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points by pintxo 1 day ago | 0 comments

European units (`Energy/100km`) work nicely here, lots of EVs are around `20 kWh/100km`, and most residential housing around here consumes less than `20kWh/day`, actually averaging `20kWh/day` would probably cover a well insulated house including heating (heatpump).

Yes, driving, compared to living, uses ridiculous amounts of energy.

[–] dllu 1 day ago

With dimensional analysis you can cancel it all the way out until it's just Newtons, which is actually just the average drag force times some efficiency multiplier.

[–] jaggederest 1 day ago

Dimensional analysis is a superpower honestly. Even in a software engineering career it's saved me from massive issues quite routinely

[–] vpmadd52huq 1 day ago

Mandatory XKCD: https://xkcd.com/3038/

[–] dllu 1 day ago

Also: https://xkcd.com/687/

[–] NetMageSCW 1 day ago

The HP 48G / 50G provides incredible unit handling. My main use for it these days.

[–] selimthegrim 1 day ago

Any war stories?

[–] jaggederest 1 day ago

Requests per second, bytes per request, bytes per second should all match up very closely under dimensional analysis or something is drastically wrong ;)

Little's law is another related phenomenon that is super critical. Measuring things when the units don't match has bitten me more than once.

[–] selimthegrim 1 day ago

Concur on Little's law

[–] gertlex 1 day ago

I'm curious if you can elaborate further? Would that make the comparison clearer, or is it just a side-note on your part?

[–] dllu 1 day ago

Work (energy) is force times distance. Since we're talking about energy divided by distance, it's actually just average force.

Actually newtons are just a constant factor (3.6) compared to Wh per km, since there are 3600 seconds in a hour while there are 1000 watts in a kilowatt. For example 144 Wh/km (or 14.4 kWh/100 km) would simply be 518 N. This also means that pushing along a Tesla on the highway at constant speed is roughly as hard as holding a 50 kg weight, giving you an intuitive sense of how "hard" it is to keep things moving.

Of course, in everyday life, since we buy electricity from our utilities in kWh rather than joules, it's more convenient to talk about kWh per km (or per mile).

[–] gertlex 1 day ago

Thanks. Was there a particular reason I'm not grasping for noting that on the comment you replied to? Seems like a comment that could be stated about any comment tackling about energy/distance.

Actually, I guess the comment you replied to was also not really clear in it's point, so I'll blame that guy for maybe diverging from the topic somewhat :)

(not trying to be negative, really! but my brain is a bit fuzzy from being sick, so thus feeling a bit dimwitted as I ponder comments)