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"Net Zero Energy" Isn't All It Seems

Matt Wald
The following is a guest post from Matt Wald, senior director of policy analysis and strategic planning at NEI. Follow Matt on Twitter at @MattLWald.

The hot new idea in energy and real estate is the “zero net energy building.” It usually means a building with enough solar panels on the roof so that over the course of a year, it produces as much energy as it consumes.

And that means the building poses no burden on the grid, right?

Well, no. In fact, the grid’s work may get harder when a zero net energy building is connected . And it means that in real life, the building still has a carbon footprint.

That’s not a fatal flaw for “zero” buildings or for solar on the roof. In fact, many aspects of a zero net energy building are unambiguously good and ought to be incorporated into a lot of structures – good insulation, high efficiency lighting and other devices, and placement of the building to make optimum use of the sun, for example.

And there’s a certain attractiveness to coming out even in the energy equation, like the squirrel who spends the fall gathering all the acorns he will consume through the winter.

The power flow between a utility and a house with solar panels.

But it’s only energy, and the building doesn’t run on just energy. It runs on a combination of energy and power. The graph above shows the power part. The purple area, above the line, shows how much power the house is demanding from the grid. The green area, below the line, shows how much power it is sending to the grid, which is electricity from the solar panels, minus household use at that instant. The grid, formerly a supplier, is now a supplier and a customer, and if the flows from the customer to the utility are large enough, the grid must be re-configured to accept them.

While the house may come out even in energy terms, it still imposes a power burden on the utility company.

Energy and power are both aspects of electricity, and the terms are frequently used interchangeably, but they should not be. Energy is typically measured in kilowatt-hours, which is a quantity. Power, also called “capacity” in the electric power industry - measured in kilowatts, is an instantaneous measurement, like speed.

Consider a really simple electric system: an island fed by a single power plant that runs on oil, delivered by tanker once a year. The energy requirement determines how big the oil tanker has to be. The power requirement determines how big the generator must be to keep all the lights, microwaves, TVs and air conditioners running and the moment of peak demand.

Rooftop solar would reduce the amount of oil needed. But it doesn’t do nearly as much for the grid’s power (or capacity) requirements, because the panel’s peak electricity production isn’t simultaneous with the period of high demand. Some systems see peak demand on winter nights; for those that peak in summer, demand around sunset is very high, because people are arriving home, and turning on their lights, air conditioners and appliances. But the sun is too low in the sky to produce much current in the panels. And some systems see a peak on cold winter mornings, when, again, the sun is too low to be helpful.

The graph above, prepared by Ben York, an engineer at the Electric Power Research Institute, a non-profit utility consortium, shows how the grid sees a house with a solar panel on the roof. For most houses, the power moves in only one direction, in. For this house, the grid is both supplying power and accepting power back again, depending on whether the solar panel output exceeds the house’s power demand.

As the graph shows, for most hours of the day, demand exceeds the panel’s output, so the utility still has to supply power. Now that the house is a producer, the grid has to be set up to accept power. This isn’t a problem if only a few houses have solar panels, but if many do, it will require some investments in the distribution system, which was designed for one-way traffic in electricity, but is now handling flows in both directions.

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