Adding up to Zero

Even with our photovoltaic system offline for 14 days in July, the Unity House can be confidently claimed "Net Zero."

We decided to do our calculations based on a year of “lived-in” data and use. Mitch’s first read date was October 5, 2008, and I recorded data on October 5, 2009.

October 5, 2008 – October 5, 2009
Production: 6441 kwh (produced by the PV system over one year)
Excess: 5190 kwh (out to grid after immediate use by the house)
In: 5179 kwh (in from grid)

Total Electricity Use: 6430 kwh ((Produced – Excess) + In = Total Use)

Net Energy Use: - 11 kwh

So it was a close one – the system produced 11 kwh more than it used for this time period.

The PV system was offline for 14 of our most productive days this summer due to a lightning strike. At an average daily production of 18.35 kwh (6441 kwh / 351 “online” days), we can comfortably assume the system would have produced AT LEAST 256.91 kwh during that 14-day “offline” period, but very likely more.