Replies

1. GBA Editor
   Martin Holladay | Mar 31, 2009 10:38am | #1

   John,
   Yes, the grid really does use any surplus electricity fed into the grid by a homeowner with a grid-connected PV or wind system. The electricity is not wasted.

   Off-grid homeowners, on the other hand, often waste electricity. For example, on a July day, it's common for the batteries of an off-grid home to be fully charged by 10:00 a.m. If the sun is shining, all of the electricity generated for the rest of the day is wasted, unless the homeowner notices that the batteries are full and starts doing extra loads of laundry.

   Whether or not the grid-connected homeowner gets any financial credit for surplus electricity is a whole other question. The answer depends on the details of the local net-metering agreement, which vary from utility to utility. Some utilities provide a credit for a maximum time period (for example, 12 months), after which the credit vanishes. A few utilities will actually send the homeowner a check for any surplus.

   If the utility sends a check, it could reimburse the homeowner at the retail rate, or perhaps only at the wholesale rate. In Germany and Gainseville, Florida, homeowners can get a check for above-retail rates, due to programs call feed-in tariffs.

   Finally, I'm not sure what you mean when you ask, "Has the zero energy home really 'zeroed out' its impact at the power plant?" I'll take a stab at answering your question, though. If a grid-connected house has a PV system, it usually needs to purchase electricity from the grid during the winter. If the local utility has a coal-burning power plant, then the grid-connected house has a co-dependent relationship with the coal-fired power plant. So it's not completely clean.

2. 
   Riversong | Mar 31, 2009 08:18pm | #2

   Even if the "net zero" house generates more excess energy than it consumes from the grid, it has to generate that excess long enough to pay back the embodied energy costs not only in its generating capacity but also in its structural materials and site impacts.

   Is it OK for a "net zero" house to have, for example, double the initial environmental impacts if it pays that debt off during the useful life of the house and its associated energy equipment? Or should be be building shelter which initially has minimal impact on our environment, using local natural materials and low-tech solutions to basic needs?

   A far better way to "zero out" our environmental impact is to dramatically simplify our lifestyle and accept basic shelter instead of the extravagance that we now call a house.