorial of the data fields that can be changed.

DC Rating (kW): The total DC rated power of the panels in kW. Enter 5000 watts as 5.0 kW. Enter 15,000 watts as 15.0 kW etc. We are using 5000 watts or 5.0 kW.

DC to AC Derate Factor: This number represents the total power generated in AC after losses for shading, wiring, soiling, inverter efficiency etc. are factored in as a percentage. Leave the default as .77

Array Type: This is the type of array tilt. Choices are fived, single or double axis. Leave the type as fixed for our example.

Array Tilt (degrees): This is the anticipated tilt angle of the array in degrees. 0 degrees would be flat against the horizon while 90 degrees would be vertical against the horizon. Our example array is mounted on a 6:12 pitched asphalt roof which calculates to a pitch of 26.6 degrees.

Array Azimuth (degrees): For systems with a tilt angle (almost all systems are tilted) this is the orientation in degrees where the panels will be facing. Optimum is South 180 degrees; however SE and SW are slightly less efficient and work well in many locations.

Cost of Electricity (cents/kWh): If left as the default setting, the electricity cost factor will represent the statewide average cost of electricity for the State selected on the location map. When manually entering electric rates, enter 15 cents as 15.0 – enter 12 cents as 12.0 etc.

Example 3

Step 4: After clicking the calculate button, you will be brought to a results screen. Let's take a look at the findings from our PVWATTS v.1 generated solar power system cost and performance estimate.

The two most important pieces of information are the total cost savings and the total amount of AC power generated in kilowatt hours by the system.

As you can see, the yearly cost savings for our system is $882.90. That number is generated by the total yearly AC kilowatt hours generated times the electricity rate. In this case, our system generated 6089 kWh and the electric rate is 14.5 cents kWh. Here's the calculation. (6089 X .145 = $882.90).

Example 4

That's all there is to it. Now anyone armed with a couple of pieces of information can easily calculate an estimate for early AC output and cost savings from a photovoltaic solar power system in 3 minutes.

By: Rick Contrata

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