500 megawatts would be 4.4 terawatt hours per year. That's in the ballpark of what they actually did produce in 02019, according to https://en.wikipedia.org/wiki/Energy_in_South_Africa. So the journalist seems to have actually been correct in this case, although it's a bit like saying "the distance covered by the runner was 12 miles per hour", when it would perhaps be more appropriate to say that that was the runner's speed rather than the runner's distance.
Sometimes energy storage devices are surprisingly reported in terms of power (the installation’s power output ends up being more relevant for whatever reason). For grid stability, being able to meet demand is more important than having days of energy, inaccessible, in reserve. So, surprising units isn’t a 100% bulletproof heuristic.
But in this case yeah, there seems to be a mix-up.
There's no mixup. Solar electric energy production in South Africa in 02019 did indeed average about 500 megajoules per second throughout the year, totaling about 16 petajoules; which is to say, it was about 500 megawatts, or, in cursed units, 4 terawatt-hours per year.
Separately, it's true that peak power is often a very important criterion for energy storage systems.
There is a mix up. It might be that the journalist is correctly summarizing the person they interviewed, but that person is wrong, or it might be that the journalist messed it up. But,
> In South Africa, for example, the total amount of energy produced from solar systems in 2019 was thought to be about 500 megawatts, Nana said
This is a very explicit phrasing “the total amount of energy produced” and the units don’t work out. You can figure out what they meant by doing additional research, but that doesn’t make it a non-error.
That's exactly the same as zelos's example of "the total amount of rope produced in 2019 was thought to be about 500 meters per second". It's a little bit odd, but it's a perfectly correct way to describe the total amount of rope produced in 02019†. Multiplying by the number of seconds in a year isn't additional research. In the same way, you can validly say "the total temperature rise of the teakettle over a minute was thought to be about 1.5° per second" or "the total GDP of South Africa in the early 02020s was thought to be about US$6400 per person per year".
There's nothing incorrect about any of these; it's mostly just a question of which units are the most convenient or unambiguous in a given context. And, I guess, which units your audience is accustomed to seeing; the SI unit for liters of gasoline per 100 kilometers would be square meters, but if you say your new car's gas mileage is 7.8 × 10⁻⁸ m², a lot of people will think you're using the wrong units, and they will surely have a hard time interpreting it.
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† Well, except that in this case the "500" is almost certainly made up; I don't think zelos was looking up South African rope production statistics!
You are mixing things up. On the one side there's the verbiage. On the other hand there's the units. You are mixing these things up in an extremely verbose way and I don't care for for it. Please just stop trying to actively mislead us all.
But hey, you reached your goal, killing the velocity of my comment way up there.
You planted the seed of "doubt". Congrats.
Because of behavior like this: HN is not a great place for discussing anything to do with energy. Fanaticism generally rules. The loudest person wins. This is sad.
Probably someone out there will be convinced by "the loudest person", but hopefully most people will instead be convinced by sound reasoning and evidence. That's my goal: understanding what is really true, rather than believing what you copy-paste the most times.
What, by giving the standard SI definitions of units so that people can see that the things you're saying are different are actually the same? You have a funny idea of what's muddy and what's clear.
You are mixing things up. On the one side there's the verbiage. On the other hand there's the units. You are mixing these things up in an extremely verbose way and I don't care for for it. Please just stop trying to actively mislead us all.
But hey, you reached your goal, killing the velocity of my comment way up there.
I was explaining why the misconception you were repeating was wrong. (It turned out to be a popular one.) I suppose that since you still haven't figured out that it was wrong, that looks like "misleading" to you, but actually you are the one doing the misleading. It's good to doubt your beliefs; that way you can abandon the false ones. Try it.
I think it is pretty common for reasoning to require a fairly long explanation to become clear. Hopefully I've achieved that for most readers, even if you are still struggling. But if you object to this thread being "extremely verbose", why are you copying and pasting the same comment into it here and in https://news.ycombinator.com/item?id=41998632?
The article explains that that number is "the total amount of energy produced" over the year, so it's averaged over the whole year including nights. The peak power is irrelevant to the total amount of energy produced except as an upper bound.
Power is king for commercial batteries. On whole sale energy markets you often see high prices in one 1/4 hour, and much lower prices 1 hour later.
You can than nicely make an arbitrage profit if you completely cycle the battery.
Also many such batteries have similar Max energy and power limits, e.g. 1.5MWh energy vs 1MW power.
Edit: noticed that HN user kragen has replaced the comment I replied to with a dot, for some reason. This doesn't seem civil to me.
You think it's clear tell that she knew what she was talking about when she wrote "the total amount of energy produced from solar systems in 2019 was thought to be about 500 megawatts".
Similarly you think it's a clear tell that I don't know what I'm talking about when I say that this is an incorrect statement, and that this is a clear tell.
Yes, because, as I explained above, I checked Wikipedia, and that figure turns out to be correct to within at worst a factor of two. Power units such as megawatts measure the rate at which energy is produced, consumed, or otherwise converted, which is to say, the amount of energy used per unit of time. For example, per year. A watt is about 31.6 megajoules per year.
So there's nothing actually wrong with the reporter's statement, even though it would be more precise to say that the average power generated was thought to be around 500 megawatts.
No, I claim that you don't know what you're talking about because you think the units were wrong in the given context and that the arithmetic values were wrong, which they weren't.
You keep repeating that as if you think it's going to convince somebody, but, in fact, when you divide an amount of energy produced, such as 4 terawatt hours, by a length of time such as a year, you get a power, such as 500 megawatts, which does indeed tell you how much energy was produced over that length of time.
We normally use joules for energy, though sometimes people do unfortunately use cursed units such as foot-pounds, calories, BTUs, megawatt-hours, electron volts, and so on. My analogy of "the distance covered by the runner was 12 miles per hour" is quite precise here. Saying that we're talking about the total energy produced in 2019 clearly excludes your hypothetical interpretations of peak output or nameplate capacity, neither of which would tell you the energy produced in a year.
It's true that someone who didn't understand the area might say "500 megawatts" when they meant "500 megawatt-hours", but the reporter in this case didn't make that error, and anyway 500 megawatt-hours in a year would be 57 kilowatts, which is an implausibly small amount of solar power to be produced by an entire medium-income country. That's more like a single large commercial building.
Domestically the most common unit of energy is calories in the countries I'm familiar with, but households also frequently measure energy in gallons of gasoline, liters of gasoline, British Thermal Units, "therms", cubic feet of gas, cubic meters of gas, kilowatt hours, cords of wood, bushels of corn, joules, and "tons", meaning tons of ice (often abused as a measure of power, the implicit denominator being "per day"). Additional common units of energy rarely used in households include barrels of oil, "megatons" (this time of TNT rather than ice), toes (ton of oil equivalents), quads (quadrillion BTU), foot-pounds, and, implicitly, horsepower seconds. The vulgar news media often use the "household-day", which is roughly as well defined as the "heap".
This is like the seventeenth century situation where cloth was sold by the "ell", but each kind of cloth was measured by a different ell. The Flemish ell was a different length from the English ell or the Scottish ell. This nonsense survives today in the custom current in the less advanced countries of weighing gold and silver in "troy ounces" and other commodities such as pepper in a different "ounce" that is about 10% smaller.
But the situation with energy is much worse, because transmuting electrical energy into gasoline, or vice versa, is enormously less inefficient than transmuting pepper into gold. Different forms of energy are far more often equivalent than different forms of mass.
The insight underlying the SI system of units is that calculations become much easier if you use a consistent set of units derived in a simple way from a minimal set of base units such as the kilogram and meter. How much kinetic energy is going to kill a 100-kg man falling from a building at 10 meters per second? Trivially, 50 kilojoules. How much energy does a 10-watt lightbulb consume in 5000 seconds? 50 kilojoules. If you are paying US$0.02 per megajoule for gasoline, US$0.01 per megajoule for natural gas, and US$0.04 per megajoule for electrical energy, what's the cheapest way to heat your house? Gas, unless your electric heat pump has a coefficient of performance over 4.
By contrast, if the different prices are expressed in different units, it adds difficulty to every kind of reasoning, to the point that there seems to be a popular misconception that megawatts are not even the right unit for measuring the solar energy production of a country.
Kilowatt hours are an especially silly unit. A watt is by definition a joule per second, so measuring energy in kilowatt hours is similar to measuring distance in mph-minutes. An mph-minute works out to be exactly 88 feet, and a kilowatt hour is exactly 3.6 megajoules. Just use megajoules! Stop making everything more difficult.
Power is energy per unit time. The statement is like saying "the total amount of rope produced in SA is 500 meters per second". Being generous, you'd assume that's (total production per year) / (seconds in a year), I guess.
Yes, I think that's a perfectly reasonable way to describe the amount of rope produced in South Africa. It uses the correct units and therefore contains no ambiguity. Saying instead that the total amount of rope produced in South Africa was 16 billion meters, leaving the "per year" implied, would, by contrast, be ambiguous and open to misinterpretation; equally plausible interpretations would be "in all of history" (the literal interpretation) or "per month". Unless, as in this case, the statement was qualified with a particular year-long time period, such as "in 2019", in which case "500 meters per second" or "16 billion meters" would be equally unambiguous ways of describing the situation.
"500 meters per second" is not just (total production per year) / (seconds in a year) but also (total production in a day) / (seconds in a day) and (total production in a month) / (seconds in a month).
Meters per second, like watts, has the advantage of being an SI unit and therefore facilitating calculations with other SI units without requiring a bunch of numerical conversion factors.
So the average power produced by those plants was 500MW.
That phrase is still nonsense. The fact that if you go, do your research, get the real numbers, and interpret them you can discover that what the journalist is trying to say is correct is meaningless. You can still cut the journalist out from the process and lose nothing.
