Do Grid-Tied Photovoltaic Systems Really Have an Advantage?
A solar electric system can be grid-connected or off-grid; it’s time to weigh the pluses and minuses of each approach
Most houses with solar electric panels remain grid-tied, meaning the house is still connected to the utility’s grid even as it has the means to produce its own power. Off-grid houses, which once accounted for the lion’s share of installations, are now in the minority.
And that seems to make sense. Grid-tied houses get the benefit of solar electricity, but also can tap into grid when panels don’t produce enough power to meet demand. Net meteringArrangement through which a homeowner who produces electricity using photovoltaics or wind power can sell excess electricity back to the utility company, running the electric meter backwards. allows houses to sell power at times when the panels are over-producing, and grid-tied systems are cheaper because they don’t require banks of deep-cycle batteries.
But is the situation really that cut and dried?
In a Q&A post at GreenBuildingAdvisor, Tristan Roberts says a chat with an expert at the Department of Energy got him poking at these widely accepted premises.
“If that electricity is feeding into the grid, we were discussing the difficulty in accounting for environmental benefits,” Roberts writes. “It's hard to know if the 2 kW that you fed into the grid at 12 p.m. really resulted in the utility being able to reduce their coal-fired output by 2 kW. In fact, there are a lot of reasons to conclude that nothing of the kind happened...The point of our conversation was that the best use of on-site renewables, from a thermodynamic and environmental point of view, is on site.”
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Does this mean, he wonders, whether homeowners should be steered more toward off-grid installations, or systems that split the electrical load between on-grid and grid-tied generation?
Yes, grid-tied systems make sense
While it’s unrealistic to think that small amounts of electricity fed into the grid will allow fossil fuel plants to be shut down, there are benefits nonetheless, Damon Lane writes.
“A lot of distributed energy would have some benefit by reducing the load and therefore the fuel consumption of those plants,” he says. “Also, while off-grid systems completely remove the building’s load and therefore many such buildings may allow a plant to shut down, all the batteries required probably offset the environmental benefit.”
There also is the issue of efficiency. Utility-produced electricity is only about one-third efficient, due mostly to losses during the conversion of fuel to electricity but also due to losses during distributing the power through the grid.
“Anyway,” he adds, “by reducing demand through efficiency or on-site generation you effectively save more than what you generate because you are also removing the losses and power plant inefficiencies associated with that power so primary power savings might be three times the on-site savings.”
Grid losses are relatively small
Roberts thinks transmission losses are about 9%. However, Ben Wilson, who identifies himself as a distribution engineer at a large electric utility, says they are probably even less.
“Most of the losses on a distribution line itself come from the transformers serving customers, and these rarely account for a loss greater than 5% or so,” Wilson writes.
“Excess power produced by an on-site renewable source will be consumed by neighbors and other users connected to a distribution circuit,” Wilson says. “The fact that power does not need to flow all the way back to a generating plant is actually the main reason why renewables are so beneficial to a utility system — the losses in transmission, distribution, and transformation are eliminated, therefore making the total power required to be produced at a generating plant much less than the offset demand provided by the renewable.”
How renewables may affect electric rates
Wilson raises another point: how net-metering may one day affect the cost of electricity. Net-zero energyProducing as much energy on an annual basis as one consumes on site, usually with renewable energy sources such as photovoltaics or small-scale wind turbines. homes, which generate as much power as they consume over a given length of time, are possible because utility customers are billed only for total consumption.
“However, as we should be well aware,” Wilson writes, “most on-site renewables cannot meet a home's demand at all times, especially during the times of highest loading. Utilities must provide this marginal power to a grid-tied home, and as renewables become more widespread, utilities will begin to bill for this benefit via real-time pricing, time-of-use rates, or something similar.
“In other words, a net-zero home might lead to no electricity bills today, but it is possible (even probable) that the electricity bills will return as utilities learn how to more properly bill their customers for the benefits they provide.”
Not so, says Kevin Dickson, who argues that a “smart grid” should make possible real-time pricing and time-of-use electric rates. That leads to another benefit of solar photovoltaics: panels have their highest output when time-of-use (TOU) pricing is the highest.
He cites an article arguing that a fair TOU rate structure will actually reduce the size of a PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. array on a net-zero energy house by 28%.
Wilson says his experience in the Midwest is that net-zero homes typically produce far more energy than they use in spring and fall, but less than required in winter and summer.
“The fact that PV produces more energy during the summer is more than offset by the usage of AC in many of these homes,” he says. “At the end of the year, the spring and fall surplus cancels out the summer and winter deficiency. With TOU rates, however, canceling out may not be enough.”
Another factor is what utilities pay in a net-zero energy arrangement. Bob Coleman says utilities often pay “market rates” for the excess power that’s generated, but are still billing consumers at a rate that can be double that.
Finally, it's important to point out that during the summer, when solar electricity production is high but off-grid homeowners' usage is low, much of the electricity produced by an off-grid PV array goes to waste. If your batteries are all topped up by 10:00 a.m. on a sunny day, there's no easy way for an off-grid homeowner to sell excess power to the neighbors.
Our expert's opinion
We asked GBAGreenBuildingAdvisor.com technical director Peter Yost for his take on this question. Here's his response:
I think all of the points made in this discussion are fair and accurate. But the decision to select grid-tied or off-grid PV is completely one of context for me. If you have access to the grid, tie into it. Of course there is no guarantee that one home’s excess PV will shut down a coal-fired plant, just as one of us biking to work won’t shut down an oil field. We all make incremental contributions that nudge us along and bank on the “Horton-Hears-a-Who” phenomenon.
- Scott Gibson
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