Quick and Dirty Solar Energy Estimating Tool
Tom Lombardo posted on February 15, 2015 | 17946 views

When someone considers purchasing a photovoltaic (PV) system to offset their electric bill, one of the first steps they take is to contact a vendor and pay for a site assessment. The assessor performs many steps, including an electric load evaluation, a visual property inspection, a shading analysis, and a financial assessment. This process helps to determine the size of the array, the optimal location for solar panels and “balance of system” equipment, the payback period, and the return on investment. While detailed assessments are crucial to a well-designed and cost-effective system, a ballpark estimate can determine whether it’s worth doing the complete site assessment. Engineers frequently do “back of the envelope” calculations to determine overall feasibility; if the ballpark figures look reasonable, then the detailed analysis will follow. If not, the idea is scrubbed and no time is wasted doing a thorough assessment.

With much of my correspondence happening electronically these days, I find myself short on used envelopes for doing ballpark estimates. Maybe it’s fitting, then, that EnergySage created a “virtual envelope” that does a quick and dirty site assessment. Upon seeing the results, the user then has the option to solicit bids from vendors, similar to online banking tools that let customers obtain bids from competing banks. Let’s take a look at what EnergySage has to offer.

User Input

Getting an assessment involves entering three things: the address; whether it’s residential, commercial, or non-profit; and the average monthly electric bill. EnergySage then presents a Google Maps image of the property and asks the user to drag the pushpin to the roof of the house:


To perform the electrical use analysis, EnergySage looks up the electric rate at that location and calculates the annual energy usage based on that rate and the user’s electric bill. That’s the easy part. Shading analysis is more complicated, and that’s where it gets interesting. John Gingrich, Head of Corporate Development at EnergySage, describes the process:

“The way we estimate roof shading is through the use of LIDAR data.  This is laser data from low-flying aircrafts and is combined with building structure data, weather simulation data, and 3D models to determine the amount of solar insolation that each square meter of roof surface will provide in a year.  The data is not 100% perfect and is not available everywhere (~40 million properties in the US), so when we do not have it, we assume a 100% usage offset for the estimate.”


After a few seconds of calculating, EnergySage gives a brief summary report and a financial analysis:

The financial report gives estimates on two options: purchasing the system or leasing it. When calculating the payback period and return on investment, EnergySage assumes an annual utility inflation rate for the specified location. The net savings represents the money saved AFTER making the purchase (total benefit minus total cost.) The net cost is the cost of the system after rebates and incentives. The increase in property value is based on studies by the the Appraisal Institute that determined that properties with PV systems sell at higher prices and sell more quickly than houses without solar. In states where Solar Renewable Energy Certificates (SRECs) are applicable, EnergySage factors that into the return on investment and payback period calculations.

Select Vendors

At this point the user can register with EnergySage and request actual quotes from qualified, pre-screened vendors. The quotes are given in a standard format, allowing the customer to do a simple side-by-side comparison. Here’s a quick overview of the whole process:


A complete site analysis is obviously more thorough and considers options that are not included with EnergySage. For example, in the electric load evaluation, an assessor would find ways to reduce energy consumption through efficient lighting and appliances. This reduces the customer’s demand and allows a smaller PV system to meet the needs. While that’s not feasible for an automatic online tool, I think the site should at least mention energy efficiency when the results are presented.

Notice that EnergySage makes its estimates based on rooftop space. While that is the most common for homeowners, a large back yard could be a good place for a ground-mounted array, especially if the roof is undesirable because of shading or orientation. Conventional wisdom says that the array should face due south for maximum total energy production, but in places where time-of-day pricing is used, it could be beneficial to point the array southwest or due west to generate more energy in the late afternoon, when demand and rates are higher.

EnergySage estimates the size of the array based on meeting 100% of the customer’s electrical needs. If purchasing a system that size is too costly and the customer doesn’t want to lease a system, one possibility is to have the array only generate part of the customer’s electricity. EnergySage doesn’t offer that option, but the user could easily get around that by entering a smaller amount for the average monthly electric bill.

With all that said, EnergySage is a good place for the average consumer to learn about solar power, evaluate its financial benefits, and get bids from competing vendors. It’s yet another tool in the effort to reduce the cost of rooftop solar, and the toolbox continues to grow!

Images and video courtesy of EnergySage

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