After the Bids, Crunching the Numbers


Inside this Article

Introductory article graphic.
A Washington DC row house
There’s no prescriptive path for solar-electric systems, since each site is different. Several configurations were proposed for this row house’s rooftop (see illustrations).
The example row house roof
The example row house roof, with the Capitol Power Plant in the background­—the largest source of pollution in the nation’s capital.
Roof showing air-conditioning compressor as obstacle to PV modules.
Unfortunately, when the air-conditioning compressor was placed, no thought was given to the placement of future PV modules.
Introductory article graphic.
A Washington DC row house
The example row house roof
Roof showing air-conditioning compressor as obstacle to PV modules.

You should no more buy a PV system for your house based only on the lowest installed cost than you should buy a water heater or refrigerator in such a manner. For energy-consuming appliances, it’s critical to consider ongoing operating costs. But for energy-producing equipment—like a PV system—once you are satisfied with the qualifications of your potential installers, considering operating efficiency of the presented systems is crucial.

As a way to provide comparative information on the cost-effectiveness of batteryless, grid-tied PV systems, DC Solar United Neighborhoods (DC SUN), a coalition of solar cooperatives, solicited bids from four installers for a rooftop system in the Capitol Hill neighborhood of Washington, DC. The results shown are specific to one homeowner and rooftop scenario, but DC residents can use the customizable spreadsheet (downloadable from the "Inside this Article" pane above) to assess the finances of installing your own PV system. (The worksheet can also be modified for the circumstances in any location.)

In this comparison, the constant is the slope and size of the roof, which in this case is the flat roof of a row house, found commonly in many DC neighborhoods. The variables are the installers and their bids, some of whom offered more than one equipment and/or financing option. The proposal analyses included:

  • Financial return. Simple payback (SPB), net present value (NPV) and internal rate of return (IRR) were all calculated. NPV and IRR are sophisticated financial metrics that consider the time value of money and are therefore more useful.
  • Production. Estimated annual production is a function of local seasonal climatic conditions, PV array size, and DC-to-AC derate values (wiring losses, module soiling, inverter efficiency, etc).
  • Energy Cost. Both dollars per nameplate watt and dollars per kilowatt-hour per year were calculated. The latter is more useful, as it factors in overall PV system efficiency.


So as not to prejudice the evaluation by having an adequately informed consumer, no direction was given to the vendors as to the goal of the PV system, be it to just offset annual household electricity consumption or to maximize energy production given the available space. Nor was a preference expressed for buying or leasing a system. While the competing vendors saw the same roof, each proposed different configurations. Though most all PV bid packages came with their own presentations of the financial benefits the homeowner would receive, each made different enough assumptions as to make any across-the-board comparisons useless without further analysis.

To compare the competitors, the bids were analyzed using PVWatts (see Access). The same key variables were extracted from the bids: number of PV modules and module rating to determine DC nameplate rating; inverter type (string or microinverters) to determine inverter efficiency; and module tilt to determine array efficiency. Except for the case of microinverter efficiency, all the PVWatts default derates were used. The PVWatts results were used in the financial calculations.

Comments (6)

whittakerj's picture

I created this Excel program to calculate your on - grid solar savings from actual numbers.

Marc Fontana's picture

I'm interested in the customizable spreadsheet mentioned on page 80 (at I visited the link but I don't see it. What is the name of the file? Thanks

Scott Russell's picture

In the meantime, you can now download this .xls file from the "Inside this Article" pane in the upper right of this page. Cheers.

Michael Welch's picture

Hi Marc. Thanks for letting us know it is not there. Should be available shortly. Michael, HP Senior Editor

Robert Winfield_2's picture

An EXTREMELY inportant consideration, especially in a city or other densly populated area are Solar Right, "Ancient Lights". Who owns your sunlight. Surprise!! YOU DON'T own your sunlight in many parts of the US.
Eden Roc vs Fountainbleu ~1951. one hotel shaded the others pool area from ~2pm in the afternoon.
I did not get any easements, conveyable or otherwise from my previous neighbors when I installed my 1.3kW array in 1998. I had a beautiful solar arc, which was one of the main reasons I bought it
In 2006 my neighbors sold and bulldozed the house 14ft away that was about 2-3ft higher.and replaced with one that was 3 times taller (it would have been 4 times taller if I had not screamed at the building inspector in a moment of unhappiness) and 5 times larger.
Now I have shade instead of sunlight and was told by city, county and state officials that they sympathized but I was out of luck unless I wanted to sue, but told i would lose (wish i had anyway) or I could move my PV array at my expense etc

Justine Sanchez's picture

Hi Robert,
Thanks for posting your story here. You bring up an excellent point, and one that we generally haven't focused on when discussing solar access. Normally we consider existing buildings and vegetation (and how vegetation will grow)...but having your neighbor's house "grow" to the extent yours did had to be quite a shock. I personally don't have experience with solar easements and your story makes me wonder what the process (and cost) for obtaining one would have been like? Regardless, thanks so much for posting your insightful comments.

-Justine Sanchez
Home Power

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