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Comment by manmal | Hacker News Reader

We have a heat pump with SOCP 5, but are still using 30kWh per day now.

On most modern EVs, that's at least 3 days worth of power if the vehicle is not being driven. Larger vehicles naturally have far larger batteries.

Mmmh no? 90kWh battery packs are not that common. An M3 or MY LR AWD have an 82kWh. Some Mercedes and some Chinese model have a 100+kWh battery but the most common setup is between 60 and 70.

Anyway I agree that in emergency conditions it can go a looong way for a house.

My bmw has 108

That's 2-3 days worth without changing your use pattern, which surely you would do if you expected a long blackout period?

Is that 30 kW each hour of the day or 30 kW per day?

Sort of neither?

It's 30kW for one hour.

(Or 1.25kW for 24 hours, or however you prefer).

[Link: https://en.wikipedia.org/wiki/Kilowatt-hour]

Pretty sure GP understands that, their point being you can't just wave away 'per unit time' when talking about it relative to generation, it matters.

"30kWh per day" isn't waving away anything.

30kW is a rate.

30kWh is a measure of energy, about 100MJ.

30kWh/day is a rate again, 100MJ/day.

It seems clunky, but it's a way energy usage in a house is typically measured.

"30kW per day" makes no sense. It's like saying "100km/h per day". W is a unit of power, Wh is a unit of energy.

It's unintuitive because a guy named Watt decided "Joule per second" is Watt, so the intuitive units Joule and Joule per second got turned on their head to W times hour and Watt. This is opposite to usual intuition of distance and speed for example, km and km/h. If I redefined the km/h to a "tick", then a km would become "tick times hour".

If you want a "better than nothing" but imperfect analogy, Wh is how much water you have in a bottle, and W is how quickly you can get the water out.

It's like you had a faucet with cross section of 2 square inches, and 15 seconds with that faucet wide open would fill a jug. That jug holds 30 squareinches-seconds of water.

You may then use that water over the course of a day. That's 30 sqin-sec per day. That is not 30 square inches for a whole day. It's second per day.

Or watt hour per week or whatever.

Yes, it can be described like 0.00034722 sqin/day, and that corresponds to no physical object and that would be largely useless.

Why not Joules or Liters? Because that's complicated for anything electrical.

30 kWh per day, while a clunky unit, is what most people would understand when talking about home consumption. It’ll be what their utility typically reports, how you’d want to size a battery, etc.

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