News stories about clean energy frequently announce, with fervor, that a new installation of wind or solar, or some other uneconomic method for generating electricity, can produce enough electricity to serve X number of homes.
This is supposed to establish that the new installation is economically sound, with the inference that these homes are now supplied with “clean energy”.
There are a few problems with this pronouncement.
First, it is frequently based on the nameplate rating of the installed generating facility.
As we know, all name plate ratings don’t generate the same amount of electricity.
Second, there is some assumption as to the average amount of electricity used by homes.
Here is an example from Bloomberg:
“Renewable-energy developer opened the 39 MW wind farm, which will generate enough power for 27,000 homes.”
This announcement relied on the frequently used assumption that an average home requires 1,500 watts of electricity.
This shorthand approach is misleading … and meaningless.
The key variable, not addressed in the Bloomberg story, is kilowatt hours (KWh), not KW or MW.
The nameplate rating is in Megawatts, which is 1,000 Kilowatts, but it’s the amount of electricity generated over a year, in KWh, that’s important.
A typical 1.5 MW wind turbine has the potential to generate 3.9 million KWh of electricity, but it doesn’t.
The wind doesn’t blow all the time, and it’s always changing its speed. Therefore, the output of a wind turbine is measured over one year to determine how much electricity is actually generated, and this is then compared with the amount that could theoretically be generated based on its nameplate rating, to arrive at a capacity factor. The average capacity factor for land based wind turbines is less than 30%, which means that a wind turbine actually produces less than one third of its nameplate rating.
This is why the nameplate rating shouldn’t be the basis for determining the number of homes a wind farm can supply.
The average US home uses around 10,000 KWh of electricity, so the 39 MW wind farm, mentioned above, can only supply electricity to around 9,600 homes … not 27,000.
This average may not apply to the area being served by the wind farm or solar power plant.
The same method can be used to determine the number of homes supplied by solar power installations, except capacity factor is usually between 16% and 22%. Obviously the sun doesn’t shine at night, so a solar installation can’t supply electricity for all twenty four hours.
Reporters aren’t engineers, so can be forgiven for using shorthand methods for determining the number of homes an installation can supply with electricity, but readers need to be alert to what is being reported.
Here’s an even more egregious quote from a news story in the Orange County Register, where the reporter accepted what he or she was told without questioning the source.
“Solar power generation on California’s electricity grid reached an all-time high Friday, totaling enough to power more than 1.5 million homes, state officials said Sunday.”
“The record of 2,071 megawatts hit at 12:59 p.m. Friday …”
“One megawatt will power about 750 homes, Greenlee said, meaning the state’s solar power generation was able to power more than 1.5 million homes at Friday’s record peak.”
In this case, not even the arithmetic works. The story, as written, is meaningless.
Energy is an area where people can be easily fooled, since most people aren’t engineers.
Every story about energy needs to be viewed with skepticism … especially if it seems to be about hype rather than facts.
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