What is your home's solar energy potential? Find out now

Matthew Grocoff

March 11, 2014

By: Matthew Grocoff, Green Renovation Expert

In: Green Living

The PV Watts Calculator from the National Renewable Energy Lab (NREL) has been around for years. But, the latest version is finally user friendly for the average consumer, or at minimum, your average solar nerd like me. The tool "estimates the energy production and cost of energy of grid-connected photovoltaic (PV) energy systems throughout the world. It allows homeowners, small building owners, installers, and manufacturers to easily develop estimates of the performance of potential PV installations."

The home page starts with a simple field for you to enter your address. It then asks you to enter the nearest weather data source. It gives you a pull down menu with nearby options. I chose the Ann Arbor Municipal Airport which is 4.4 miles from my home.

On the Systems Information page you are asked to enter the fun stuff. First, you select a size of system you are considering. My page defaulted to 4kW, which is about the size of the average residential array. Since we already have an 8.1kW array on our net zero energy home, I decided to see how accurate PVWatts was compared to our actual data.

The Array Type is the type of racking system that holds the solar panels. A fixed open rack is one that sits on a constructed frame on the ground or roof. Roof mount is what we have and is typical on residential roofs. They lay on a rack flat against the roof. Axis Tracking is the expensive little robot rack that moves with the sun throughout the day and/or season.

The Derate Factor is the conversion of the energy from your panels to your house. Some panels and inverters are more efficient than others. If you're entering your fantasy system and don't know the actual derate, then stick with the default of .77.

The Tilt (deg) is the angle that your panels tilt toward the horizon. PVWatts defaults to the optimum angle for your latitude. For example, if you are at 45 degrees latitude, the calculator will default to 45 degrees Tilt. Chances are your roof angle is not exactly the same as your latitude. If you know your roof angle then enter that number. You can also just give your best estimate. Ann Arbor is at 42 degrees latitude, but my roof pitch is exactly 45 degrees (which by the way helps shed snow in the winter quicker than a roof with a shallower angle).

Azimuth is the degree angle on the map in relation to south. 180 degrees faces due south. If you're lucky, your house is like mine and you have a lovely 180 degree south facing roof. If it's slightly off one direction or the other, that's okay. You may still be able to get a good amout of solar production. That's exactly what this calculator is designed to tell you.

There's also an optional section to input your baseline electricity cost. This will help you decide how much adding solar panels will benefit your bottom line. The Initial Cost box asks you to enter the cost per watt of your potential system. The system defaulted my input to $3.70 per watt. But, I know that in Ann Arbor solar contractors are installing for as little as $3.00 per watt. That means an 8,100 watt system (8.1kW), which is the size of our large system, today would cost $24,300 before the 30% Federal tax credit or other incentives.

There is also a section that puts many of the available incentives available to you specific to your location. This is a wonderful resource since every region has different and ever-changing incentives.

So, how did the PVWatts do compared to our actual results? Nearly dead on. PVWatts does not account for any possible shading on the property. We have quite a bit of early morning and late afternoon shade. PVWatts said we should produce about 9,155 kWh per year.

You can check out my live and historical solar data here: America's Oldest Net Zero Energy Home

Check out how much renewable energy you can produce on your house and find out how much you can save!: PV Watts Calculator


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