To get to net zero energy use, we must produce enough energy to offset our energy usage over a year. We originally planned to live in the house a few years before installing photovoltaic (PV) panels on the roof, but after we ran the numbers we decided it made sense to do it now rather than wait.

First a little context. How much energy should we be trying to offset? Our energy consultants estimated our total site energy usage at 5,995 kW hours per year (kWh/y). Based on this, they recommended a 6 kW array which they estimated would produce roughly 6,000 kWh/y. At $0.14 kWh that’s roughly $840 per year total energy costs (not counting the Basic Service Charge of $16.21/month and misc. charges). That includes heating, cooling, hot water, well pump and plug load (it’s an all electric house).

It is important to note that we’re using site energy numbers rather than source. Source energy is the energy that the power company must generate in order to supply power to the site. Roughly 2/3 of this power is lost on it’s way to the site. Some net zero homes try to offset their usage based on source energy rather than site energy. Our estimated source energy usage is approximately 16,187 kWh/y. This would require a much larger array, more space and, of course, a much bigger wallet!

The next step was to determine if we have enough space on our roof to generate that much power. We already made sure that the roof was facing due south and pitched the the correct angle to maximize production over the year. The rule of thumb is to pitch the panels at the same angle as your latitude. Our latitude is 43 degrees and our roof pitch is 45 degrees. We also have no shading during the primary energy production hours (unless you count snow as shade.)

We have approximately 646 sf of south facing roof area.This is enough room for roughly 30 panels, 3 rows of 10. One of the most popular panels being used right now is a 230 watt panel. 30 panels x 230 watts = 6,900 watts or 6.9 kW. This is DC power. Factoring in some losses converting DC to AC, and the solar potential for our geographic location, this equals roughly 7,000 to 8,000 kWh/y AC.

This is more energy than we require, but it makes good use of our available area and gives us some extra capacity for a future barn and electric UTV. I must note that over-producing is not necessarily economically beneficial for a homeowner. Any excess we produce one month is subtracted from our next bill, dollar for dollar. This is good for offsetting usage, but if we produce more than we use over a period of a year, the power company is only obliged to offer us payment at avoided cost. Defining avoided cost is for lawyers, but it’s safe to assume it means pennies on the dollar. (If you know how to calculate the avoided cost and where to get the data, please let me know in the comments!)

I originally estimated the system would cost in the range of $20k just for the panels and inverter, not counting all the other wiring, roof clips and rails, inspections and labor. I figured design and installation costs could double that number. I knew there were state and federal tax credits but I was surprised to find out how much federal stimulus dollars are available. In New York, NYSERDA administers this money, and rebates roughly a third of the cost of a solar PV system (including labor). I pay taxes, so it’s nice to get a little something extra back.

Th rebate brought the total cost of ownership down to a level that made sense to go forward with the installation now rather than wait. I called a few solar installers and settled on GroSolar. (GroSolar recently sold the residential part of their business to SolarCity.) Once the rebate and tax credits are figured in, the entire 6.9 kW system is less than $15k. Assuming we just count the $840 calculated above as saving per year, then the system would pay for itself in roughly 18 years, assuming electricity stays at $0.14 kWh.

We are scheduled to get our system installed in April. I’ll post again when I have more details on all the components of the system.