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Comment by tromp | Hacker News Reader

> The reactor must be able to generate an uninterrupted electricity output of at least 10 kilowatts. The average U.S. residential home, according to the U.S. Energy Information Administration, uses about 11,000 kilowatt-hours per year.

Would be more helpful to express that as 11000/365/24 = 1.256 kilowatt, so you understand that the reactor could only power 8 U.S. homes.

Btw, what are non-residential homes?

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Holiday homes, homes held as a capital investment and not leased out, homes of people who have entered long-term care but not sold the primary residence...

Ah yes; rarely-occupied homes would indeed have much lower power consumption...

The snowbirds keep them nice and frosty, even in the off season, unfortunately.

I've always found the use of kWh/year so absurd. I mean Wh is kinda weird as a unit for energy, but this is just plain asburd.

Not if you work in the power industry.

Wh is directly equivalent to joules. It makes perfect sense as a unit of energy.

1 Wh = 3600 joules

That's the point though. Just refer to it in joules.

It's kind of like radians vs. degrees. Degrees seem ridiculous if you're doing trig, but working with irrational fractions of pi is ridiculous if you're trying to measure with a protractor. If you need to convert to radians to do some trig (and let's be honest you usually don't) you are going to end up converting something like degrees that are marked on the protractor anyway.

> Degrees seem ridiculous if you're doing trig

Degrees are ridiculous if you're doing calculus. They don't really pose any problems if you're doing trig.

> but working with irrational fractions of pi is ridiculous if you're trying to measure with a protractor.

Sure, that's true, but that's only because irrational numbers are by definition infinitely precise. It's no more difficult to use a protractor labeled in increments of 0.02 radians than one labeled in increments of one degree.

30/60 degrees are round numbers which are very useful for a lot of applications. Your .02 radian protractor doesn't have a mark on it for either.

Realistically, it probably would; those marks would just not fit into the progression of .02 radian marks. Ever seen a meter stick that also measured inches?

Are you trying to measure an angle, draw a particular angle, or confirm that an angle meets a particular standard?

360 is evenly divisible by 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, and 20.

0.02 radians would absolutely be a ridiculous unit of measurement if you were a carpenter or draftsman in the age before computing.

> 360 is evenly divisible by 1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, and 20.

But if you're trying to measure an angle, this is totally irrelevant. The angle is whatever it is.

That's a general statement I'd agree with, but it's a losing battle. So many consumer products are rated by power that "I turn this 2000 W thing on for 1 hour, thus it consumes 2000 Wh of energy" is kind of convenient, if weird.

My real surprise and problem comes with Wh/x, where x is any unit of time. Why not cancel h/x for any dimensionless factor? This is what the grandparent is referring to.

I know. And while I would prefer J as the unit for energy, I accept the convenience of Wh. The problem comes with Wh/x, where x is any unit of time. Why not cancel the h/x for a dimensionaless factor in this case?

Most utilities charge by the kWh, so this measure makes it easy to compute costs.

Watt hours are the most widely used unit for energy in science and engineering ...

I know (in the case of engineering anyway; I would hold joules are more used in science). I accept the convenience of Wh. The problem comes with Wh/x, where x is any unit of time. Why not cancel the h/x for a dimensionaless factor in this case?

It happens all the time in reporting. It seems to be part of the journalistic style guide to provide context to quantities by comparing them to another quantity. Here, both units contain the word "kilowatt" so they are used as comparisons.

Of course it's innumerate, but that's a broader problem for our society.

I never understood why kWh/yr is so commonly used.

watt-hour itself is already a somewhat silly unit, since joule already exists and is part of the definition of watt. but people can easily multiply an appliance's watt rating by the amount of time they use it to estimate its monthly energy use, which makes it a convenient unit for billing. I would guess kWh/yr is an extension of this idea. you could use J/yr, but most people don't have an intuitive sense of how much power that is.

I'm just surprised that the U.S uses such metric units as Watts and Joules, rather than foot-pounds or calories

I'd imagine it has something to do with the timeframe that terms describing units of electricity came into common usage.

The U.S. is a signatory of the Metre Convention (1875). All of our customary units are defined as linear conversions of corresponding SI units. The Watt and Joule (also Ohm) were not standardized until 1893. Funnily enough, the international conference where these were officially decided happened in Chicago.

The USA more or less fully adopted the metric system in the 90s. All food is labeled in metric units for example. We just never abandoned imperial weights and measures. This leads to some amusing things like large bottles of soda and wine being measured in metric, while cans are measured in ounces. Then again we're not alone. The UK is considered officially metric, but they still pour beer in pints and weigh people in stone.

There are good arguments to be made for everyday use of Fahrenheit over Celsius. The 0-100 range in F cover pretty much all of the temperatures one is likely to experience in the USA. It also has a finer resolution than degrees Celsius in everyday weather temperature ranges.

The SI units are designed for scientific and engineering use and they excel for the purpose they are designed.

> There are good arguments to be made for everyday use of Fahrenheit over Celsius. The 0-100 range in F cover pretty much all of the temperatures one is likely to experience in the USA.

You're mostly right, but there are plenty of places in the US where the daily low temperature drops well below zero for significant parts of the year.

I guess the inconvenience with Celsius is that you're more likely to go to negative numbers? Is there a problem other than the having to say "minus ten degrees" vs "fourteen degrees"?

> It also has a finer resolution than degrees Celsius in everyday weather temperature ranges.

I'd make the opposite argument about everyday use. Most people I know would barely differentiate 25C from 26C, so there's not really a need for finer gradations. And the 10 degree increments are surprisingly convenient:

   -20 (or less)    dangerously cold
   -10 -- -20       damn cold
   -10 --  0        winter
    0  --  10       chilly
    10 --  20       brisk
    20 --  30       hot
    30 --  40       sweltering
    40 --  50       damn hot
    50 (or more)    dangerously hot

I like that table, and when I’ve been in Europe it’s pretty much what I’ve used. Oddly though while 30 degrees in southern Germany feels unbearably hot, 86 degrees in southern California is warmer than ideal, but not what I’d call scorching. I wonder to what extent it’s physical factors besides temperature and if there is a psychological effect from the temperature scales.

Humidity is a major factor. We cool our body below ambient temperature through evaporation cooling (sweat), which works much better in dry air.

From experience I'd say a sauna with 45°C and 80% humidity is about comparable with one at 80°C and 10% humidity.

And height as well. It is hard to remember mostly 1.x metre high (rarely 2m). But in feet you can have change in the first unit. After all feet is about foot. More natural.

Vast majority of people are between 5 foot and 6 foot, so I'm not sure what your point is.

Most beer pints are not a full pint. And they keep getting smaller!

Don't jinx us! haha

Calories are metric as well: 1 kcal is the amount of energy required to heat 1 kilogram of water by 1 degree Celsius.

>I would guess kWh/yr is an extension of this idea. you could use J/yr, but most people don't have an intuitive sense of how much power that is.

Just using kW for your average power usage over a year seems more intuitive to me.

Seems to make sense for capacity planning. The considerable daily and seasonal variation in use argues in favor of using the aggregate rather than expressing it as an average.

If you're doing capacity planning, wouldn't you have to plan for the peaks? (Kind of like highways and rush hour.)

For the moon, I'd plan on batteries being used to meet demand peaks (like we are starting to do here).

Weird units like that are usually inertia from the pre SI days.

But electricity consumption was never measured in anything else. Here, it's because the aggregate unit, kWh, is present on the bill, so the derivation for kWh/year is easy to determine.

Because a home that uses 11000 kWh/yr doesn't use 1.25 kW. During low power usage times (middle of the night) it uses substantially less, and at peak times it uses substantially more. And even these values will shift with time of year and weather. kWh/yr implies you are averaging energy use over a substantial period of time, whereas kW is ambiguous on its own. MJ/yr would work as well and have a similar order of magnitude, but it doesn't really have any advantage in its favor.

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